CA1313694C - Transmission and reception apparatus for automobile - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
An antenna element including a loading coil is composed so as to be expanded and contracted like a telescope upward from a housing tube. A characteristic impedance of a transmission line from a lower end part of this antenna element to a cable is equal to one of a cable. A part of the loading coil is reinforced.
A branching filter, which is set between the antenna and a communication means using a different frequency band, suppresses a mutual interference between signals of communication means.
An antenna circuit, which is set between the antenna or the branching filter and the communication means, converts an impedance of a lower part in a frequency band, and reduces a loss due to a capacitive antenna impedance.

Description

13~3~
This inve~tion relates to an apparatus employing a single antenna to transmit and receive at low loss and without mutual interference, signals in different frequency bans, such as mobile telephone signals and radio broadcasting, and is p~eferably mounted to a car.

Reference is now made to the accompanying drawings, in which:

Fig. 1 is a longitudinal sectional view of a conventional whip antenna 1 in an extended state;

Fig. 2 is an equivalent circuit diagram in which whip antenna 1 is used for the reception of frequency-modulated broadcast;

Fig. 3 is a equivalent circuit diagram in which whip antenna 1 is used for the reception of amplitude-modulated broadcasts;

Fig. 4 is a longitudinal sectional view of another conventional whip antenna 31 in an extended state;

Fig. 5 is a block diagram of a conventional transmission and reception apparatus;

Fig. 6 is an electric circuit diagram showing the equivalent of an antenna 53 and a low pass filter 52 of a transmission and reception apparatus 50;

Fig~ 7~is an equivalent circuit diagram in a frequency band of AM broadcast in a conventional antenna 61 and a cable 62;

Fig. 8 is an overall schematic of a mobile transmission and reception apparatus according to the present invention;

. '~

13~ 3~

Fig. 9 is a sectional ~lie~ of one embodiment of a multi-band r~hip antenna according to the present invention as shown in an extended state;

Fig. 10 is a sectional view taken along line A-A in Fig. 9;

Fig. 11 is a sectional view taken along line B-B in Fig. 9;

Fig. 12 is a sectional view of another embodiment of a multi-band whip antenna according to the present invention as shown in an extended state;

Fig. 13 is a sectional view taken along line C-C in Fig. 12;

Fig. 14 is a sectional view of a further embodiment of an multi-band whip antenna according to the present invention as shown in an extended state;

Fig. 15 is an electrical circuit diagram of an embodiment of a branching filter according to the present invention;

Fig. 16 is a graph showing frequency characteristics of a band inhibiting filter;

Fig. 17 is a schematic of an embodiment of an antenna circuit according to the present invention;

Fig. 18 is an equivalent circuit diagram of an antenna circuit for explaining the principle of the present invention.

Fig. 19 is an equivalent circuit diagram for explaining the principle under consideration with respect to the capacity of CF in the equivalent circuit shown in Fig. 18;

~3~L3~
Fig. 20 is a graph showing -the relation bet~een reception frequenc~ f and output ~oltage leve] V41 in the equivalent circuit shotin in r ig. 15;

Fig. 21 ls an equivalent circuit diagram in an AM radio signal frequency band f2a of an antenna circuit; and Fig. 22 is a schematic drawing of a further embodiment of an antenna circuit.

Fig. 1 is a sectional view of a typical conventional car-mount whip antenna 1 in its extended state. This whip antenna 1 is mounted, for example, near the rear trunk of an automobile car body 2, and is used commonly of the transmission and reception of signals for a mobile telephone and the reception of radio broadcasts. An antenna element 3 of this whip antenna 1 comprises a first antenna element part lS 4 having a round tubular shape, and a second antenna element part 5 telescopically disposed within the first antenna element part 4. The antenna element 3 is accommodated in a housing tube 6 fitted in a mounting hole 14 formed in the car body 2. The housing tube 6 is composed of a tubular body 7 made of electric insulating material such as resin, an outer conductor 8 and an inner conductor 9 made of conductive materials.

The first antenna element part 4 is composed of a sequential connection of a first conductor 15, a phase shifting coil 18, a second conductor 16, a band separating coil 19, and a third conductor 17. These conductors 15 to 17 and coils 18 and 19 have identical outside diameters. The phase shifting coil 18 functions as a phase shifter on frequency fl of a mobile telephone, so that the current distribution in reverse phase may be suppressed low, while the normal phase portion is emphasized in the current distribution profile. The band separating coil 19 has a high impedance against frequency fl 1~3~
of a mobile telephone, and a low impedance against frequency f2 of a radio broadcast Therefore, a colinear array antenna is constituted by conductors 15 and 16 and the phase shifting coil 18, which may be used for the transmission and reception of mobile telephone signals. The overall length of the antenna element 3 is used in the reception of radio broadcasts.

A leaf spring 28 is fixed at a lower end part 15a of the antenna element 3. By this leaf spring 28 the antenna element 3 is supported so as to be slidable in the axial direction, while it is electrically connected with the inner conductor 9. At an upper end part 6a of the housing tube 6, the outer conductor 8 is fixed to the car body 2 by way of metallic fixing tubes 21 and 22 and fixing plate 23, and thereby connected electrically. The connections of the housing tube 6, fixing tubes 21, 22 and the fixing plate 23 are filled with sealing resin 24, and a nut 25 is screwed thereover.

Beneath the housing tube 6, a connection hole 26 is formed near the lower end part 9a of the inner conductor 9. In the connection hole 26, an inner conductor 12 of a coaxial cable 11 is connected to the inner conductor 9, and an outer conductor 13 of the coaxial cable 11 is connected to the outer conductor 8. The coaxial cable 11 is supported by a cable support member 30 fitted to the outer conductor 8.
This coaxial cable 11 is connected to a branching filter 27, and this branching filter 27 is connected to the transmitter/receptor of the mobile telephone and the radio set by the coaxial cable 29a and 29b.

This whip antenna 1 is erected, for example, near the rear trunk of the car body 2. Therefore, the are a large number of restrictions imposed due to the shape of the car body 2, 1 3 ~
such as on the width of the rear fender, and the size of the mounting hole 14 for mounting the housing tube 6. Besides, if the outer diameter of the antenna element 3 is reduced too much in order to resist the wind pressure while traveling, the tubular body 7 made of electric insulation material becomes thin, and the spacing between the inner conductor g and the outer conductor 8 becomes small.

Therefore, as mentioned below, the characteristic impedance Z2 from the upper end part of the housing tube 6 to the lower end part 9a of the inner conductor 9, that is, in the section 2 up to the current feed point P is lowered. On the other hand, if the mobile telephone is used in a state in which the impedance at the current feed point P is mismatched, the signal sent out from the he transmitter is reflected, so that the coil in the transmitter may be burnt.

Therefore, by forming the length of this section ~ 2 at about 15 cm or half of the wavelength ~ 1 of the mobile telephone the impedance matching is achieved. Therefore, the current feed point P cannot be set at an arbitrary position. Such construction of the whip antenna 1 in accordance with the above-mentioned restriction causes the following problems.

Fig. 2 is a equivalent circuit diagram in which the whip antenna 1 is used for the reception of frequency modulated (FM) broadcasts. In this antenna element 3, supposing the characteristic impedance of the section ~ 1 projecting from the upper end part 6a of the housing tube 6 to be Z1, and the characteristic impedance of the section ~ 3 of the coaxial cable 11 to be Z3, the characteristic impedance Z1 of section 1 is nearly equal to the characteristic impedance Z3 of section ~ 3, and is, for example, about 50 ohms. Moreover, the characteristic impedance Z2 of the section ~ 2 is expressed as follows, assuming the outside diameter of the inner conductor 9 to be d, the inside diameter of the outer ~3~3~

conduc-tor 8 to be D and the specific dielectric constant of the tubular body 7 to be r r:

Z2 = ~,_ log~ d [.`~ .................. (1) On the other hand, because of the restrictions imposed by the S shape of the car body 2 as mentioned above, there is not a large difference between the outside diameter d of the inner conductor 9 and the inside diameter of the outer conductor 8, and therefore as is clear from eg. (1), the characteristic impedance Z2 in the section ~ 2 is lowered, and the impedance matching between the section ~ 1 or antenna element 3 and the section ~ 3 or the coaxial cable 11 is worsened, whereby transmission loss lncreases. Accordingly, the length of the section ~ 2 becomes too long to be ignored with respect to the wavelength ~ 2 of FM broadcast, and the band width is consequently narrowed.

Fig. 3 is an equivalent circuit diagram in which the whip antenna 1 is used for the reception of amplitude-modulated (AM) broadcasts. The length of the antenna element 3 is formed in accordance with the mobile telephone and FM
broadcast, so that it is extremely short for the wavelength of AM broadcasts, and the radiation resistance almost becomes null, and the characteristic impedance Zl becomes capacitative.

Supposing the capacity of section Q 1 to be Cl, that ~ 3 ~

o~ section R 2 to be C2, and that of section ,f 3 to be C3, the relation bet~.~een a voltage V1 induced in the antenna element 3 and a voltage V2 at the powder receiving end obtained by way of the coaxial cable 11 is shown in the following equation:

v2 Cl + C2 + C3 vl ......... ....... ,....... , (2) where the capacitance C1 of section 1 and the capacitance c3 of section 3 are constant, and the power receiving end voltage V2 may be raised by reducing the capacitance c2 of section ~ 2. However, the capacitance C2 of section ~2 is, supposing the specific dielectric constant in a vacuum to be Eo, expressed as follows C2 = 2 ~D~/2-Q2 iF]............. ........ ....... (3) and the ratio of the inside diameter D of the outer conductor 8 to the outside diameter d of the inner conductor 9 cannot be increased too much as stated above, and therefore the power receiving end voltage V2 cannot be increased too much.

Fig. 4 is a sectional view of another conventional whip antenna 31 in an extended state. This long bar-shaped whip antenna 31 is mounted near the rear trunk of an automobile car body 32, and is commonly used for the reception of ratio broadcasts and the transmission and reception of mobile telephone signals. An antenna element 33 of this whip antenna 31 is composed in a sequential connection of a firs conductor 34, a phase shifting coil 38, a second conductor 3S, a band separating ~3~3~$~
coil 39 ~ ri conci~_t~r 36, and a ~o~ conduct~r 37.
The fi-st conductor 3! and the second co)ld~ctor 35 ila;e -i~ a round cylindricalshape, and the third conductor 36 is formed like a cap.
~ 1ithin a space 43 formed by the ~irst conductor 34, the phase shifting coil 38, the second conductor 35 and the band separating coil 39, the fourth conductor 3/ is accom~
modated. The outside diametersof the first to third con-ductors 34 to 36, and coils 38 and 39 are identical, and are kept in a housing tube 40 provided in the car body 32.
The housing tube 40 is composed of an electric insulating tube body 40a, an outer conductor 4Cb, and an inner conductor 40c. An outer conductor 44a of a coaxlal cable 44 is connected to the outer conductor 40b, and a inner eonductor 44b of the coaxial eable 44 is conneeted to the inner conduetor 40c.
At the high frequeney fl of a mobile telephone or the like, the phase shifting eoil 38 functions as a phase shifter, and the normal phase portion is emphasi~ed by suppressing the current distribution in the reverse phase, while the band separating coil 39 has a high impedance, thereby forming a colinear array antenna by the first conductor 34, the phase shifting coil 38, and the second cndUctr 35 to be used for the transmission and reception of .L~ r~ f ~L ~t ~r t}l~

r~ti~ L ,~ - lv~ p~la~ nd '.il `; .' S' '.~ vll~''UCt')L_. .,; ':' 3 / 1 ._ :,~iia lG ~1-'8 '?
~L-~ arlt~lln~ ~or i~ L~ ttl-~ r~
l c ;~
~ ,irlc~ trl~ ?ortiorls of COl ' S 3G dll'i 3? ~ llOit low strellgth, -he~ are li~:ely to be brok~n, a~ tne~ are reinEorce~l b~ molding resins 41 and 42 .hereto. The resin portions 41 and 42 have the same outside diameters as those of first to third conductors 3-~ to 36 so as not to form an oostruction ~whell the antenna eJement 33 is put into the housing tube 40.
In the thus composed whip antenna 31, the resin portions 41 and 42 are bulged out, inward in the raaial direction of coils 38 and 39, in order to obtain a desired strength. Therefore, such bulaing would interfere with the displacement of the fourth conductor 37 into the space 43, and it is difficult to provide resin portions 41, 42 with a thickness sufficient to obtain a desired strength. Besides, after the coils 38 and 39 are once molded with resins 41 and 42, it is difficult to adjust the length of the coils 38 and 39. Furthermore, since the first to third conductors 34 to 36 are metallic, thus being of material different from thc resin 41 and 42, the antenna is deemed to be unaesthetic.
Fig. 5 is a block diagram of a conven~ional transmission/reception apparatus 50 tor a mobile telephone.

1 3 ~L tJ .~ ~J ~

'.;r)~dca-ats i, or1er to z.,lar~ ~he ~: .:.'.~!iil-^. in ~IllS '~Ja';, ~h"
.l'Ji~ t ::tlr` !nooil~ t~ n~ a _ ~n!lr~r`~ 'ti~l th~
.-jL-lllli lina 5. ~he radio set. Ther-: e, wilen a radio broadcast ~ L-ceived ;rhile using ~ne mobile tele~hone, the ,o-called b-at noise is mi~:e-d in the sound reproduced b~f the radio set. To prevent the generation o~ such beat noise, the elelnellts sho~/n in Fig. 5 have been us-d r.itherto.
The frequency band f2 of radio broadcasts is, in ~il broadcasts, frequency band E2a, that is, 500 to 1620 kHz, and, in F~t broadcasts, frequency band f2b, tAat is, 76 to 90 ~IHz. In the mobile telephone, on the other hand, for radio communication ~./ith the ground station connected with the telephone line, a frequenc1f band fla of 870 to 890 ~Hz is used in receiving, and a frequenc,~ band Elb of 920 to 9~0 ~1~z is used in sending. The prior art sho~ln in Fig. ~ makes use of such a diEference in frequency band.
In other words, a radio set Sl is connected to an antenna 53 by way of a low pass filter ~2, and the mooile telephone 5~ is connected to the antenna 53 by way of a high pass filter 5;. The signal line connected to the mobile teLephone ~ is joined to the signal line connected to the 131~
radio set ~ cas~ radio cOn~unicatialls b,s the mobile telephor~e 5~!, since thc freauenc-~ band ~l of the ai~,~.als .~ns.~i--e,.
or receive~l bv ~he mobile telephone 54 is relativel~ hignj the ra~io set 51 ~ill not generate beat noise hy the 1nterfer-ence wlth the signal ln the frequency band ~2 used in the mobile telephone 54 o~lng to the low ?ass -il er 52A

The equivalent circuit of the antenna 53 and the typical circuit composition of the low pass filter 52 are shown in Fig. 6. A capacitor Cll is connected in series to a signal source 56, and coils Lll and Ll2 are connected in series to this capacitor Cll. The contact point 57 of coils Lll and Ll2 is grounded by way of another capacitor Cl2.
The relation between voltage Vll generated in signal source 56 and output voltage Vl2 of the low pass filter 52 due to electrostatic capacity of capacitors Cll and Cl7 is as follows:

V12 ~ Cll + cl2'Vll ......... ,,..... ,,,,, That is, in the low pass filter 52, since the capacitor C12 is provided between the signal line and the ground, the output voltage V12 of the low pass filter 52 unfavorably becomes smaller than the generated voltage Vll in the signal source 56. In e~. 4, since it is supposed that radio broadcasts are to be received the attenuation of signals by coils Lll, Ll2 is ~3.~v~
ass~me~i to be sllfficio~tl~ small.
~ is. 7 is all equiialent circuit ~ii.gram in the frequenci baaa t2a of A;i broad_ast of an aatenna 61 an~ a cahle 62 in a different prior device.
In a car-mountedradio set, it will ~e very convenient if FM radio signa~, ~1 radiosignals, and mobile telephone signa~ can be r^ceived by one antenna. In a construction in which the antenna is expanded or contracted by a motor or the like, a signal cable cannot be attached to the lower end of the antenna, and it is difficult to shorten the signal cable.
Accordingly, the cable capacity of the signal cable increases, and the impedance derived from the cable capacity becomes high. In parti^ular, in radio signals of a relatively low frequency band such as AM radio signa~, the effect of cable capacity bscomes larger. Therefore, in a car-mounted antenna, signals in a wide frequency band must be sent out to the radio set while suppressing the loss Dy the signal cable.
The antenna 61 can be represented by antenna effective capacity Ce and antenna reactive capacity Ca, and the ~M radiosignals receive~ by this antenna 61can be represented by an alternating-current power source V21.
The cable 62 can be shown as a line ~ll between terminals Al and Bl, and this line Qll is groundsd by way of cable capacity Cb. The signal at the terminal Bl is fed into a radio set. The voltage V22 at this terminal Bl is 1 3 ~
e~?-essed as follo/s ~ + ~a + Cb ~21 ........................ .

As e~:pressed in eq. 5, supposing that the cable capacity Cb is large, the gain of the AM radio signal of relatively low frequency received by the ant2nna 61 is ~owered so that the cable capacity Cb makes the receiving sensitivity and the ratio of signal to noise (S/N ratio) drop.

To prevent such a drop in receiving sensitivity and S/N ratio, an amplifier is placed between the antenna 61 and the cable 62, that is, at the position of terminal Al, so that the receiving sensitivity and S/N ratio are improved.

In such an antenna, since active e].ements are used, they give rise to an increase of cost, and also involve other problems such as maintaining a circuit characteristic of suppressing only the distortion of signa~ at the time of input of a strong electric field.
In addition, new problems may be also experienced, such as loss due to impedance conversion in the amplifier, and insufficient matching of impedance.

3 ~

~ A~ t ~1r,l~,ar, o~ ct ~ t:t ~ 5.~ _5 :prz.,zn-: a r~ , im?ro;ed ~ransm ssi,n a?.~ '-'Ce?i- 5n ~:~?r~ra-Us GL- ~tt~m30ilcs ',lhl-i-l sol ;cs ' ''i'' -~3~;z-~ii.,~,lls-iz~
prsoicms.
J t is a~lother ob~ec~ o~ this in~;en~3n . 3 ?reseQt a multi-band whl? antenna havinG relati;zl, low .-ansmis.sion Loss, capable cf matching ~hz impec~ancc fa~-orabLy, while COnfOrmillq t3 restrictions im?osed bv the car bod-~ sha?e.
To achieve the above objects, in an multi-band whi?
antenna of the present invention, having a housing tube which is connected and fi~ed to a car body of an automobile, an antenna element which is disposed in the housing tube, is electrically insulated from the housinq tu~e, and c~n be e:ctended and retracted like a telescope upward from the housing tube, and a cable which is electrically connected to the lower end part of the antenna element, in a state where the antenna element is drawn upward and e~tended from the housing tube, the improvement comprising:
a lower end part in the housing tube has a first lower end part which is smaller in diameter than the ?ortion of the antenna that extends above the housing tube when the antenna element is ectended, and a second lower end part which is larger in diameter than the J i ~

c~r ~ r ~ ~ _ r r~

se ~ r~ iCi. i J ~_r~tl ! -~ 'i,,~cent _ 'r~ r~ ~' ~~ ~ ~
io:e- end -ar~ -~~ i5 lar~er .. diame~e~: -han ~ rr--lo/er eni ?a-- Ti.e housing tu~e nas a irst oll~er - -e part sur-ound~l.- the firs~ lcler en~i par~ b~,/a; 5, an elecL-ic insulaclcn tube bod~, and a second outer tu3e part . .
surrounding the second lower end part b~ ~./ay o tr.e elect-lc insuia~ion tube bod-y, the second Guter tube part being disposed adjacent the first ou~er tube ?art and naving , larger inside diameter than the first outer tube part.
The outside diameter of the first lower end part and insi~e diameter of the first outer tube part, and the outside diameter of the second lot~er end part and inside diameter of the second outer tube part are selected so that the characteristic impedance due to the first io~ler end part and first outer tube part, the characteristic impedance due to the second lot~er end part and second outer tube part, and the characteristic impedance due to the antenna elemen~ and cable may be nearl~/ equal to each other.
Thus, according to this invention, if the antenna element is used for the transmission and reception of mobile telsphcne signals and for the reception of FM broadcasting, the impedance matching of antenna element and cable may be achieved ' D ~ ` O '1, i !' .'~ u o r~ Eirs- ~o;~r ~.~
~ar- ~;n -n , maller ln diamecer th_n C,`'' D5r-'On 0.' ''.^"
-~nten.;a -`ie'-le!l_ ?r_lec~in~ ;-om cn-- ;.olsill~ c~ke, and ~
se~onc iowe: ''!lC ?art ~nicn i, directi, adiacen_ c~e f -s.
iower ena ~ar~ -nd is larger ~n clamerer .han ~he r,e lower end part. Tne housing tube has a .irst outer .~be oart surrounding the f irst losler end ~art by way of an elec rlc insula~isn .ube body, -snd a second outer ~ke ~art surrounain~ the second losfer end part oy way of the electrlc insulation tube body, the second outer tube parl oeinc disposed adjacent the first outer tube part and navina a larger inside diameter tnan the first outer tube part.
The outside diameter of the first lower end part and insiae diameter of the Eirst outer tube part, and the outside diameter of the second lower end part and inside diameter of the second outer tube part are selected so that the characteristic impedance due to the first iower end part and first outer tube part, the characceristic impedance due to the second lower end part and second outer tube part, and the charac_eristic impedance due to the antenna element and cable may be nearly equal to each other.
Thus, according to this invention, if the antenna element is used or the transmission and reception of mobile telephone signals and for the reception of F~l broadcasting, the impedance matching of antenna element and cable may be achieved r.~.inlsa ~ ?~;lcn-~?. ~.-, '3r ~::am?l?, .~ S ~n~?.~.~a ei~?ln~nr ~ ,e~ E~ ?~e?tl3n 'Od'i-ls-i, Cne capac t-,- oi~ .,s;e -3-- -r. ma~ ee - ?dl!C'?'i, ~O ~_~d~ the Jol~aqe -~t i-.- ? ? ' -?C:''l' _oie- -e5e~iln~
end ma~ i~e ~ sed. ;lore3:er, ~he an~?n.n,~ c~n _c~or~oda~e for restricsions imposed thereon due .o ~he c;r bod~ shape.
In a prererred embodiment, an lnsertion hole pi~ces the first and second lower end parts i; communication, and d wire for driving the antenna elemen[ ~s set in .his insertion hole.
In ano~her preferred embodiment, a brush touches a contac. piece connected to the cable and instailed in the housing tube when the antenna element is extended, and supports the antenna element in the second lower end part while sliding on the inner wall of the housing tube durinq the extenslon and retraction of the antenna element.
In a different preferred embodiment, the first lower end part is covered at the outer circumference thereof with electric insulation material so as to be nearl~y equal to the inside diameter of the housing tube.
In other preferred embodiment, the upper end part or the housing tube is arranged to be level with or lower than the lower end part of the antenna element when the antenna element is in the extended state.

~L 3 1 ~
Tn another ~/L~-er~ed embodiment, t~se housinc3 tube comprises a tubular il~l r conductor electricalll connected to the lower end part of the antenna elemeni-, and a tubular outer _onductor accommodating this inner conductor by way of a space defined therebetween.
According to this invention, the housing tube for accommodating the antenna element comprises the tubular inner conductor and outer conductor, and the antenna element is stored in the inner conductor. The antenna element is electrically connected with the cable by way of this inner conductor. The outside diameter of the inner conductor and the inside diameter of the outer conductor are selected so that the charac'eristic impedance due to the transmission line of the inner conductor and outer conductor, and the_har-acteristic impedance due to the antenna element and cable may be nearly equal to each other.
Thus, according to this invention, since the space between the inner conductor and outer conductor has a small specific inductive capacity ~r, the characteristic impedance of the transmission line of the inner conductor and outer con-ductor, and the characteristic impedance of the antenna element and cable may be equalized, so that impedance matching may be effected favorably. Besides, it is not necessary to increase the outside diameter of the outer conductor too much, and the antenna may accomodate for restrictions of the car body shape.

~ 3 1 ~

In a certain preferred embodiment, the outer conductor is fitted to the car bod~, and an electric insulating member is disposed in the space so as to support the inner conductor.

It is a further object of this invention to present a multi-band whip antenna exhibiting sufficient strength and an aesthetic appearance.

According to one aspect of this invention, there is provided a multi-band whip an antenna element including a first antenna element part having a tubular conductor and a coil for operating electrically connecting the tubular conductor in the antenna, and a second antenna element part telescopically extendable in the first antenna element part;
and a covering tube made of an electric insulation material for covering the first antenna element part along its axial direction.

The antenna element of this invention comprises a first antenna element part having a tubular conductor and a coil for operating electrically connecting this conductor in the antenna in the axial direction to be mounted on the car body, and a second antenna first antenna element part. The first antenna element part is covered with a covering tube made of an electric insulation material along is axial direction.

Thus, according to this invention the first antenna element part having the coil exhibiting a small amount of strength 3 ~

is reinforced by the covering tube. Risk of breakage thereof may be eliminate~, and ~eflection or deformation hardly occurs, so that stable transmission and reception may be realized. F'urther, the first antenna element part is covered with a homogeneous covering tube, and has an aesthetic appearance.

In a further preferred embodiment, the antenna element comprises the first antenna element part extending from the lower end part and the second antenna element part which can be stowed in this antenna element part, the first antenna element part, the first antenna element part having a plural tubular parts composed telescopically.

In another preferred embodiment, the first antenna element part is composed of two tubular parts which are telescopically extendable and retractable.

In a different preferred embodiment, an end of the wire is fixed at the lower end part of the second antenna element part, and another end of this wire is wound on a take-up shaft of a motor. The motor may be driven to extend and retract the antenna element telescopically.

It is other object of this invention to provide branching filter capable of suppressing the mutual interference of signals between plural communication means using different frequency bands.

According to another aspect of this invention there is provided a branching filter comprising:

~ 3 ~
a first communication means for transmltting at least in a ~irst fre~uency band f 1; a second communication m~ans for receiving at least in a second frequency band f2 which is different from the first frequency band fl; and a band inhibiting means possessing an electrostatic capacity which has a larger impedance in the first frequency band fl and is connected in series to the signal line of the second communication means.

The branchlng filter of this invention has the signal line from the commullication means facilitating the transmission or reception of signal at least in the first or second frequency band fl, f2 connected to a common antenna.

The signal line of the second communication means is provided with band inhibiting means having an electrostatic capacity in series with the signal line and having large impedance in the first frequency band fl. Therefore, electrostatic capacity does not occur between the signal line of the second communication means and the ground, and the signal level will not be reduced by the band inhibiting means. Besides, the signal in the first frequency band fl at least transmitted by the first communication means is inhibited by the band inhibiting means, so that there is no adverse effect on the reception signals b~ the second communication means.

~3~
Thus, according to this invention, the effect of the transmiss.ion signal or the first communication means on the reception signal of the second communication means can be suppressed without lowering the level of reception by the second communication means, and mutual interference bet~,leen the transmlssion and reception signals of the antenna commonly used in different frequency bands fl, f2 can be suppressed.

In a further different preferred embodiment, the band inhibitlng means is a parallel resonance circuit connected to the signal line, and its resonance frequency is selected in the first frequency band fl.

` r~

In another preferred embodiment, the first communication means transmits and receives signals for a mobile telephone, while the second communication means is a radio set for receiving signals in the frequency band f2 lower than the frequency band fl of the first communication means, and the band inhibiting means is designed to inhibit signal within the transmission and reception frequency band fl of the firsL
communication means.

In a further different preferred embodiment, the band inhibiting means is a connection of parallel resonance circuits for resonating in the reception frequency band fla and the transmission frequency band flb of the first communication means.

13~ 3~9~

In another preferred embodiment, a bypass filter for allowiny signals in the first frequency band fl to pass and blocking signals in the second frequency band f2 is provided in the signal llne connecting the flrt communicating means and the antenna.

It is an advantage of this invention that it provides an antenna circult capable of enhancing the reception sensitivity and S/N ratio in a wide frequency band.

According to another aspect of this invention there is provided an antenna circuit which is provided between the antenna and an antenna input circuit of a radio set for receiving a first radio signal in a first frequency band f2a and a second radio signal in a second frequency band f2b which is a higher frequency band than the first frequency band f2a, the improvement comprises:

a signal cable;

a first impedance conversion circuit connected between the signal cable and the antenna for converting the impedance in the first frequency band f2a from high impedance to low impedance;

a first filter circuit connected between the signal cable and the antenna for allowing signals in the second frequency band f2b to pass;

a second impedance conversion circuit connected between the signal cable and the antenna input circuit for converting the impedance in the first frequency band f2a ~ 3 ~
from low i~pedance to high impedance; and a second filter circuit connected between the signal cable and the antenna input circuit for allowing signal in the second frequency band f2b. Between the antenna and the signal cable may be disposed means for adjusting the impedance, said means being composed of a first filter circuit for allowing the first radio signals in the first frequency band f2a, and a first impedance conversion circuit for converting the impedance in the second frequency band f2b from high impedance to low impedance. And between the signal cable and the antenna input circuit of the radio set is disposed means for adjusting the impedance, said means being composed of a second filter circuit for allowing the second radio signals in the second frequency band f2b, to and a second impendance conversion circuit for converting the impedance in the first frequency band from low impedance to high impedance.

The second radio signals are sent out to the radio from the antenna by way of the first filter circuit, while the first radio signals are converted with respect to impedance by the first impedance conversion circuit. Thus, loss due to cable capacity in the signal cable is reduced, and the signal is transmitted to the radio set. The second radio signals are then transmitted to the antenna input circuit of the radio set through the second filter circuit, while the first radio ~3~

signals are converted into an impedance matched ~Jith the antenna input circuit of the radio set by the second impedance conversion circuit, and are transmitted to the antenna input circuit of the radio set. Therefore, radio signals over a wide frequency band can be transmitted to the radio set without increasing loss in the antenna and signal cable.

In this way, according to this invention, when radio signals are received by the antenna, the loss of reception signal due to capacitative impedance of the signals cable may be reduced. Therefore, the reception sensitivity and S/N ratio in a wide frequency band can be outstandingly enhanced.

In a preferred embodiment, the first and second filter circuits are series circuits of a coil and a capacitor.

In a different preferred embodiment, the first and second impedance conversion circuits are transformers .

In a still further preferred embodiment, at least one of the primary and secondary windings of the transformer is connected in series with a coil for reducing the loss due to the stray capacity of the transformer.

- 2~ -Preferred embodiments of this invention are described in detail below.

Fig. 8 is over all schematic drawing of a mobile transmission and reception apparatus 101 according to the present invention.

On an automobile car body 102 is erected a multiband whip antenna 103 which is used commonly in transmission and reception of signals for a mobile telephone and for the reception of radio broadcasts. This antenna 103 is telescopically driven by a motor 104 installed at its lower end part. The antenna 103 is connected to a branching filter 106 by way of a coaxial cable 105, and signals for the mobile telephone transmitter/receiver 108 by way of coaxial cable 107 while the reception signals of a radio broadcast are transmitted to a radio set 111 by a coaxial cable 109 through an antenna circuit 110.

Fig. 9 is a sectional view of one embodiment, a multi-band whip antenna of one embodiment of this invention in an extended state. Fig. 10 is a sectional view along cut section line A-A in Fig. 9. Fig. 11 is a sectional view along line B-B in Fig. 9. This antenna 103 is set up, for example, near the rear trunk of the automobile car body 102.

An antenna element 123 of this 3 4 ~

antenna 103 is composed of a first antenna element part (hereinafter called first part) 124 of a roun~ tubular fro~, and a second antenna element part (second part) 125 telescop-ically formed within the first part 124. This antenna element 123, in a contracted state, is stored in a housing tube 126 disposed on the car body 102.
The first part 124 is composed in a sequential connection of a first conductor 145, a phase shifting coil 148, a second conductor 146, a band separating coil 149, and a third conductor 147. These conductors 145 to 147 and coils 148 and 149 are formed in an identical outside diameter.
The outer circumference of thus formed tubular first part 124 is covered with a covering tube 171, while a tube body 172 is inserted into the inner circumference, so that the first part 124 is reinforced, thereby preventing deflection or deformation of the coils 148 and 149. The covering tube 171 and the tube body 172 are made of electric insulating synthetic resin such as glass fiber, which will not affect the transmission and reception characteristics of the antenna 103.
As shown in Fig. 9, in the extended state of the antenna element 123, a lower end part 120 of the first con-ductor 145 positioned in the housing tube 126 is composed of a first lower end part 120a formed in a round tubular shape in a smaller diameter than the first part 124, and a second ~ 3 ~ "J ~ ~J ~

lower end part 120b formed like a cap in a ~arger diameter than the fi~st lower end part 120a, being consecutive beneath with the first lower end part 120a. The outer circumference of the first lower end part 120a is molded by a resin 135 so as to be identical with the outside diameter of the first part 124. As a result, the antenna element 123 can be expanded and contracted smoothly. On the outer circumference of the second lower end part 120b, a brush 134 is mounted in order to support the antenna element 123 and slide on a contact piece 130 which is described later.
The housing tube 126 is composed of an inner tube 127 made of electric insulation material, for edample, resin, and an outer tube 128 made of conductive material. The outer tube 128 comprises a first outer tube part 128a corresponding to the first lower end part 120a, and a second outer tube part 128b corresponding to the second lower end part 120b.
In the extended state of antenna element 123, a connection hole 129 is formed, communicating through the second outer tube part 128b and inner tube 127 toward the lower end part 120b, and the contact piece 130 contacting with the second lower end part 120b is fixed in this con-nection hole 129. To the contact piece 130, an inner conductor 132 of the coaxial cable 105 is connected, and the antenna element 123 and the inner conductor 132 are electrically connected. An outer conductor 133 of the ~L3 ~.3~
coaxial cable 105 is connected to the outer tube 128 of the housins t~be 126, ~nd this outer tube 128 is electrically connected with the car body 102 as mentioned below, and the outer conductor 133 is connected thus to the car body 102.
The vicinity of the current feed poit P where the contact piece 130 is disposed is reinforced by resin 136.
At the upper end part of-the outer tube 128 of the housing tube 126, a step 137 is formed, and external threads 138 are formed upward from this step137. At the upper end part of the housing tube 127 where external threads 138 are formed, a connecting member 140 with a metallic ring 139 is inserted. The upper end part of the housing tube 126 where the connecting member 140 is thus inserted is inserted in a mounting hole 141 formed in the car body 102, and is projected from the surface of the car body 102. In the projected part from the surface of the car body 102 of the housing tube 126, a resin-made seat 142 is fitted, and a nut 143 is set. The end part of the car body 102 side of the connecting member 140 is formed in a sawtooth shape, and therefore the outer tube 128 is electrically connected with the car body 102, and the outer conductor 133 of the coaxial cable 105 is grounded, while the housing tube 126 is securely fitted to the car body 102.
Flanges 173 and 174 are formed at both ends of the second part 125 of the antenna element 123, so that the ~?~

second part 125 ls prevented from slipp1ng out of the firs.
part 124 or falllng into the first part 124. At the flange 173 at the lower end part side of the second par, 125, one end of a flexible wire 175 telescopically driven by the motor 104 is fixed. The other end of this wire 175 is wound on a take-up reel or the like mounted on the output shaft of the motor 104 which is passing through an insertion hole 176 and formed on the inner circumference of the tubular first lower end part 120a and the cap-shaped second lower end part 120b, so that the antenna element 12~ is extended or contracted by the driving of the motor 104 in the normal or reverse direction, and may be stored in the housing tube 126.
The signal transmitted nad received by thus composed antenna element 123 is led into the branching filter 106 from the coaxial cable 105, and the frequency band is separated. The separated signal is led into the transmitter/
receiver 108 of the mobile telephone through the coaxial cable 107, and is also led into the receiver 111 of radio broadcast from the coaxial cable 109 through the antenna cicuit 110.
In the antenna element 123, supposing the wavelength of mobile telephone to be ~1, the first conductor 145 is formed in a length of 3 x ~1/8 (approx. 11 cm), while the developing length of the phase shifting coil 148 is formed in ~1/4 (about 9 cm), and the second conductor 146 is formed 1 3 3~

in 5 x ~1/8 (about 20 cm). Thus, a colinear antenna array is composed by first, ~econ~ conducto~s 145 and 146, and the phase shifting coil 148.
The overall length in the state of developing the phase shifting coil 148 of this colinear array antenna is about 40 cm, and in other words it is seclected at 5/4 times the wavelength Al in the frequency band 860 to 940 MHz of mobile telephone in Japan. The phase shifting coil 148 functions as a phase shifter for the wavelength of ~1, and suppresses the current distribution in the reverse phase at a low level, so that a current distribution possessing an amplitude largely emphasized in the normal phase portion is obtained. The band separating coil 149 has a high impedance against the short wavelength ~1 for mobile telephone, and a low impedance for a long wavelength ~2 for radio broadcast-ing. Thus, transmission and reception of mobile telephone can be effected by using a colinear array antenna.
The winding length of the phase shifting coil 148 is about 4 cm, and therefore the overall length of the colinear array antenna is about 35 cm. The length from the lower end part of the band separating coil 149 to the upper end part of the second part 125 is selected at about 38 cm, and therefore the overall length of this antenna element 123 is about 73 cm, and in order words it is selected at a length of 1/4 of the wavelength ~2 in the frequency band 76 to 90 MHz of FM broadcasting in Japan. Thus, at a relatively long wavelength ~2 of radio broadcasting, the radio broadcast is received by using the overall length of the antenna element 123.
In this whip antenna 103, supposing the section of the projected portion from the upper end part of the housing tube 126 of the antenna element 123 to be Q31, the section from the upper end part of the housing tube 126 to the current feed point P to be Q32, and the section of the coaxial cable to be Q33, the outside diameter dl of the first lower end part 120a of ther lower end part 120 may be set sufficiently smaller than the inside diameter Dl of the first outer tube part 128a of the outer tube 128. Besides, corresponding to the outside diameter dla of the second lower end part 120b sliding on the contact piece 130, the inside diameter Dlb of the second outer tube part 128b may be formed largely. Thus, from eq. 1, the characteristic impedance Z2 in the section Q32 may be increased.
Therefore, when transmitting or receiving mobile telephone and receiving FM broadcast, from eq. 1, a favorable impedance matching may be obtained by properly selecting the ratio of the inside diameters Dl and Dla of the outer tube parts 128a and 128b to the outside diameters dl and dla of the lower end parts 120a and 120~, so that the characteristic impedance Z2 in the section Q32 may be nearly equal to the 1 3 ~

characteristic impedance Zl and z3 in the sections ~31 and ~33.
As a result, t~e transmission loss ma~ be reduced, and the reception frequency band may be prevented being narrowed.
Besides, when receiving AM broadcast, as stated above, since the ratio of the inside diameters Dl and Dla of the outer tube parts 128a and 128b to the outside diameters dl and dla of the lower end parts 120a and 120b may be set larger, the capacity C2 in the section Q32 may be reduced as indicated in eq. 3 and eq. 2, so that the power receiving end voltage V2 may be increased.
Furthermore, the outside diameter of the first outer tube part 128a of the outer tube 128 will not enlarged, and the current feed point P may be set at an arbitrary posi-tion, so that it may be suited to restrictions by the shape of the car body 102, and to any model of the autmobile.
In addition, since the first part 124 of the antenna element l23 is reinforced by the covering tube 171 and the tube body 172, breakage of the antenna element 123 may be prevented, while deflection or deformation may be also avoided, so that stable transmission and reception may be realized.
Moreover, a favorable appearance is attained by covering the first part 124 comprising the coils 148 and 149 with a covering tube 171 made of a homogeneous material, and it can be smoothly put into the housing tube 126. By mounting by inserting the tube body 172, the second part 125 may be ~L 3 ~ s ~ v ~

; lo c n l~r . . c . i~ 5 ~ ! ^,; ~ r i r. ~J

-~ ! J !.; ~ a ~ d~ * ~ ~ i l; .
~ iiL lur~:~er~ b~i Lorinir~:l ~ne if:l~.r elld Dar~ Oa dS a r~ '.~'{~ or dnd Corliling dn ins~-L~ OlQ i/'~
second lcwer end ~-~aL- i20b, rain~a~er oeno~ra.lng ,âS~ ~he ~ir,t part 12~ may be discharqed, and the im~edance matchinq may be furthcr enhanced.
Fig. 12 ia a sectional Vie~J of allother embodiment of a mu1ti-band ~hip dntellna 201 according .o the present in~ention as shown in an extended sta~e, and rig. ;3 is a sectional vie~,~ ta~:en along line c-c in ~ig. 12. ~his embodiment is similar to the foregoing embodiment, and the corresponding parts are identified ~ h same reference numbers.
In this embodiment, a nousing tube 202 ccmprises an inner conductor 203 having a round c~lindrical shape, an outer conductor 20~ having a round cylindrical shape with a larger inside didmeter D2 than an outside diame~er d2 of the inner conductor '03, and support members 205 and 206 made of electric insulation material and interposed bet~een the conductors 203 and 204 at both ends of the inner conductor 203.
A brush 134 fitted to a lo~ler end part 120 of the antenna element 123 slides on the inner circumference of the inner conductor 203. And an inner conductor 132 of a ~L 3 ~

coaxial cable 105 is connected at a current feed poirt P on an outer circumference. At the current feed pont P, a c~n-necting hole 129 is formed in the outer condcutor 20~, and in this connecting hole 129, an outer conductor 133 of the coaxial cable 105 is connected to the outer conductor 204.
Thus, in the housing tube 202, by forming a space 207 ketween the inner conductor 203 and outer conductor 204, the specific dielectric constant Er in eq. 1 may be reduced to the value of air, that is, nearly 1.0, and the character-istic impedance Z2 in a section Q41 can be increased while a capacity C2 can be reduced without enlarging the outside diameter of the housing tube 202, so that the same effects as in the foregoing embodiment may be obtained.
Fig. 14 is a sectional view of a multi-band whip antenna 301 in a further different embodiment of this invention in an extended state, and this embodiment is similar to the foregoing embodiments, and the corresponding parts areidentified with same reference numbers. In this embodiment, an antenna element 302 is composed in three stayes, and a second conductor 146 between a phase shifting coil 148 and a band separting coil 149 is divided into a lower side part 146a and an upper side 146b. The outer circumferences of the conductors 145 to 147 and coils 148 and 149 are covered with a covering tube 171 and 172.
By thus dividing into three stages, the size of ~ 3~J~i~
the antenna element 302 ln the contracted state can be reduced, and the length of the housing tube 202 may be shortened.
Fig. 15 is an electric circuit diagram OI a branch-ing filter 106 in a certain embodiment of this invention.
The antenna 103 mounted on an automobile is connected to a band inhibiting filter 413 by way of a cable 105 which is a signal line. The output of the band inhibiting filter ~13 is applied to a radio set 111 which is second communication means. The coaxial cable 105 is connected with a transmitter/
receiver 108 for mobile telephone, which is first communica-tion means, by way of a high pass filter 415.
The transmitter/receiver 108 for mobile telephone performs radio communications with the ground station con-nected in the telephone line network in a first frequency band fl, that is, in a frequency band fla of 870 to 890 MHz in receiving, and in a frequency band flb of 920 to 940 MHz in transmitting. On the other hand, the radio broadcast received in a radio set 111 using a second frequency band f2, that is, a frequency band f2a of 500 to 1620 kHz for AM broadcast, and a frequency band f2b of 76 to 90 MHz for FM broadcast. Therefore, in reception of radio broadcast by radio set 111, if a mobile telephone is used, it is enough when the signals in the frequency bands fla and flb in reception and transmission be inhibited by the band inhibiting filter 413.

~3~

The high pass fllter 415 intervening between the coaxiai cable 10~ and the transmitter/receiver 108 fo~ mobil~
telephone is connected in series to capacitors c23 and C24, and a connecting point 417 of these capacitors C23 and C24 is grounded through a coil L23, thereby allowing to pass the signal in the freuqency band fl of mobile telephone and cutting off the signal in the frequency band f2 of radio broadcast.
Meanwhile, the band inhibiting filter 413 is composed of a first band inhibiting filter 418 for inhibiting the frequency band fla of 870 to 890 MHz, and a second band inhibiting filter 419 for inhibiting the frequency band flb of 920 to 940 MHz.
The first and second band inhibiting filters 418 and 418 are connected in series to the coaxlal cable 105, individually. The first band inhibiting filter 418 comprises a coil L25 and a capacitor C2S, while the second band inhibiting filter 419 comprises a coil L26 and a capacitor C26. The inductance of coils L25 and L26, and the electrostatic capacity of capacitors C25 and C26 are properly selected so as to inhibit the signals in the above frequency bands fla and flb.
Fig. 16 is a graph showing the frequency character-istics of the band inhibiting filter 413. The band inhibit-ing filter 413 operates during use of mobile telephone, and inhibits the signal from the antenna 103 in reception mode, and the signal from the transmitter/receiver 108 for 3L 3 ~

rnobile telephone in transmisslon :mode. In the radio set lll, generation of its noise does not matter if less than 110 d~J
~v (~3 dBmW) at input voltage. On the other hand, the transmission outputof transmitter/receiver 108 for mobile telephone is 5w (+37 dsmW~ in Japan. Therefore, the band inhlbiting filter 413 is composed so that the input signal level may be attenuated more than 34 dB and delivered in the frequency bands fla and flb of 870 to 890 MHz and 920 to 940 MHz. Fig. 16 shows the frequency characteristics with respect to the input signal level VI.
Thus, in this embodiment, during use of moblle telephone, interference of reception signal (870 to 890 MHz) into the radio set lll is prevented by the first band inhibiting filter 418, whereas the interference of transmis-sion signal (920 to 940 MHz) into the radio set 111 is prevented by the second band inhibiting filter 419. In addition, between the signal line of the radio set 111 and the ground, there is no intervening electrostatic capacity such as capacitor, so that drop of voltage level induced by antenna 103 by band inhibiting filter 413 in reception mode of.radiobrodcast will never occur.
In this manner, without lowering the reception signal level of the radio set 111, effects of transmission and reception signals of mobile telephone on the reception signal of radio broadcast may be suppresed, and mutual . - 41 -~ 3 ~

lnterference bet-,ieen transmission and reception signals of the antenna co~only used in different frequency bands fl and f2 may be suppressed.
Fig. 17 is a structural drawing of an antenna circuit 110 in a different embodiment of this invention, and Fig. 18 is an equivalent circuit diagram in AM radio frequency band f2a of an antenna circuit 501 for explaining the principle of this invention. The antenna 500 is expressed as a composition of an antenna reactive capacity Ca existing agaisnt the ground level, and an atenna effective capacity Ce existing in series, and an AM radio signal which is a first radio signal received by this antenna 500 is expressed as an anternating-current power source V31. A coaxial cable 109 i~,s expressed to comprise a line Q61 betweern terminals B2 and P2, and this line Q61 is grounded by way of a cable capacity Cb. Between the antenna 500 and the coaxial cable 109 is intervening a transformer 502 for converting the impedance. The signal at terminal P2 is transmitted to the antenna input circuit in the radio set 111. The voltage V41 at this terminal P2 is expressed as follows, supposing the turn ratio of input side to output side of the transformer 502 to be H:

Ce + Ca + Cb/n2 V31 --.................. (6) As understood from eq. 6, by additionally installing the transformer 502, the effect relating to the cable capacity Cb may be reduced to l/n2 of the expla~ation ln ~ig. 7.
Therefore, the impedance derived from the cable capacit~ Cb as seen from the terminal A2 converted to l/n2 by the transformer 502, so that the loss at the coaxial cable 109 may be reduced.
The antenna circuit 110 is composed of an antenna 103, the coaxial cable 109, an impedance adjusting circuit 513 intervening between the antenna 103 and the coaxial cable 109, and the impedance adjusting circuit 517 interven-ing between the coaxial cable 109 nad the radio set 111.
In Fig. 8, meanwhile, the impedance adjustlng circuit 513 is built in the branching filter 106.
The output from the antenna 103 is applied to the impedance adjusting circuit 513 through the branching filter 106. The impedance adjusting circuit 513 has a low impedance in the frequency band f2b of FM radio signal, and comprises a FM radio signal filter circuit 514 which is a first filter circuit, and transformer 522 and others, and an impedance conversion circuit 515 which is a first impedance conversion circuit is connection in parallel to make up the composition.
The FM radio signal received thus by the antenna 103 is delvired to the coa~ial cable 109 through FM radio signal filter circuit 514.
The FM radio signal filter circuit 514 is composed, .

for example, in a series connection o~ a coil 520 and a capacitor 521, and functions as a hish pass filter with a low impednace agalnst FM frequenc~ band f2b.
The radio signal from the coaxial cable 109 is given to the impedance adjsuting circuit 517. The impedance adjusting circuit 517 is composed of an FM radio signal filter circuit 518 which filters FM radio signal and is a second filter cirucit, and an impedance conversion circuit 519 which has an impedance conversion action on AM radio slgnal and is a second impedance conversion circuit.
The FM radio signal filter circuit 518 is connected in parallel to the impedance conversion circuit 519, and the FM radio signal from the coaxial cable 109 is led out into the antenna input circuit of the radio set 111 through the FM radio signal filter circuit 518. The FM radio signal filter circuit 518 is, for example, composed of a coil 523 and a capacitor 524, and functions as a high pass filter for filtering a relatively high frequency signal such as a FM radio signal. The impedance conversion circuit 519 comprises a transformer 525 and others, same as the first impedance conversion circuit 522 mentioned above.
Therefore, the inductance of coils 520 and 523 in the FM radio signal filter circuits 514 and 518, and the electrostatic capacity of capacitors 521 and 524 are properly selected so as to possess the resonance frequency in the FM

~3~3~
radio signal frequency band, respectlvely.
In the construction shown in Fig. 18, howe~er, there is actually an effect of the capacity in the FM radio signal filter circuit 514 in Fig,. 17. An equivalent circuit diagram to show the principle in consideration of such capacity component Cf is shown in Fig. 19. For the sake of simplicity, the antenna effective capacity Ce and the antenna reactive capacity Ca are collectively expressed as CA. Incidentally, the transformer 502 corresponds to a transformer 522 in Fig. 17, while the antenna 500 corresponds to the antenna 103.
A self-inductance Ll is provided at the input side, a self-inductance L2 is provided at the output side, and there is a mutual inductance M between the input side and the output side. Therefore, between the alternating-current power source V31 derived from the radio signal received by the antenna 500, and the voltage level V41 applied to the radio set 111, the following relation is established, assuming the current from the antenna 500 to be il, the current flowing in the capacity component Cf to be i2, and the current due to cable capacity Cb to be i3:

V31 = (j C + jWLl)il + (jwM - jwL1)i2 + jwMi3 ... (7) O = jwMil + (jwLz - jwM)i2 ~ (jwL2 + jWcb)i3 -----------~--- (8) V31 = j C il + j Cf i2 -- j Cb i3 ................ t9) ?
And, V41 = - jW1 b i3 ................... , .... (10) Therefore, solvlng them, we obtain:
41 = ~ {~4CACf (L1L2--M2)--~2CAM}V31 . . (11) (CACf~CACb+C~Cf) (L1L2-M2)-~2{L1(CA+Cf)+L~Cb+Cf)-2MCL-~+1 where ~ denotes the angular frequency of the received radio signal.
At this time, when the denominator of eq. 11 is zero, V41 reaches the maximal value. Supposing here that the mutual inductance M be expressed as k~LI L2 (where k is a coupling coefficiency of transformer 502), the maximal value of V41 is expressed as follows:
f = 1 ~ Y + X ~ ................... (12) where X = (CACf + CACb + CbCf)(l - k2)LlL2 .. (13) Y = -{Ll(CA + Cf) ~ L2 (Cb + Cf) - 2Cf-k ~ }.. (14) Z = 1 ................................. (15) Thus, as shown in eq. 12, the voltage level V41 comes to possess the maximal value with respect to two values differing in frequency f. Supposing the frequencies corre-sponding to the maximal value of voltage level V41 to be fll, fl2 (fll < fl2), the relation between frequency f and voltage level Vc is expressed in Fig. 20. As understood from eq. 12 to eq. 14, as the coupling coefficient k becomes smaller, the frequency fl2 becomes lower. Therefore, by increasing ~3~

the coupling coefficeint k possessed by the tr~nsformer 502, when the A~ radio signal frequency band f2a is adjusted to settle within frequency fll and frequency fl2, a flat reception characteristic will be obtained in the AM radio signal frequency band f2a. As the transformer capable of increasing the coupling coefficient k, for example, the tranformer 502 of so-called sandwich winding or bifilar winding may be used.
Fig. 21 is an equivalent circuit diagram in an AM
radio signal frequency band f2a of the antenna circuit 110 in Fig. 17. The antenna 103 may be expressed by a capacity CA comprising the antenna effective capacity possessing a series electrostatlc capacity with respect to the radio signal, and the antenna reactive capacity generated between the radio signal and grounding level. The radio signal received by antenna 103 may be expressed as alternating-current power source V32.
The AM radio signal received by antenna 103 has a high impedance in the FM radio signal filter circuit 514, and therefore it is fed into the impedance conversion circuit 515. In the impedance conversion circuit 515, the turn ratio of the number of turns at the input side and the output side of the transformer 522 is n:l. Accordingly, the voltage of the AM radio signal is reduced to l/n and the impedance is reduced to l/n2 by the transformer 522.
The coaxial cable 109 contains the cable capacity Cb between the radio signal a~d a groundin~ level.
Relative to a high frequency signal, for exmaple, a FM radio singal, the coaxial cable lO9 has a low impedance.
However, to a relatively low frequency signal such as a AM
radio signal, the impedance of the coaxial cable 109 due to cable capacity Cb is large. In this embodiment, the impedance of the AM radlo signal is reduced by the impedance conversion circuit 515, so that the loss relating to cable capacity Cb may be reduced.
The signal in a relat`ively low frequency band f2a such as AM radio signal from the coaxial cable 109 is high in impednace in the FM radio signal filter circuit 518, and it is applied to the impedance conversion circuit 519. In the transformer 525 of the impedance conversion circuit 519, the ratio m against the number of turns 1 at the input side to that at the output side is set, and the AM radio signal fed to this transformer 525 is amplified in voltage, and ic delivered into the antenna input circuit of the radio set 111 .
The relation between the alternating-current power source V32 and the output voltage V42 is expressed in the following equation.

n CA + Cb/n2 '''' ---. (16) A capacity CTA of the antenna circuit 110 as seen from the ~ 3 ~ ~31~
radio set lll is expressed as follows:

C CA n + Cb ............... ................. (17) For example, this capacity CTA is defined at 80 pF in relation with the impedance matching with the radio set, and the capacity CA and the cable capacity Cb are determined by the length of the antenna 103 and the coaxial cable lO9.
Therefore, the turn ratios n and m of the transformers 522 and 525 are selected so as to satisfy eq. 17 above.
The equivalent circuit of antenna circuit llO as seen from the radio set lll may be expressed as the composi-tion of inductance Lo/2 and capacity CTA connected in parallel, assuming the inductance at transformers 522 and 526 to be Lo. Suppbsing the resonance frequency of such circuit compsoition to be fp, the inductance Lo may be expressed as follows:

(2~fP)2-CTA (18) It is desired to flatten the frequency charactersitics in the AM radio signal frequency band f2a by selecting the resonance frequency fp at, for example, 250 kHz or other outside the AM radio signal frequency band f2a. Accordingly, the inductance Lo of the transformers 522 and 525 is determined by eq. l8.
Thus, in the antenna circuit llO, for exmaple, 13~3~

when receiving an A;~l radio signal and a E'M radio slgnal commonl~
by one antenna 103, the loss of the ~ radio signal at the coa~ial cable 109 may be lowered. For instance, assuming the antenna effective capacity Ce to be 15 pF, the antenna reactive capacity Ca to be 5 pF, the cable capacity Cb to be 120 pF, and the turn ratios n and m to be 4, the gain is improved by about 9 dB as calculated according to eq. 5 and eq. 6.
In the foregoing embodiments, the loss will be greater if a too large value is set for the turn ratios n and m of the transformers 522 and 525, or the effect will be smaller if a too small value is used. According to the experlment by the present inventors, practically, favorable results are obtained when a numerical value of 10 or less is selected for the turn ratios n and m.
Fig. 22 is a strcutural drawing of an antenna circuit 531 in a still different embodiment of this invention.
The parts corresponding to the foregoing antenna circuit 110 are identified with same reference numbers. In the antenna circuit 531, in the impedance adjusting circuit 513a, the impedance conversion circuit 515a comprises coils 532 and 533 and the transformer 522, and in the impedance adjusting circuit 517a, the impedance conversion circuit 519a comprises coils 534 and 535 and the transformer 525. In order to reduce the loss due to the stray capacity possessed by the 1 3 ~
transformers 522 and 525, coils 532 to 535 are added ~o ~he input end and the output end of the transfor~ers 522 and 525, respectively. As a result, the loss attributable to the stray capacity of the transformers 522 and 525 is prevented, and the reception sensitivity and the S/N ratio may be further enhanced.
In the foregoing embodiments, thus, the loss in the AM radio signal frequency band f2a due to stray capacity, in particular, can be reduced, while the reception sensitivity and the S/N ratio in the radio ~eceiver may be outstandingly enhanced. Therefore, when receiving signals in a wide frequency band by a single antenna, for exmaple, when receiv-ing both FM and AM radio signals by a car-mount antenna, it is particularly effective.
Besides, depending on the type of antenna, generally, the antenna reactive capacity varies more significantly than the antenna effective capacity. When this invention is applied in an antenna with a large antenna reactive capacity, its effect will be manifest. Meanwhile, the polarity of the transformers 522 and 525 may be either normal phase or reverse phase, but according to the experiment, a greater effect will be obtained when transformers 522 and 525 of normal phase are used.
In this embodiment, it is explained to receive FM
radio-signal and AM radio signal, but it may be also favorably embodied also in the application of recelving radio signal and other signal such as a mobile telephone signal at the same time.
The invention may be embodied in other specific forms without departlng from the spirit or essential charac-teristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not re-strictive, the scope of the invention being indicated by the appended claims rather than by the foreging description and all changes which come within the meaning and the range of equivalency of the claims are therefore intended to be embraced therein.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A branching filter comprising: a first communication means for transmitting at least in a first frequency band f1;
a second communication means for receiving at least in a second frequency band f2 which is different from the first frequency band f1; and a band inhibiting means possessing a electrostatic capacity which is increased in the impedance in the first frequency band f1 and connecting in series to the signal line of the second communication means.

2. A branching filter according to claim 12, wherein the band inhibiting means is a parallel resonance circuit connected to the signal line, and its resonance frequency is selected in the first frequency band f1.

3. A branching filter according to claim 12, wherein the first communication means possesses the composition for transmission and reception with the mobile telephone, while the second communication means is a radio set for receiving the frequency band f2 lower than the frequency band f1 of the first communication means, and the band inhibiting means is designed to inhibit the signal of the transmission and reception frequency band f1 of the first communication means.

4. A branching filter according to claim 12, wherein the band inhibiting means is composed by connecting in series parallel resonance circuits for resonating in the reception frequency band f1a and the transmission frequency band f1b of the first communication means.

5. A branching filter according to claim 12, wherein a bypass filter for passing the first frequency band f1 and blocking the second frequency band f2 is provided in the signal line connecting the first communication means and the antenna.

6. In an antenna circuit provided between the antenna and an antenna input circuit of a radio set for receiving a first radio signal in a first frequency band f2a and a second radio signal in a second frequency band f2b which is a higher frequency band than the first frequency band f2a, the improvement comprising: a signal cable; a first impedance conversion circuit connected between the signal cable and the antenna for converting the impedance in the first frequency band f2a from high impedance to low impedance; a first filter circuit connected between the signal cable and the antenna for passing the signal in the second frequency band f2b; a second impedance conversion circuit connected between the signal cable and the antenna input circuit for converting the impedance in the first frequency band f2a from low impedance to high impedance; and a second filter circuit connected between the signal cable and the antenna input circuit for passing the signal in the second frequency band f2b.

7. An antenna circuit according to claim 17, wherein the first and second filter circuits are series circuits of a coil and a capacitor.

8. An antenna circuit according to claim 17, wherein the first and second impedance conversion circuits are transformers.

9. An antenna circuit according to claim 17, wherein at least one of the primary and secondary windings of the transformer is connected in series with a coil for reducing the loss due to the stray capacity of the transformer.