US2881430A - Multi-band tuned antenna - Google Patents

Description

\ 7,1959 I i G. A. BIRD 2 881,430

MULTI-BAND TUNED ANTENNA Filed se t. 20. 1956 4 ZShets-Sheetl omo v 70' INVENTOR Gonna/v 194 F'R'fl 80w QYKWMW ATTORNEYS April 7, 1959 G. A. BIRD MULTI-BAND TUNED ANTENNA Filed Sept. 20, 1956 2 Sheets-Sheet 2 w a 2 v o 5 3 3 m k ,W.\o O Z 9 a 2 7 6 7 3 3 3 v 8 a z\ INVENTQR GORDON HAP/P50 BIRD BYM W AT'rQRNeYS United States Patent O MULTI-BAND TUNED ANTENNA Gordon Alfred Bird, Croydon, England Application September 20, 1956, Serial No. 611,018

Claims priority, application Great Britain November 23, 1955 7 Claims. Cl. 343-747 This invention is concerned with aerials and in particular to aerials which are effective at a plurality of frequencies or over a plurality of bands of frequencies simultaneously.

Radio stations are frequently required to transmit and/or receive on a plurality of frequencies or within a plurality of bands of frequencies. Generally it is necessary, in order that transmission and/or reception shall be as effective as possible, that a separate aerial be employed for each frequency or band of frequencies.

When a directional aerial, that is an aerial which is more effective for transmission or reception in one direction than others, is to be employed it is usually effective only at one frequency or over one band of frequencies.

The plurality of aerials' which are thus necessary at such a radio station require a considerable space in which they can be erected. If the various aerials are erected in close proximity to each other the characteristics of any one aerial will be affected by the presence of the other aerials.

This latter effect causes difiiculties when the radio station is located in a confined area, such for example as often occurs in the case of an amateur radio station. In some cases the space available may even precludethe erection of more than one aerial.

It is an object of the invention to overcome or minimize the aforesaid difiiculties.

More particularly it is an object of the invention to provide an aerial system which is automaticallyeffective for operation on any one of the plurality offrequencies or bands of frequencies without adjustment.

The invention provides an aerial comprising anelectric conductor resonant at a first frequency and having associated therewith a first reactance means whereby the aerial is resonant at a second frequency and a second reactance means whereby at the first frequency the first reactance means is rendered ineffective whereby the aerial is resonant at both the first and second frequencies;

The invention also provides an aerial which is re'so nant at each of a plurality of frequencies and comprises a conductor resonant at a first frequency, a first reactance means whereby the conductor is resonant at a second frequency and a second reactance means whereby the first reactance means is rendered ineffective at the first frequency. v p

Further, the invention provides a directive aerialwhich is effective at a plurality of frequencies and which c'omprises a plurality of electric conductors at least one of which is resonant at a first frequency and has asso= ciated therewith a first reactance means whereby it is resonant at a second frequency and a second reactance means whereby the first reactance means is rendered ineffective at the first frequency.

'Still further,v the invention provides a directive aerial which" is'effective at each of three frequencies and which comprises a driven element and two parasitic elements; each parasite element comprising a conductor resonant at one of the three frequencies and having associated therewith a first reactance means whereby the element is resonant at another of said three frequencies arid a second reactance means whereby the first reactance means is rendered ineffective at the one frequency.

In order that the invention shall be more readily understood one particular embodiment thereof will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a schematic diagram of a directive aerial, in accordance with the invention,

Figure 2 is a schematic diagram of one form of inipedance transformer adapted for use with an aerial according to the invention, 1

Figure 3 is a view of the aerial shown schematically in Figure 1 showing the mechanical arrangement thereof,

Figure 4 is a plan view of another directive aerial in accordance with the invention,

Figure 5 is a view of a modified driven element for an aerial in accordance with the invention, and

Figure 6 is a view, partly in section, of another aerial in accordance with the invention.

The embodiment shown in Figure 1 is arranged as a directive aerial, that is an aerial which is more efiec tive for transmission or reception in one direction than in others.

This embodiment is a three element beam aerial system comprising a radiating element 1, a director element 2 and a reflector element 3. The system is arranged to be effective ov'er three frequency bands namely the 14, 21 and 28 mc./S. amateur bands. Each of the elements of the system will be described separately.

The resonant frequency of an element can be varied by varying its length or by adding capacitance or inductance in series with the conductors of the element. When capacitance is added the resonant frequency of i the element will be increased whilst if inductance is added the resonant frequency will be decreased.

The first element which will be described is the reflector 3. This element is of a length such that it is effective at about 21 r'nc./s. The elementis cut approximately in the centre and the two resulting halves 4, 5 are connected by a first reactance means comprising an inductor 6 the inductance of which is such that the element is then effective at a frequency a little below 14 mc./s.

This inductor has to be rendered ineffective before the reflector element 3 is again effective at 21 mc./s. To do this a second reactance means is arranged to present alow impedance path in shunt with the inductor 6;

The low impedance path is provided by a quarter wave stub 7 resonant at about 21' mc./s. This stub comprises a length of low impedance twin feeder presenting two substantially parallel conductors each approximately a quarter wave length long at 21 mc./s. One end of each conductor is connected to an end ofthe inductor. The other ends of each conductor are left unconnected. At 21 mc./ s. the stub presents alow impedance path inshunt with the inductor thus rendering it ineffective at this" frequency. At 14 mc./s. the stub will behave as a small capacity shunting the inductor. The effect of this'is that the inductor need possess less inductance to ma'lietlie reflector element remain effective at 14 mc./s.

At 28' mc./s. the reflector element is such that it behaves somewhat as if it were physically three half wave lengths long, the centre half wave portion and pa'rto'f each of t-he-t-wo remaining half wave portions being-represented electrically bythe inductor and thus relatively ineffective: The effect of this is to produce at 28' mcl/s'. two current maxima in the reflector element, one about eight feet fr'o'rri each end so that the reflector element effectively presents twig) half wave reflectors operating in phase with each ot er.

It will be seen that the reflector element is now simultaneously effective at 14, 21 and 28 mc./ s. The length of this element is 23 feet.

The next element which will be described is the director element 2. This element is very similar to the reflector element. However in this case the physical length of the element 2 is such that it is effective at about 28 mc./s. The element is cut approximately in the centre and the two resulting halves 8, 9 connected by an inductor 10 which has an inductance such that the element is also effective on 21 mc./s. A quarter wave stub 11 is again connected across the inductor; in this case the stub is resonant at about 28 mc./s. The length of the director element is 16 feet.

The driven element 1 which is centre fed will now be described. It is customary to use a driven element which .is about a half wave length long at the desired frequency of operation but this is by no means essential as the physical length'of the driven element has little effect on either the gain or the front to back ratio of the aerial system. The only effect of changing the physical length is to alter the feed impedance and band width of the system.

The length of the driven element chosen in the present embodiment is about 24 to 26 feet. This length is such that it is etfective at 14, 21 and 28 mc./s. although the driven element is aperiodic at these frequencies. At 28 mc./s. there is a current maximum in each of the two portions of the element which lie behind the director element. At 21 mc./s. this length is such that the feed impedance is advantageously increased.

The'driven element 1 is fed approximately at its centre "by means of an open wire twin tfeeder of about 450 ohms impedance. This impedance gives the lowest average standing wave ratio over the three frequency bands at which the aerial is effective. The losses in this type of feeder due to standing waves are much lower than in the case 'of a low impedance twin wire or coaxial feeder.

The spacing between the driven element 1 and the director element 2 is feet and the spacing between the driven element 1 and the reflector element 3 is 7 feet.

An aerial tuning unit is used to couple the aerial system to a transmitter or receiver. By a suitable choice of lengths of the feeder the aerial tuning unit can be arranged to act as an impedance transformer which is effective to transform the impedance presented by the feeder to a substantially constant, predetermined impedance at each of the three frequencies at which the aerial is effective.

The'length of feeder suitable is found by determining a point on the feeder which is approximately a number of quarter wavelengths from the end of the driven element to which the feeder is connected at each of the frequencies at which the aerial system is effective. In the present embodiment a suitable convenient length is between 38 and 40 feet.

Figure 2 is a schematic diagram of the aerial tuning unit which comprises an inductor having two parts 12, 13 and capacitor 14 connected in series, the free end of the inductor parts 12, 13 being connected each to one of terminals 15, 16 to each of which one wire of the feeder is connected. When a suitable length of feeder is connected to the aerial tuning unit the reactance of the feeder in combination with the reactances of the inductor and capacitor in the aerial tuning unit combines automatically to resonate the arrangement on each of the three bands of frequencies. The capacitor 14 is a trimmer capacitor provided for tuning out any reactance which may be introduced due to incorrect choice of feeder length and after initial adjustment need rarely be adjusted further.

A further inductor 17 is arranged in the aerial tuning unit to, provide inductive coupling to a low impedance feeder at terminals 18, 19 and which connects the aerial tuning unit, to the transmitter or receiver. In order to provide a relatively uniform coupling on each of the three bands the inductance of the further inductor 17 is such as v to give adequate coupling on 21 and 28 mc./s. but insufficient coupling on 14 mc./ s. To increase the coupling on 14 mc./s. to an adequate value a capacitor 20 is included in series with the further inductor, the capacitance of the capacitor being such as to resonate the further inductor at about 14 mc./s.

It will be seen that the aerial coupling unit is simple and thus can be made relatively compact and is capable of operation, in conjunction with the aerial system, on any of the three bands without adjustment.

The mechanical construction of the aerial system will now be described in connection with Figure 3.

A horizontal boom 21 is arranged to carry the three elements of the aerial system. This boom 21 is supported on a mast 22 or any other suitable support. On the underside of the boom three short, relative to the elements, inverted U-section metal channels 23, 24 and 25 are arranged substantially parallel to each other approximately at right angles to the boom 21 and spaced along the length of said boom substantially in the same disposition as the three elements 1, 2 and 3. The elements are supported from beneath the U-section channels by means of standoff insulators 26. By arranging the U-section channels in this way they form protective covering for the inductors associated with the director and reflector elements.

The inductors 6 and 10 are coils wound of heavy gauge copper wire or tubing. If desired they can be further protected from the weather by covers of a material such as polythene.

The stubs, which are of low impedance twin feeder as described above, are passed down the inside of the boom 21 where they are protected from the weather and also hidden from sight so that the aerial system is of a tidy appearance.

The feeder, by which the driven element is connected to a transmitter or receiver, is shown at 38.

Figure 4 shows a further embodiment of the invention which is a directive aerial effective on two frequencies or hands of frequencies. In this arrangement the driven element 27 and a parasitic element 28, which is arranged to be effective as a reflector element, are each arranged in two halves each of which is secured at one end to one of two substantially parallel metal members 29, 30 which form the boom.

The driven element 27 is of a length such that it is resonant at the higher frequency of the two at which the serial is effective. The driven element is resonated at the lower frequency by means of a first reactance means which comprises an inductive reactance in the form of part of the two metal members 29, 30 between the ends of each half of the driven elements 27 and an adjustable shorting bar 31. The shorting bar 31 is adjustable along the length of the members 29, 30 thereby to vary the inductive reactance presented thereby. A second reactance means is arranged in shunt with the first reactance means to render it ineffective at the higher frequency. This second reactance means comprises a quarter wave stub at the higher frequency, 32, and is length of twin feeder.

The reflector element is similar to the driven element. In this case the first reactance means for the reflector element is variable by means of the shorting bar 33 and the second reactance means comprises a further quarter wave stub at the higher frequency, 34, which is also a length twin feeder.

As will be seen from the drawing both the second reactance means 32 and 34 are arranged inside the metal members 29 and 30.

The aerial is supported upon a mast (not shown) or a similar support by means of the support member 35.

The driven element is connected to a transmitter or receiver by means of a coaxial feeder 36, the outer conductor of which is connected to the shunting bar 31 or another part of the metal member 29. The inner con ductor is connected to one end of a conductor 37 through a capacitor 37a the other end of the conductor 37 being connected to one half of the driven element 27 part way between its ends. The conductor 37 forms an impedance transformer "between the coaxial feeder and the driven element 27.

It will be appreciated that the arrangement of the first reactance means as spaced, substantially parallel conductors electrically connected to one end shown in Figure 4 is equally applicable to the arrangement shown in Figure 3 if the boom 21 is replaced by two metal members similar to the members 29 and 30 shown in Figure 4.

In Figure there is shown an arrangement of a driven element, which can be used with the arrangements shown in Figures 3 and 4, in which the driven element is effective as a half-wave dipole at each of the frequencies at which the aerial is effective.

The driven element shown in Figure 5 comprises a conductor 39 which is elfecetive as a half-wave dipole at the highest frequency at which the aerial is effective. The inductors 40 are arranged to present a high impedance at this highest frequency thus isolating the remainder of the driven element from the conductor 39 at the highest frequency.

To each of the inductors 40 there is connected a further conductor 41. These conductors is conjunction with the conductor 39 are effective as a half-wave dipole at the next highest frequency at which the aerial is effective. At this frequency the inductors 40 present a low impedance relative to that which they present at the highest frequency and so are ineffective. It will, of course, be appreciated "that at the next highest frequency the inductors have some loading efiect so that the combined lengths of the condoctors 39 and 41 will be somewhat less than a half-wave length long at the next highest frequency.

The inductors 42 act at the next highest frequency similarly to the inductors 40 at the highest frequency. The additional conductors 43 together with the loading effects of inductors 42 and the remaining inductors 40 and conductors 39 and 41 make the driven element effective at the lowest frequency at which the aerial is effective.

The conductors 44 which are arranged parallel to the conductor 39 are connected to the conductor by means of connections 4-5. This arrangement constitutes an im pedance transformer between the co-axial feeder 46 and the driven element.

Figure 6 shows a further embodiment of the invention as applied to an aerial which is effective at both band I and band III to enable reception of a television signal in each band.

The conductors 47, which are hollow tubes, are arranged as a half wave dipole at the desired frequency in band III. The inductor 48, which is wound upon a core of ferrite material, acts as a loading coil in conjunction with the conductors 47 so that they are resonant at the desired frequency in band I. At the band III frequency the inductor 48 is rendered ineffective by means of two quarter wave stubs 49 which are a quarter wave at the band lII frequency. The feeder 50 is connected to tapping points on the inductor 48. It will be seen that the arrangement of Figure 6 provides a two-band television aerial to which connection is made by a single feeder and which is physically small, the length of the conductors 47 being determined by the higher frequency which is to be received.

It will be appreciated by those skilled in the art that the driven element can also be made resonant at a plurality of frequencies in the same manner as the reflector and director elements.

Further, an impedance network can be used at the feed point of the driven element automatically to transform the impedance of said driven element to a low impedance at each of said resonant frequencies of the aerial system.

I claim:

1. An antenna resonant at a first frequency and a second frequency comprising two spaced elements, a reacltance means connecting an end of one element to an adjacent end of the other element whereby the antenna is resonant at said first frequency, and a transmission line an electrical quarter wave length long at said second frequency having two ends, one of said line ends being free of electrical connections and the other of said line ends being connected electrically in parallel with said first reactance means whereby the antenna is resonant at said second frequency.

2. A directive antenna eflective at a plurality of frequencies which comprises a driven element which is resonant at a frequency different from any of said plurality of frequencies, a parasitic reflector element and a parasitic director element, said parasitic reflector and director elements each comprising two "spaced conductive members, a reactance means connecting an end of one conductive member to an adjacent end of the other conductive member whereby the antenna is resonant at said first frequency and a transmission line an electrical quarter wavelength long at said second frequency and connected electrically in parallel with said reactance means whereby the antenna is resonant at said second frequency.

3. A directive antenna effective at first, second and third frequencies, comprising a driven element which is resonant at a frequency different from any of said first, second and third frequencies, a parasitic reflector element resonant at said first and second frequencies and including two spaced conductive members, a first reactance means connecting an end of one conductive member to an adjacent end of the other conductive member whereby said reflector element is resonant at said first frequency, a transmission line an electrical quarter wavelength long at said second frequency and connected electrically in parallel with said first reactance means whereby said reflector element is resonant at said second frequency, a parasitic director element resonant at said second and third frequencies and including two spaced conductive members, a second reactance means connecting an end of one conductive member of the director element to an adjacent end of the other conductive member of the director element whereby said director element is resonant at said second frequency, and a transmission line an electrical quarter wavelength long at said third frequency and connected electrically in parallel with said second reactance means whereby said director element is resonant at said third frequency.

4. A directive antenna effective at first, second and third frequencies comprising a driven element which is resonant at a frequency different from any of said first, second and third frequencies, a parasitic reflector element resonant at said first and second frequencies and including two spaced conductive members of substantially equal length, a first inductor connecting an end of one conductive members to an adjacent end of the other conductive member whereby the reflector element is resonant at said first frequency, a transmission line an electrical quarter wavelength long at said second frequency and connected electrically in parallel with said first inductor whereby the reflector element is resonant at said second frequency, a parasitic director element resonant at said second and third frequencies and including two spaced conductive members of substantially equal length, a second inductor connecting an end of one conductive member of the director element to an adjacent end of the other conductive member of the director element whereby the director element is resonant at said second frequency, and a transmission line an electrical quarter wavelength long at said third frequency and connected electrically in parallel with said second inductor whereby said director element is resonant at said third frequency.

5. A directive antenna effective at first, second and third frequencies comprising a driven element, a parasitic reflector element and a parasitic director element, said driven element comprising two spaced conductive members of substantially equal length terminating in two adjacent ends and a twin feeder having a length substantially corresponding to a number of electrical quarter wavelengths at each of said first, second and third fre quencies and connected to said adjacent ends of said spaced conductive members, said parasitic reflector element being resonant at said first and second and comprising two'conductive members of substantially equal length terminating in two adjacent ends, a first inductor connecting said adjacent ends of the conductive members of the parasitic reflector element whereby the reflector element is resonant atsaid first frequency, a first transmission line an electrical quarter wavelength long at said second frequency and connected electrically in parallel with said first conductor whereby the reflector element is resonant at said second frequency, said parasitic director element being resonant at said second and third frequencies and comprising two spaced conductive members of substantially equal length terminating in two adjacent ends of the conductive members of the director element whereby the director element is resonant at said second frequency, and a second transmission line an electrical quarter wavelength long at said third frequency and connected electrically in parallel with said second inductor whereby the director element is resonant at said third frequency.

6. A directive antenna effective at first, second and third frequencies comprising a driven element resonant at a frequency different from said first, second and third frequencies, a parasitic reflector element and a parasitic director element, said driven element including two spaced conductive members of substantially equal length terminating in two adjacent ends, an impedance transformer, a

twin wire feeder having a length substantially corresponding to a number of electrical quarter wavelengths at each of said adjacent ends and said impedance transformer, said parasitic reflector element comprising two spaced conductive members of substantially equal length terminating in two adjacent ends, a first inductor connecting said adjacent ends whereby, the reflector element is resonant at said first frequency, a transmission line an electrical quarter wavelength long at said second frequency and connected electrically in parallel with said first inductor whereby the reflector element is resonant at said second frequency, said parasitic director element including two spaced conductive members of substantially equal length terminating in two adjacent ends, a second inductor connecting said adjacent ends whereby the director element is resonant at said second frequency, a transmission line an electrical quarter wavelength long at said third frequency connected electrically in parallel with said second inductor whereby the director element is resonant at said third frequency, said impedance transformer including a third inductor presenting two portions, a capacitor connecting one end of one portion to one end of the other portion, said twin wire feeder connecting the other ends of each of said portions to said spaced conductive members of said driven element, a pair of terminals, and a fourth inductor electromagnetically coupled to said third inductor and connecting said pair of terminals for presenting between them a substantially constant,

predetermined impedance.

7. A directive antenna effective at first, second and third frequencies comprising a driven element resonant at a frequency different from said first, second and third frequencies, a parasitic reflector element, a parasitic director element, a supporting boom for said elements, said supporting boom including two substantially parallel conductive members, said driven element including two spaced conductive members of substantially equal length, insulators insulatingly supporting each of said conductive members of the drive members from one of said substantially parallel conductive members, said conductive members of the driven member terminating in two adjacent ends, an impedance transformer, a twin wire feeder having a length substantially corresponding to a number of electrical quarter wavelengths at each of said first, second and third frequencies and connected between said adjacent ends and said impedance transformer, said parasitic reflector element including two spaced conductive members of substantially equal length terminating in two adjacent ends and each carried by one of said substantially parallel conductive members, a first shorting bar coupling said substantially parallel conductive members for forming a first inductive reactance connecting the adjacent ends of said two conductive members whereby the reflector element is resonant at said first frequency, a transmission line an electrical quarter wavelength long at said second frequency and connected electrically in parallel with said first inductive reactance whereby said reflector element is resonant at said second frequency, said parasitic director element including two spaced conductive members of substantially equal length terminating in two adjacent ends and each supported by one of said substantially parallel conductive members for forming a second inductive reactance connecting said adjacent ends of the conductive members of the director element whereby the director element is resonant at said second frequency, a transmission line an electrical quarter wavelength long at said third frequency, said impedance transformer including a third inductive reactance presenting two portions each having two ends, a capacitor connecting one end of one of said portions to one end of the other of said portions, said twin wire feeder connecting the remaining ends of said portions to the conductive members of said driven element, a pair of terminals, and a fourth inductive reactance electromagnetically coupled to said third inductive reactance and connecting said pair of terminals --for presenting between them a substantially constant, predetermined impedance.

References Cited in the file of this patent UNITED STATES PATENTS

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