US2267445A - Electric high frequency signaling system - Google Patents

Description

Dec. 23, 1941. E. c. CORK ETAL' 2,267,445

- ELECTRIC EIGH FREQUENCY SIGNALING SYSTEM Filed A' ri; 21 1938' 9 m 9 ngnm INVENTORS EDWARD CFC/L CORK /QSEPH o mwsn' ATTORNEY Patented Dec. 23, 1941 ELECTRIC HIGH FREQUENCY SI'GNALING SYSTEM" Edward Cecil Cork, Ealing, Lo'ndtimahdilldsephf Lade Pawsey, Hillingdon, England; assignors to Electric &, Musical Industries Limited; a

British company Application April 21, 1938, Serial No;- 203,244

In Great Britain May 1, 1937 4 Claims. (01. 250-33) The present invention relates to high frequency signaling systems and more specifically to such systems which? are required to transmit a wide band of signals.

In the complete specificationof British appli-' cations Nos. 32,339 35 and 16,993 36 (Patent No. 469,245), itis explained that in high frequency signaling systems *such as television systems, it is generally impractical to match the transmitter impedance to the input impedance of the feeder terminated by the aerial and at the same time obtain the necessary band width It is pointed outin our prior specification that even if the reactance of the aerial is small or is neutralized by suitable circuits the radiation resistance of the aerial will, in general, vary with frequency. The aerialisusl'ially matched to the feeder at the aerial frequency, but due to the vari-- at ion of the radiation resistance" with frequency;

theaerial will notbe' matched at side band 'fre' quencies. This mismatching" causes variation in" the 'impedance presentedto the'transmitter by the aerial system, and hencev'a'riations in the radiated power over the wide band of frequencies constituting the side bands of a televisiontransmitter. I r

Theph ase angle of current reflected at the aerial end of the feeder when it reaches the transmitter depends on the length of the feeder I having regard to the width of the band of modu lation frequencies. r

In the aforesaid priorspecification, methods are described of substantially eliminating the variation of the aerial impedance with frequency so that there is at all relevant frequencies a substantially: constant resistance, by I means of lengths of lineandresonant circuits, sothat it can be connected to the associated transmitter byany length of transmission line."

Where it is necessary forthe transmitting -sys tem' as awhole to have a, band pass characteriStiCyihB-rCOIlStEtIlt resistance aerial provided by the methods of -the above applications would be coupledvtothe final tuned (tank) circuitof the transmitter by a further-tuned circuit. 1 Such arfeeder "wto lcompensate for the reactaii'ce' change of -th 'tank circuit of v the transmitter,- and the 50 "companying drawing in which othertuned circuit being inserted towards the aerialend of the feeder-td annul reactance' change due to thefaerial.

Now the relative variations of the impedance of i the aerial and of the tank circuit of the transmitter are of the same order of magnitude and for this reason in some "cases a simplificaf tion of the band -pass arrangement can be, ef-

fected; in that the variation oftheimpedanc'e of r the aerial maybe caused to "appear from "the transmitter as a-reactance variation'of opposita'f senseto that of the variatio'nin the reactance "of the transmitter itself, whereby variations I in the impedance of the aerial with changing fre quency-tend to compensate the variations in thej outputimpedance of the transmitter} thusfif the length of the feeder is correctly choserif the tuned circuits provided inthe feeder 'described in the prior case may be omitted."

Thus, according to the present invention, 'a radio signal transmitting'or "receiving arrangement more especially for use on short waves is provided comprisinga radio transmitter or receiverso associated with an aerial-that for -a' band of frequencies in the pass range of the? arrangement, the variationof the-impedance of the aerial as viewed fromthe transmitter or receiver appears as a reactance variation substantially opposite in sense'to the variation in the reactance of the transmitter" or receiver itself, whereby variationsin the impedance of the aerial with changing frequency tend to compensate the variations in'the output impedance" of the transmitter. V

In the preferred arrangement according to the invention the variation of the impedance of the aerial as viewed: from transmitterappears as a reactance substantially equal in value to the variation of ther reactanc'e -of the transmitter over said band of"frequericiesi In certain circumstances,- in an, arrangement according to-theinvention; a transmitter and its aerial may be, connected without the inter position of a feeder, or a simple half wave length feeder may be'provided. V i

v The nature'ofthe invention and the method of carrying -the same 'into effect; willbe more clearly understoool from the following descrip tion" detaiP reference being made to the Figure 1 shows a schematic diagram of a bandpass aerial system embodying the invention described in our prior specification above referred to.

Figure 2 is a similar representation of an arrangment according to the present invention.

Figure 3 is an explanatory equivalent circuit of the arrangement of Figure 2.

Figure 4 is a vector diagram applicable to the arrangement of Figure 2, I

Referring to Figure 1, it will be seen that the output circuit of a transmitter including tube l and tank circuit 2 is shown connected over transformer 3 to feeder 4 which is directly coupled to dipole aerial 5. In order to compensate for the variation in the reactance of the tank circuit of the transmitter the secondary winding of the transformer 3 is associated with condensers 6, which form a resonant combination with the said secondary winding, and in order to compensate for the variation in aerial reactance the series tuned combination I of inductive and capacity elements is provided.

Now the changes in the magnitude of the reactances which are compensated in the above arr' rangement are of the same order, and in accordance with the present invention, instead of providing tuned combinations 3,6, 6,andl,the length of the feeder is chosen so that the reactive change of the aerial may have the effect of a series tuned circuit located at the point where the feeder is connected to the transmitter. For example, referring to Figure 2, aerial 5 is'shown connected over feeder 4 to tappings on the inductance 3' of the tank circuit 2 of the transmitter the inductance 3 thus serving as an auto-transformer;

Figure 3 shows the equivalent circuit in'this case. In this figure, the tank circuit 2 of Figure 2 is represented by the tuned circuit comprising inductance L and capacity C, while L, and C, and R represent the equivalent circuit of the feeder and aerial.

In such, a circuit the constant K band-pass I condition is given by 2= R 0 0. and Lcwo2=l where am is the angular velocity, corresponding to the midfrequency f0 of the band.

The circuit comprising elements L1, C1 and R1 The input impedance of the feeder at any given frequency may be obtained-from the vector diagram of Figure 4 in which line 00' is a'vectorial representation of the characteristic impedance of the feeder which is made equal to RA, and the vector O'P is the resultant change of impedance due to the increments of resistance reactance of magnitude OP: the angle 20 is given by the equation The aerial impedance viewed from the transmit ter will then be given by the expression Equation 3 can satisfy the bandpass condition given by Equation 1 if dR 2 dXAF} 2 4712C (4) Now for a half wave dipole aeria1 with a tuned impedance at mid-band frequency of 72 ohms 26 360 dgs. cotis approximately and is approximately Z (GA) where ZOA is the characteristic impedance of the aerial. Hence the value of ZOA for a given value of C can be found and the electrical length of feeder required to produce the apparent rotation of OP is given from the Equation 3 as 2 5 =n 360 cot where n is any integer, including zero. It is apparent from the form of this equation that the same physical length of line approximately satisfies the equation over a band of frequencies.

By way of example, it may be noted that in the case given if C=l09 ,u f. the characteristic impedance of the aerial Z0A=72 ohms, and for 0:440 i, ZoA=400 ohms. These are the approximate limits over which the aerial may conveniently be varied.

It will be noted that the physical length of line required to satisfy Equation 5 is slightly dependent on frequency, since the electrical length required is constant. In practice, therefore, a single feeder will not be efiective equally at all freplane transmitter. -In many cases, this restric- I tion on the use of theinvention makes it prefer-able to use an arrangement such as that shown in Figure 1 of the drawing.

Certain special cases of the application of the invention are of interest. For example, if the variation of the resistance with frequency nt is zero or negligibly small compared withthe variation an f as for example in a condensed array, the Equation 4 above becomes and the electrical length of the feeder required is given by :11. 180 degs. where n is any integer including zero. The simplest case is that in which n and consequently 0 is zero, in which case the transmitter would be directly connected to the aerial but if a direct connection is inconvenient,

of conditions'for the application of the inven-v tion may be determined in the manner described above for any particular case; The invention will, of course, be applicable in any case where it is possible to transform the variation of the impedance of an aerial to substantially a linear reactance variation of the correct magnitude.

In the arrangement described above, the feeder is chosen to match the aerial impedance, so that the input impedance to the line is equal to the tuned aerial resistance. If it is desired to work into a different impedance, an unmatched line may be used of suitable impedance and electrical length, so that it acts as a transformer and produces the desired bandpass action. The invention is not only applicable at the transmitting end of signaling system, but is equally applicable to a receiver.

Having now particularly described and ascertained the nature of our said invention and in what manner the same is to be performed, we declare that what we claim is:

1. In combination, an aerial having an impedance comprising a reactance which varies with the frequency of a wave applied thereto and a substantially constant resistance, a feeder con nected to said antenna and a coupling circuit connected to said feeder for coupling a source of reactance of said output circuit high frequency oscillations to'said aerial, the reactance of said coupling circuit varying with the frequency of a wave applied thereto, said feeder having an electrical length equal to a half wave length of the operating frequency so that the reactance variation of said aerial viewed from said coupling circuit is substantiallyopposite in sense to the reactance variation. of said coupling circuit whereby said variations compensate for each other. I

2. In combination, an aerial having an impedance comprising a reactance which varies with the frequency of a wave applied thereto and a substantially constant resistance, a feeder connected to said antenna and an output circuit of a transmitter connected to said feeder, the reactance of said output circuit varying with the free quency of a wave applied thereto, said feeder having an electrical length equal to a half wave length of the operating frequency so that the reactance variation of said aerial viewed from said output circuit is substantially opposite in sense to the reactance variation of said output circuit whereby said variations compensate for each other.

3. A broad band transmitting arrangement including an aerial having an impedance comprising a reactance which varies with the frequency of a wave applied thereto and a substantially constant resistance, a feeder connected to said antenna and an output circuit of a transmitter connected to said feeder for coupling said transmitter to said aerial, the reactance of said output circuit varying with the frequency of the wave appearing therein, said feeder having an electrical length equal to a multiple including unity of a half Wave length at the middle frequency of said band so that the reactance variation of said aerial viewed from said output circuit is substantially opposite in sense to the variation in whereby said variations compensate for each other. i i

4. A broad band antenna arrangement including an aerial having an impedance comprising a reactance which varies with the frequency of a .wave applied thereto and a substantially constant resistance, afeeder connected to said antenna and a coupling circuit connected to said feeder for coupling a transducer to said aerial, the reactance of said coupling circuit varying with the frequency of the wave applied thereto, said feeder having an electrical length equal to a multiple including unity of a half wave length at the middle frequency of said band so that the reactance variation of said aerial viewed from said coupling circuit is substantially opposite in sense to the variation in reactance of said coupling circuit whereby said variations compensate for each other.

EDWARD CECIL CORK.

JOSEPH LADE PAWSEY.

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