US2195717A - Electrical high frequency signaling system - Google Patents

Description

April 2, 1940. c, RK 1- AL 2,195,717

ELECTRICAL HIGH FREQUENCY SIGNALING SYSTEM Filed Oct. 30, 1937 2 Sheets-Sheet l I I P l 53 E I G 2 l u! a I m a: Q a (If a: u] U I 0 V) Mm I U Z l p,- 9 l .3 l b E l g 5 u f I E FREQUENCY l .Xip :1

Fly 211 z l a n to l a l a n M N 3 l IE .20. Fg: 300' y INVENTORS EDWARD (50/ L CORK AND By W was PAWSEY A TTORNEY April 2, 1940- E. c. CORK ET AL 2. .717

ELECTRICAL HIGH FREQUENCY SIGNALING SYSTEM Filed Oct. 30, 1937 2 Sheets-Sheet 2 u] d E u 15 M'rN. Ki um 1 I 5 P 5 I g: Q P a: I 5 R R I F Faaaumcv I l- I L I 3, I I Faeeusucw. E B .5- N j I 3%. m I g I w l n.

l- /4 5 i M. 4 i i F159. 4. A f A A I L I XII L l I I' L IkVVENTORS LADE PAWSE),

Patented Apr. 2, 1940 PATENT OFFICE ELECTRICAL RIG}; ran cuunNovv SIGNALING Edward Cecil .Cork,-Ealing, London, and Joseph Lade Pawsey, Hillingdon, England,' assignors to Electric & Musical Industries Limited,

Hayes, Middlesex, England, a' company of Great Britain Application October 30, 1937,

' In Great Britain November 13,

18 claims.

This invention relates to electrical high frequency systems and is particularly concerned with short wave transmitting and receiving systems and'equipment. Y

In short wave transmitting systems it is de sirable that the impedance of any piece of apparatus should be matched to the impedance of any other'piece of apparatus with which it is connected. In particular cases it may be necessary, for-example, to match the impedance of a feeder systemwith that of anaerial or of another feedersystem, or again, it may be desired to 'match and tune a transmitter to a feeder system. I

In British Patent No. 469,245, accepted July 21, 19 37, corresponding to pending United States application Serial No. 110,652, filed November 13, 1936, the consequences of unmatched impedances are fully dealt with, and the desirability of transforming an'impedance varying with frequency to a constant resistance is discussed; and circuitsare shown for obtaining this result making use of lengths of transmission line in combination with reactive elements such as lengths of transmission line. 1

. The present invention is also concerned with matching impedances which vary with frequencybut has particular reference to cases in which the variation in reactance is substantially linear and the resistance variation is symmetrically parabolic about a mean frequency. p

In general the variation of resistance of an aerial over a range'of side-band frequencies is not-symmetrical about a'mean frequency, but a length of transmission line can be chosen such that when terminated by the aerial its input impedance has a series resistive component which is a maximum at'the mid-frequency and a reactive component which varies with frequency in a substantially linear manner, the reactance at the mid-frequency being not necessarily zero. In certain common cases the deviation of resistance with frequency variation from the midfrequency is a parabola and in general a smoothly varying curve closely approximating to a parabola over a required'range of frequencies.

According to the present invention in a high frequency signaling system including apparatus theresistance of which varies in an approximately parabolic manner about a given frequency in the frequency range of signals fed to said apparatus, correcting reactive elements are provided which render the resistance of said apparatus substantially constant and equal to the resistance at the said frequency. The repensate for reactance due to correcting reactive elements connected in series or parallel respec- 1 tively with said apparatus, additional reactivei elements are providedconnected in parallel with said apparatus in the first case and in series with said apparatus in the second case. The reactive elements may be constituted by short lengths of transmission line. V y In order that the invention may be more clearly .understoodlkand' readily carried into effect, compensating" circuits embodyingthe invention -will now be described by'way of example with reference to Figs. 1 to 5fof the drawings, where-, in Figs. 1a and lb' illustrate cases in which the present invention is concerned, where the re-' ,actance variation is substantially linear and the resistance r variation approximately parabolic Fig. 2a shows a compensating circuit; Fig. 2b

shows the seriesresis'tance and reactance values be found in the feeder system connected to a.

short wave aerial at which the resistance varies with frequency in a parabolicmanner as shown in Fig. lrnof the drawings and the reactance variation is ,linear. In applying the present invention. a series resonant circuit is added having such values of inductance andv capacity that at the centre frequency Q the total reactance is zero, and further that the reactance at frequency P is such that the equivalent parallel resistance at this frequency has the same value as that at the frequency Q. Since the total reactance varies linearly through zero at Q the equivalent parallel resistance at P will also be equal to that at Q. In the particular case of a parabolic series resistance curve the equivalent parallel resistances are also constant at the intermediate frequencies. There remains, 'how-. ever, a parallel susceptance varying linearly through zero which may be annulled by a par-,-

allel tuned circuit having the correct ratio of inductance and capacity and tuned to frequency Q.

An inverse method may be employed for the compensation of "a resistance varying in the manner shown in Fig. .lb.

Fig. 2a shows diagrammatically the method of applying the invention. In this figure Z represents the aerial impedance with series resistance varying in a parabolic manner as shown by the curve PQP in Fig. 2b and reactance varying linearly as shown by the line L in this figure. The series circuit consisting of inductance ii and condenser 2 is added in series. "The series impedance at M has the same resistance/frequency curve as before, but the reactance/frequency curve is now modified to the .line M.

The equivalent parallel resistance atM is now constant as shown by Fig. 2c and the parallel susceptance at M is shown by the curve M. The

sign convention is such that the susceptance of.

a'capacity is positive. This susceptanceis neutralized by the equal and opposite susceptance provided by the tuned circuit consisting of inductance 3 and-capacity 4 so that the impedance at M is a constant pure resistance equal to the original resistance at frequency Q.

The circuit for compensating 1h isshown in Fig. 3a. Fig. 3b shows the equivalent parallel resistance'and'susceptance at Lof the-impedance Z and the modified values at M. The susceptance at L is shown passing through but if this is not the case, the susceptance added at M must be such that the total susceptance at M is zero at the mid-frequency. Fig. 3c shows the series values of resistance and reactance at the point M of Fig.3a'an'd the final value of impedance at N i. e. a constant pure resistance.

The values-of the components to be used follow from the following considerations. In the case of Fig. 2b the reactance at M must pass through zero at the'frequency Q of the peak of the resistance-curve. At another frequency P, at which the parallel resistance is to be made equal to 4'6} the reactance am at M must be given by If the reactance, at L is known, theamount to be contributed by'the series tuned circuit at these two frequencies is also known, and determines the values of inductance Land condenser 2 required. The parallel tuned circuit .3, 4 must be tuned to the frequency Qand have a value of inductance capacity to neutralise thev equivalent parallel reactance of TP in series with :2? at the frequency P.

From the Equation (1) it follows that the resistance variation is exactly compensated if at M The invention may be applied to the compensation of variations of impedance in which the variation of reactance is linear whether or not it passes through zero at the peak of the resistance curve provided'that the rea'ctance at frequency P or P is not already greater than the required value and is in the same sense as the appropriate first tuned circuit. If the reactance at frequency P or P is in the correct sense but greater than that required, then the alternative method can be used.

In the correcting circuits described with reference to Figs. 2 and 3, tuned circuits including liunped reactances have been employed. There are, however, instances in which it would be preferable to use transmission lines in place of the tuned circuits. Figs. 4 and 5 of the drawings show circuits utilising approximate quarter wavelength transmission lines to replace the lumped reactances shown in Figs. 2 and 3 in which short lines equal toquarter of a wavelength are employed. I I

If greater selectivity is required than that given conveniently by single quarter wavelength lines as shown, abnormally high or low values of characteristic impedance being indicated, lines of multiples of a quarter wavelength greater than one may be used. In this case an open circuited multiple half wavelength or a short circuited odd multiple quarter wavelength acts similarlyto a parallel tuned circuit and a short circuited multiple half wavelength or open circuited odd multiple quarter wavelength as a series tuned circuit.

The compensating arrangements described may usefully be combined with the property of transformation, and for this purposethe circuits 'of 'Figs. 2 and 3 for example may be modi fled so that the paralleltuned circuit is appropriately tapped to form an 'auto transformer.

We claim:

' l. A high frequency signaling system including apparatus the impedance of which isasubstantially pure resistance which varies'in an approximately parabolic'manner about a given frequency in the frequency range of signals fedto said apparatus, correcting reactive elements-of such magnitude and being so connected to said apparatus as to renderthe resistance of said apparatus substantially constant over said frequency range and equal to the resistance atthe said given frequency.

2. A high frequency signaling system according to claim 1 wherein said reactive elements comprise inductance and capacity connected in series with each other and in :series with said apparatus.

3. A high "frequency signaling system according to claim 1 wherein said reactive elements comprise inductance and capacity connected in parallel with each other and in parallel with said'apparatus.

4. A high frequency signaling system according to claim 3. whereinin order to compensate for reactance due to correcting reactive elements connected in series with said apparatus "additional reactive elements are provided connected in parallel with said apparatus.

.5. A high frequency signaling system according to claim 1 wherein in order to compensatefor I reactance due to correcting reactive elements connected to parallel with said appaartus, ad-

ditional reactive elements are provided connected in series with said apparatus.

6. A high frequency signaling system accord ingv to claim 1 wherein said reactive elements are constituted by short lengths of transmission line.

'I. A shortwave transmitting system including which is such that the total reactance is zero at a selected frequency in said range and atfa side frequency the reactance is such that the equivalent parallel resistance is the same as at the selected frequency in said range.

8. A shortwave transmitting system according to claim 7 wherein a parallel tuned circuit resonant at a selected frequency in said range, is

connected across said feeder in order to annul susceptance due to said series resonant circuit.

9. A shortwave transmitting system including an aerial connected by a feeder to transmitting apparatus wherein in order to render the resist ance of the said aerial constant over a range of frequencies a parallel resonant circuit is connected in parallel with said feeder, said resonant circuit including inductance and capacity the value of which is such that the total susceptance is zero at a selected frequency in said range and at a side frequency the susceptance is such that the equivalent parallel resistance is the same as at the selected frequency of said range.

10. A short wave transmitting system according to claim 9 wherein a series tuned circuit resonant at a selected frequency in said'range is connected in the feeder in order to annul reactance due to said parallel resonant circuit.

11. A shortwave transmitting system according to claim 7 wherein said series and parallel resonant circuits are constituted by a short circuited line a multiple of a quarter wavelength long including unity at the selected frequency in said range.

12. A shortwave transmitting system. according to claim 7 wherein said series and parallel resonant circuits are constituted by open circuited lines a multiple of a quarter wavelength long including unity at the selected frequency in said range.

13. The combination with aresonant load and a source therefor, of a corrective network for insertion between said source and resonant load, said network comprising a pair of transmission line sections an odd multiple of a quarter wave at the operating frequency, each section having 1 input terminals, connections between said source and load, the terminals of one line section being connected in series with said connections between said source and load, and the terminals of the connected in series-with said connections between said source and load, and the terminals of the other line section being connected in shunt to the circuit connecting said source'and load, the shunt connected section having its output terminals chosen to render the effective impedance presented to said source substantially uniform over a band of frequencies. v

15. A high frequency signaling system includconnected together,and the series connected section having its output terminals open, the char- 'acteristic impedances of said line sections being ing apparatus the impedance of which'throughout a range about a given frequency is a substantially pure resistance having an extremum at said given frequency, leadsfrom said apparatus to a pair of terminals, a network between said terminals and said apparatus including a series circuit tuned to said given frequency connected in series with one of said leads and a parallel circuit tuned to said same given frequency in shunt'to said leads,

the tuning elements of said circuits being so chosen as to make the impedance between-said ,terminals at a second frequency in said range equal to a pure resistance of the same value as the resistance at the-given frequency.

. 16. An impedance network for presenting between a pair of terminals a substantially pure resistance over a range of frequencies, comprising a load circuit having resistance whose value passes through an extremum at a frequency within said range and a reactance network comprising series and shunt reactance elements connected respectively in series and shunt to said load with respect to said terminals, said series and shunt elements being so chosen as to make the impedance between said terminals at said extremum frequency and at two other frequencies in said range equal to said extremum of resistance.

17. An impedance network for presenting be tween a pair of terminals asubstantially pure resistance over a range of frequencies, comprising a load circuit having resistance whose value passes through an extremum at a frequency within said range and a reactance network comprising series and shunt reactance elements connected respectively in series and shunt to said load with respect to said terminals, said series and shunt elements being individually so chosen as to make the total reactance between said terminals vanish at the frequency of said extremum,

while they are furthermore relatively so chosen as to make the reactance between said terminals vanish at two other frequencies in said range.

ing a load circuit having resistance whose value 1 passes through an extremum at a frequency within said range and a reactancenetwork comprising series and shunt reactance elements connected respectively in series and shunt to said load with respect to said terminals, said series and shunt elements being individually so chosen

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