US2428290A - Approach signal system - Google Patents

Description

Sept. 30, 1947. L. H. PECK APPROACH SIGNAL SYSTEM Filed April 30, 1942 4 Sheets-Sheet 1 mz Zzom 2m; h A 8 8n .3 mm

M nm 1 INVENTOR LEE H. PECK BY Wa ATTORNEY Nv mm hm won om awn om NN MN om I? Sept. 30, 1947.

L. H. PE CK APPROACH SIGNAL SYSTEM Filed April 30, 1942 4 Sheets-Sheet 2 lNVENTOR LEE H. PECK W 2/ 710 4 ATTORNEYS 8 uw= 0 150 mm C no. 2. q: 2

Sept. 30, 1947. L, PECK APPROACH SIGNAL SYSTEM Filed April 30, 1942 4 Sheets-Sheet 3 N2 NE INVENTOR LEE H. PECK BY ATTORNEYS Sept. 3.0, 1947'. L. H. PECK APPROACH SIGNAL SYSTEM Filed April 30, 19 2 4 Sheets-Sheet 4 GROUsLINE FIG-5 INVENTOR LEE H. PECK BY ATTORNEYS Patented Sept. 30, 1947 UNITED STATES; PATENT OFFICE 2,428,290 I ifilfifif 2 Claims. 1

An object of the present invention is to provide a signal and alarm system adapted to indicate the approach of a person to a guarded boundary, such as a fence around a guarded area and more particularly to provide such a system as will prevent 'a person crossing the uarded line in any fashion without giving a signal and to prevent the cutting of the guard wire without giving a signal and to provide such a system as will operate with maximum eiliclency through all kinds of weather changes.

Further objects and advantages of the present invention will be apparent from the accompanying drawings and description and the essential features will be summarized in the claims.

In the drawings, Fig. 1 is a diagrammatic view of a systemwherein the capacity of a body appreaching the guarded line will produce a change in voltage in my improved system so as to produce a signal; Fig. 2 is a diagram of a modified system wherein the capacity of the body of a person approaching a guarded line will produce a change in frequency in the system so as to produce a signal; Fig. 3 is another modification wherein the capacity of the body of a person approaching the guarded line produces a change in current or voltage in the system so as to produce a signal; Fig. 4 is a diagrammatic view illustrating generally how such a signal system might be installed; and Fig. 5 is a diagrammatic view of a protection against tunneling under a fence.

My improved signal system is particularly useful for outdoor installation where growing weeds, rain and snow storms and other changing electrical conditions have hindered the most eflicient operation of prior known protective systems of this sort. The fundamental principle upon which the various modifications of my system are based, involves a double antenna system having two wires of substantially equal electrical capacity to ground, with means for balancing one antenna system against the other so that the end output of the two systems is normally substantially zero in so far as electrical flow is concerned and no signal is produced. Various arrangements are here disclosed whereby any unbalanced change in either system causes a flow of electricity and only such changes as are due to the approach or other activity of a person near the antenna systems are permitted to produce a signal. Other changes which occur more slowly because they are due to natural causes are taken care of in other ways.

Referring first to Fig. 4, various forms of systems will be described applicable to a situation wherein a property is guarded by the fence i0 (Cl. 177-352) g with a guard located at the house II and with protective units along the fence system as shown at l2, l3, M, iii and It. The limits of the protective antenna systems areindicated by the suftlxes a and b appended to the different reference characters for the various units. For instance the antenna systems for unit 12 run from the unit to 12a and lib. The other units have been marked in like fashion.

A unit constructed according to Fig. 1 would be housed in a single casing protecting it from atmosphere and located at the point l2, it, etc. The wire I! represents a wire running from I! to l2a and the wire 18 represents one running from I! to lib. These wires might well be made of the same size wire and of exactly the same length and supported on insulators at thesamc height above the ground. This would give approximately the same electrical capacity to ground for each of the wires with provision in the unit to balance for small inequalities by means of tuning condensers. It will be understood by those skilled in this art that one of the wires H or ill might be longer and placed higher above the ground than the other and still give equal electrical capacity to ground for the two wires, considering each as an antenna system. Such obvious variations will not be further discussed.

In the system shown in Fig. 1, I have made use of an electrical bridge to balance one antenna system against the other and to permit a relative change in voltage between the two antenna systerms to produce a signal at the guard house II. On one side of the electrical bridge is the resistance l9 and the variable condenser 20 which is connected to ground. On the other side of the bridge is the resistance 2| and the parallel connected condensers 22 and 23 which are also grounded. The two sides of the electrical bridge are normally balanced against each other roughly by the proper construction of the double antenna system with final adjustments made in the field by the setting of the condensers 20, 22 and 23. Current is supplied to the entire unit from lines L1 and La which are connected to anysuitable source not shown and extend to the units l2, ll,

it. etc. Only two wires are needed around the entire fence system as each unit is connected in parallel to the same two wires L1 and Lo. It is here assumed that these wires are supplied from a source of A. C. current at to volts.

In the unit l2, for instance, is provided a recthrough line 28 to oscillator 28 and through lines 30 and 44 to provide the B current for the electronic circuits later described. Tubes 26 and 2'! are voltage regulator tubes to regulate voltage in these lines. The oscillator 29 is of a standard type adapted to supply an oscillating current stant frequency-under all weather conditions.

The unit shown at 33 will be recognized as. a 'bufier or isolation amplifier to prevent changes in the main circuit from afiecting the frequency transmitted to the double antenna system,

The frequency supplied by the oscillator may have a wide range, it being understood that the length of wires connected at I! and i8 and the sensitivity of the device will depend upon the frequency selected. For instance, if 300 cycles were supplied, the wire I! might be two miles long and the sensitivity would be so low that a person would have to touch the wire to produce a signal. However I have used in actual practice 180 kilocycles which permits the wire I! to be any length up to 1,000 feet and which can provide a system of suificient sensitivity that a signal will be given if a person approaches within ten feet of the wire H or a signal will be transmitted if so much as one inch of the wire I! is cut off. The sensitivity and length of wire used for antennas are proportional to the frequency generated by the oscillator 29. Also the sensitivity is afiected by the bias on the control grid of the thyratron trigger tube 43. If the frequency is lowered, the length of wire can be increased but the sensitivity will be reduced.

Normally when the system is installed and properly balanced and the weather is dry there will be practically no current flowing at the output end of the electrical bridge. However upon a change in capacity to ground on either side of the double antenna system either in a positive or negative direction current will flow from the bridge through the system about to be described.

Following the electrical bridge I have shown two stages of radio amplification at 34 and 35 tuned to the frequency of the oscillator. Since these are standard circuits further description is believed unnecessary. Next in the system is a double rectifier tube 36. Upper and lower pairs of elements are indicated in the diagram. The plate 36a of the upper pair and the cathode 35c of the lower pair are connected to the source of input voltage. The D. C. output will then be positive from the cathode 36b and will be negative from the plate 36d and from a properly designed amplifier the positive and negative components will be of equal magnitude. Two resistances 31 and 38 of substantially equal value are connected in on the two sides of the output circuit. These are connected together at the common point 39 and normally the resulting voltage at this point would be zero, as the positive and negative values in the two legs of the voltage corrector 31, 38 would cancel each other. The resistance 38 in the negative leg, however, is connected through condenser 40 to the ground at 4| for a purpose to be later described.

The next device in the circuit shown at 42 is a low pass filter to prevent lightning from operatin the trigger tube shown at 43. This filter has the usual air core and iron core chokes connected through condensers to ground and cuts off all frequencies above 50 or 60 cycles per second.

The trigger tube 43 as here shown is one known as a thyratron which normally is nonconducting when its grid is below 1.4 volts negative. The normal bias for the grid of this tube is here below 1.4 volts negative and is provided through line 44 and the adjustable resistance 45. .By adjusting this bias the amount 01' positive voltage which must be received from the rectifier to overcome the bias of the tube sufllciently to permit it to conduct a signaling current is adjusted. This adjusts the'sensitivity of the system. If an amplifier. (3'4, 35) having a--gain"of'-25,000 to 50,000 is used, the sensitivity is extremely high and a small fraction of a mmfd. change in the antenna wire will produce a signal. Testing jacks are shown at 45 and call for no further description.

A relay 4 i is provided in the plate circuit of the tube 43 for the purpose of closin the contacts shown at 48 to provide a signal.- 0n small installations two wires might run from contacts 48 to a signal device in guard house II. A system is shown here for reducing the wiring cost on large installations. The contacts are shown as closing a circuit from line L1 through line 49, the winding of motor 50, line 5|, contacts 48 and line 52 back to L2. The shaft of motor 50 isdiagrammatically indicated as driving cam wheels 53, 54 and 55. Means is provided insuring sufficient length of operation of motor 50 to send a signal even though a person approaches wire H or i8 only momentarily. To this end a maintaining circuit is provided from L1 through line.49, winding of motor 50, line 55, normally open contacts 51 and lines 58 and 52 to In. The contacts at 51 are normally held open by a small projection a on the cam wheel 55. As soon as motor 50 starts this cam 55a permits the contacts at 51 to close and motor 50 will continue to operate even though the contacts at 48 should immediately open. This operation will continue until-wheel 55 has made one revolution when cam 55a will again open the contacts at 51. Meanwhile the cam 53a on wheel 53 has opened the normally closed contacts at 59, thus deenerg'izing the tube 43 and relay 41, setting these parts for another operation. The wheel 54 is utilized to send a signal'to the house I I. To this end multiple projections 54a are provided on wheel 54 in a predetermined pattern adapted to engage and operate normally closed contacts which control a circuit from lines 49 and 8! to line 62 which leads back to the guard house H. A suitable signal device, not shown, is

provided at the house il actuated by the opening of the contacts 60. The signaling projections 54a may be made different at the points [2, l3, l4, etc., so that a different signal will be given at the house I l depending upon which section of the fence has been approached by an intruder. This system using the motor 50 and the signal wheel 54 is advisable on large installations because a single wire 82 running around the entire guarded area may be utilized to send a coded signal from any one ,of the units I! to Hi. Another system involving an electronic timer and two signal wires running from each unit l2, I3, etc., back to the house II is described in connection with Fig. 2 but it will be understood that either of these devices might be energized by the tube 43 to produce a signal in the system here described in connection with Fig. 1.

In some cases a fourth cam wheel 55 is necessary having a cam controlling contacts 56' These contacts are normally held open and are in series between a source of negative voltage and the conr trol grid of tube 43. After the current to the plate of tube 48. is broken at contacts 58, the cam on wheel 48' permits contacts 56' to close thus applying a negative bias to the control grid to regain control of tube 43. This prevents repeated triggering of tube 43 by inductive surges of motor I.

The double rectifier 36 and the cor-rector 31, 88 are provided to prevent slow changes in the double antenna system producing operation of the tube 43 and the sending of a false signah Such changes take place due to unequal efiect of rain or snow on the two wires l1 and I8 or phase shift from changes in ground resistance, the growth of weeds. etc. These changes due to natural causes occur very slowly and over a period of several minutes or hours as distinguished from the changes utilized to cause asignal such as the approach of a person to one of the wires H or I 8. Any slow changes in the wires i! or [8 of the type just described will cause a flow at the output end of the electrical bridge and these changes will cause a slight flow of positive and negative currents in the resistances 31 and 38. However, since plenty of time is available due to the slow anges, those voltages cancel each other at 39 and no signal is given. In distinction to this, a sudden surge, such as that caused by the approach of a person to the wire, meets a time lag in the negative leg 38 of the corr'ector due to the condenser 40 and resistor 38. This prevents the negative voltage reaching the junction point 39 in time to cancel the positive voltage and so this positive voltage continues on to the trigger tube 43, overcomes the negative bias of the tube, and causes a signal.

I havehere shown tube 43 biased on the negative side of its conducting point and rendered conducting by a positive current passing through resistor 31; Those skilled in the art will understand that tube 43 might be biased on the positive side of its conducting point. Then resistor 31 would be connected through a condenser to ground and on a sudden surge a negative current would reach tube 43 suflicient to change its conducting condition. A proper relay and contact arrangement'in the plate circuit of tube 43 would 4 then send a signal.

In Fig. 5 I have shown a manner of protecting against cutting of a fence or tunneling under it. A very fine wire I80 such as thirty gage or smaller, insulated with glass so as to be almost invisible is woven along the fence and then run below ground as shown at 160a and then returned to the unit housed at I2, l3, etc. A relay iGI is in series with wire I80. This relay has self-closing contacts I82 held open so long as relay [5| i energized from lines 5| and 52 through wire I80. If the wire ISO is cut, contacts I82 will close and send a signal the same as if contacts 48 were closed.

Another manner of utilizin my invention is illustrated in Fig. 2 where a double antenna system is set up as before consisting of the guard wires l1 and i8 erected on insulators along a fence or the like so that said wires have substantially equal capacity to ground. The unit shown diagrammatically in Fig. 2 would be housed in one of the casings i2, l3, l4, etc., mounted upon the fence and supplied with power from a source L1. 14, such as illustrated in Fig, 1, comprising wires strung along the fence. This part of the diagram has been omitted from Fig. 2. the power supply being designated B+ which will be understood to 6 ting voltage regulator tubes 24 and 2! which are not necessary here.

The principle utilized in the system shown in Fig. 2 includes the impressing of oscillating currents of different frequencies on the wires l1 and i8 by means of two oscillators. The output of both oscillators is fed into a converter tube whose output thus has a beat frequency of the difference in the frequencies of the two oscillators. As shown in Fig. 2, the oscillator 10 impresses a frequency of say 250 kilocycls on the guard wire II. The oscillator ll impresses a frequency of 200 kilocycles on the wire H. The principle underlying the selection of these frequencies is that they must be sumciently separated so that they will not couple through the antenna system so as to cause the oscillators to lock together and maintain the same frequency.

The output of oscillator 10 is fed through line 12 to the grid 18 of converter tube 14. The output of oscillator ll is fed through line 15 to the grid 16 of the converter tube. The output of this converter tube is fed through line 11 and a tuned transformer 18' through line I8 to the grid of a second converter tube 8!. A third oscillator 82 is provided having a frequency equal to 'the nor'maldifference between oscillators HI and H, or in the present instance 50 kilocycles. The output of this oscillator is fed through line 88 to the grid 84 of the tube 8|. Obviously if the frequency output of tube 14 is exactly 50 kilocycles and if the output of oscillator 82 is held exactly at 50 kilocycles, then the normal output from tube 8i will be zero. The variable condensers 85 for wire I1 and 86 for wire l8 may be utilized to obtain this exactly balanced condition.

If a person approaches near one of the wires i! or [8 or if a portion of the guard wire should be cut off for the purpose of making an entry, the capacitance of wire H or I8 is ailected and the frequency changes. This causes the output from tube '14 to vary from thepredetermined frequency of say 50 kilocycles and since the output of oscillator 82 is held constant at 50 kilocycles. the output of tube 8| is no longer zero and a voltage appears in the output line 81. This is fed through a stage of audio-frequency amplification at 88 to a thyratron tube 89. This tube is like that already discussed in connection with In the plate circuit of tube 89 is a relay 92 which is adapted to close a normally open set of contacts at 93 which connect with an alarm signal circuit GI, 82 which is like the similarly numbered circuit in Fig. 1.

As explained in connection with tube 43, the tube 88 might have a grid bias on the positive or conducting side of its cut-oil point so that it would be rendered nonconducting by the negative swing 'of an oscillating current sent along line 8|. The relay 8! and its associated contacts would have to be redesigned to take care of this change.

Means is provided to hold the alarm for a sumcient length of time to be noticed by a man in the house II in case the inciting cause, such as a person approaching wire il or [8, is of very short duration. The means here might be similar to that shown in connection with motor 58 in Fig. 1. Instead, however, I have here illustrated an elecbe the power at the point B+ of Fig, 1, omit- 7 tronic timing device 84. A second set of nor.

mally closed contacts 95 maintains a voltage of about 100 volts negative on the control grid of tube 99. This tube is thus nonconducting because of the highly negative grid. the tube 89 tube 99 leaks off through resistance 98. The value of condenser 96 and resistance 98 determines the I time required. When this negative charge has motor 50 for the purpose of holding the signal for an appreciable length-of time. g

Means is provided in connection with the device of Fig. 2 to correct for changes due to weather and other natural conditions which change slowly over a period of time, so as to avoid operation of the alarm signal because of weather changes only. The output from tube I4 is fed through lines I1 and I02 through transformer I03 through a stage of radio frequency amplification at I04 to the discriminator I05. This discriminator is tuned to the normal frequency encountered at the output from tube I4 or in the present instance 50 kilocycles. If the guard wires II and I8 are subjected to rain or snow so as to change their capacity to ground slowly and at different rates the output from tube I4 will gradually change from 50 kilocycles and this will result in a change in the output through line I06 from discriminator I05. This will be either positive or negative depending upon an increase or decrease in the frequency output from the converter tube I4. This output through line I06 is fed to a reactance modulator I01 of known type. It is well known that a change in the input to grid I08 will result in an apparent change in inductance of the circuit associated with tube IIO. Line I09, leading from the plate circuit of the tube I I0, is fed back to one of the oscillators I0 or II. In the present instance it is fed throughline III, condensers H2 and H3 to grid II4 of the oscillator I0. This produces a change in frequency of the oscillator I0 in a direction to restore the normal balance to converter tube I4. Therefore the output of this tube through lines TI and I02 will return to 50 kilocycles so that no further change takes place in the frequency of oscillators 10 or II until some further change calls for it. A time delay circuit between the discriminator and reactance modulator slows the correction of any rapid change in guard wire capacity such as would be caused by a person approaching the guard wire. This consists of the resistance I I5 and condenser II6. As a rapid change in guard wire capacity caused by a person approaching the guard wire II or I8 results in large changes in frequency, the time lag is sufficient to allow a voltage to reach the thyratron 89 so as to result in an alarm signal.

Connected with line 9| is an inductance choke III with a series resistance II8 to ground. This passes all low frequencies to ground and prevents them from reaching th thyratron 89'. Its value may be adjusted by the resistance II8 so as to control those frequencies which reach the thyratron.

An advantage of the system shown in Fig. 2 over that shown in Fig. 1 is that changes in voltage of the oscillators I0 and II do not affect 8 the system which depends upon frequencies only. A third system'is' shown in the diagram'of Fig. 3. Here=oscillations of the same frequency are impressed upon the guard wires I1 and I9 which form a double antenna system where each wire has substantially the same capacity to ground as before. The currents in the'two-wires I1 and I8 are rectified and balanced against each other by means of a discriminator connection; The capacity of a foreign body approaching one of the wires I! or I8 unbalances the system'causing a current to flow which gives a signal.

As shown in'Fig. 3, an oscillator I30 supplies current through a buffer or isolation amplifier I3I 'to transformer I32 and through the secondary of this transformer the same frequency is impressed upon wires I1 and I8. It will be understood that the current supply for the oscillator indicated at B+ is connected atthe point B+ of Fig. 1. In other words the unit I2, I3 or I4 on the fence receives power through lines L1 and La which is rectified for use in the oscillator. The mid point of the secondary I32a of the transformer is grounded as shown. Each end of the secondary is then connected to a stage of radio frequency amplification as indicated at I33 and I34. The output of these two stages of amplification is fed to the discriminator I35. The output of this discriminator through line I36 is normally zero and this balanced condition is attained by adjustment of the variable condensers I31 and I30 connected with the wires I1 and I8 respectively,

The output through line I36 (rectified in the discriminator I35) is fed to two thyratrons I39 and I40 which have their grids biased so that I39 is normally conducting and I40 is normally nonconducting. In the plate circuits of the thyratrons are the relays I39a and NM. The contacts at I4I are self-closing and the contacts at I42 are self-opening. Since the tube I39 is normally biased to conduct when the system is in balance, relay I39a is energized and its contacts are held open. Since tube I40 is biased to nonconducting when the system is in balance, relay MM is normally deenergized and its contacts remain open. The relays I39a and MM each operate two sets of contacts. in the same direction, i. e., in each set the two pairs open or close together. Across one set of contacts of each relay is connected a reversing'motor I43 of a type such as is used to tune radio receivers. If either thyratron changes .from its normal condition, i. e., if 39 changes to nonconducting or if I40 changes to conducting, then one of the circuits of motor I43 is closed and the motor will operate in one direction orfrom its normal condition, i. e., if I39 changes to nonconducting, relay I39a is deenergized and the contacts at I4I being self-closing will complete a circuit from L1 through line I44 and winding I43a of the motor to L2. If tube I40 becomes conducting, relay I40a will become energized so as to close contacts I42 thus completing a circuit from line L1 through line I 45 and the motor winding I43b back to line L2. A variable condenser I45 has movable plates connected with the shaft of motor I43. This condenser is connected to one of the wires I! or I8 so that as the motor operates in one direction or the other the capacity of wire I8 (in the present instance) is either increased or decreased. This restores the system to balance at which time the output in line I36 becomes Zero and tubes I39 and I40 return to their original condition and stop the motor.

Across the second set of contacts shown at I4 Ia and I42a is connected a time delay relay I41 which operates to close a set of normally open contacts I48 so as to operate the alarm circuit I, 62 similar to circuits already described. This relay I4! is of the delayed attract type. An electronic timer delay relay of known type could be used here. In other words it does not close for some predetermined time after current is supplied to it. Since changes due to weather affecting wires I1 and I8 differently occur very slowly, these changes will cause motor I43 to operate for such short periods of time that contacts I48 will not be closed and the alarm will not be sounded. However, if a person approaches one of the guard wires, a larger change of voltage would result which would take the motor I43 much longer to correct, whereupon the relay I41 would act to close the contacts I48 and give the alarm. The two condensers I50 across the output of the discriminator I35 will hold a charge for an ap-, preclable length of time so that even if a person jumped quickly over one of the guard wires the resulting surge of voltage would charge these condensers and keep the motor I43 operating long enough for relay I41 to close.

Explaining more in detail, an example of the operation of the thyratrons I39 and I40 may now be given. -These thyratons might be of a well known type Whose grid bias cut-off voltage is 1.4 volts negative. Therefore if I39 is biased to 1.3 volts negative and if I40 is biased to 1.5 volts negative, then if a change in line I36 occurs which is 1.0 volt negative, then the condition of I39 would be' changed to 2.3 volts negative, thus changing from conducting to nonconducting. At the same time I40 changes from 1.5 volts negative to 2.5 volts negative so that it remains nonconducting. The negative bias of grids I39 and I40 is regulated by the variable resistances I52. If the oscillator I30 is set to apply a small constant voltage of say 0.1 volt to wires I1 and I8 and if the amplifiers I33 and I34 have a gain of 1000, then a change in the wire capacity which produces a change as small as .001 volt will result in a change of 1.0 volt in the output of the amplifiers and will result in a 1.0 volt change either positive or negative in line I36. The system is thus very sensitive because a Very slight change in the capacitance of either wire I! or I8 will result in changing the condition of tubes I39 and I40 and a change in either direction will cause an operation of the signal.

While I have here shown thyratron tubes for controlling the sendin of a signal, this is for purposes of illustration only and it will be understood that other electronic tubes might be used. Many electrostatically or electromagnetically .controlled tubes could be used for this purpose if the circuits were properly designed by one skilled in this art. It is only necessary to provide a tube having a controlling element to receive current from one of my improved protective systems. so arranged and adjusted that when the current is built up by a person approaching the guard wire, the tube will change its conduct n condition and send a signal.

It should be noted that in every case I have shown a double antenna system. It would be possible but impractical to use a single antenna. The reason for this is that with two antennas balanced against each other in the systems as shown, most of the changes due to natural causes are substantially equal in the two antennas and therefore cancel each other because of the way the systems are built. This leaves only a small amount of correction for the automatic means shown in the different circuits. If a single antenna were used and corrections made in the system itself to take care of changes due to natural causes, the corrections would be of such magnitude that the correction devices would destroy the useful characteristics of such a protective system.

What I claim is:

1. In a signal system, a double antenna system comprising two separate conductors .of substantially equal electrical capacity to ground, two oscillators respectively impressing on said conductors oscillating currents of different predetermined frequencies, the said frequency difference being suflicient to preventoscillator interlocking due to coupling between the antenna conductors of said system, a converter tube having grids connected respectively with the output circuits of said oscillators whereby the output circuit of said converter tube has a beat frequency equal to said frequency difference, a second converter tube, means feeding the output of said first converter tube to a grid of said second tube, a third oscillator having a frequency equal to said frequency difference, means feeding the output of said third oscillator to a. grid of said second converter tube so that the output of said second converter tube is normally substantially zero, a signal device, a circuit connecting said device to receive energy from the output of said second,

converter tube to cause actuation of said signal device when a humanbody nears one of said antenna conductors and causes a change in frequency of the current impressed thereon.

2. The combination of claim 1 including a circuit having in series an amplifier, a discriminator and a reactance modulator, means feeding the output of said first named converter tube to said amplifier, and means feeding the output of said modulator toone of said two first named oscillators, whereby to automatically maintain said frequency difference during'slow changes of capacitance insaid double antenna system.

'LEE H. PECK.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Draper et al. Sept. 28, 1937

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