US2290314A - Feedback system for traveling wave antennas - Google Patents

Description

July 21, 1942. P. s. CARTER FEEDBACK SYSTEM FOR TRAVELING WAVE ANTENNAS Filed Sept. 24, 1940' 5 Sheets-Sheet 1 (Ittorneg July 21, 1942. P. s. CARTER 2,290,314

` FEEDBACK SYSTEM FOR TRAVELING WAVE ANTENNAS I Filed Sept. 24, 1940 5 Sheets-Sheet 2 \Fl/EE TEHVEL//YG Wil/E Gttorneg July 21, 1942. P. s. CARTER 2,290,314

FEEDBACK SYSTEM FOR TRAVELING WAVE ANTENNAS FiledA Sept. 24, 1940 5 Sheets-Sheet 5 Zmventor Pluim .Si 6mm Gttorneg July 21, 1942. P. s. CARTER y 2,290,314

FEEDBACK SYSTEM FOR TRAVELING WAVE ANTENNAS Filed spt. 24, 1940 5 sheets-sheet 4 Gttorneg July 21, 1942. P. S. CARTER FEEDBACK SYSTEM FOR TRAVELING WAV ANTENNAS Filed Sept. 24, 1940 5 Sheets-Sheet 5 Snventor f/zlv S. Carle:-

Cttorneg Patented July 21, 1942 FEEDBACK SYSTEM FOR TRAVELING WAVE ANTENNAS Philip S. Carter, Rocky Point, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application September 24, 1940, Serial No. 358,059

(Cl. Z50-33) 14 Claims.

The present invention relates to traveling Wave antennas and, more particularly, to a feedback system therefor whereby termination losses are avoided.

Many end fire antennas have the serious disadvantage from the transmitting viewpoint in that a Vlarge portion of the radio frequency energy is dissipated in the resistor located at the far end of the antenna system.

An object of the present invention is to save the radio frequency energy heretofore wasted by returning it to the input of the antenna in such a manner that the characteristics of the antenna remain unchanged.

A further object of the present invention is to improve the efficiency of traveling wave antennas.

Another object of the present invention is to eliminate the terminating resistor at the far end of traveling wave antennas.

The present invention contemplates the coupling, through an impedance matching network when necessary, of a transmission line to the far end of a traveling wave antenna instead of a terminating resistor. The transmission line is then so connected to the input end of the antenna that energy arriving at the far end is reapplied to the input end and again used to energize the antenna.

Further objects and features of the present invention will be understood from the following detailed description, which is accompanied by drawings in which Figure 1 illustrates one form of the invention; Figure 2 is a curve showing the effect of varying the dimensions of the antenna of Figure 1; Figure 3 illustrates a modication of the embodiment of Figure 1, while Figures 4 and 5 are block diagrams of antennas for a mathematical anlysis of the problems involved; Figure 6 illustrates a form of phase shifting and voltage transferring network applied to a rhombic antenna and includes a showing of the current and voltage relationships encountered, while Figures 7a and '7b show the current and voltage relationships vectorially; Figures S and 9 illustrate modified forms of transforming devices and their use in the present invention; Figures 10, 11 and 12 illustrate the application of the present invention to other forms of traveling wave antennas, while Figure 13 shows an equivalent circuit diagram of the `antenna of Figure 12; Figure 14 illustrates the application of the present invention to a Wave type or Beverage antenna, while Figure 15 shows still another form of the present invention.

A schematic diagram of one form of the antenna system according to the present invention is shown in Figure 1. The antenna comprises a pair of conductors IIJ, 20 disposed in a generally rhombic configuration. At one end the antenna is energized from a transmitter (not shown) through a transmission line TL. At the other end is connected an ancillary transmission line I4 Which is connected back into line TL. Across line I4 at a distance equal to a quarter of the operating wavelength from its connection to transmission line TL is connected a transformation loop I5. Line I4 and line section I5 must have such lengths that the energy is applied to' line TL in phase with the energy therein.

The physical principles involved may be more clearly understood by resorting to mathematics. If both ends of an antenna of this type are connected by means of a branched transmission line equal power will ow into each end of the antenna, standing Waves will exist in the antenna system and bi-directional radiation will be obtained. This, of course, is not the desired result. Let us suppose the antenna to be loaded with a resistance which matches the effective surge impedance of the antenna system and that the antenna under this condition radiates half the'power fed to it. 50% of the power then fiows into the resistance, the voltage and current in the resistance each being (1h/) times the input voltage and current respectively. If, in place of this resistance, feedback line I4 is substituted and the feedback line is coupled to the input line 'I'L through an ideal transformer I5, the ideal transformer having such ratio that it steps up in the ratio of V5 to 1 in proceeding from feedback line I4 to input line TL, and the feedback line I4 is of such a length that the phase of the voltage at the junction with the input line is the same as the phase of the voltage in the input line when the antenna is loaded with a. resistance, the energy previously lost in the resistance is returned to the antenna input. The antenna then operates exactly as before with a resistance load excepting that the power radiated is noW substantially doubled.

At the frequencies usually employed in antennas of the type described the characteristics of an ideal transformer cannot in practiceY be entirely attained. However, I have found that the proper transformation and phase shift of the returning energy may be obtained by a very simple combination of transmission line sections. Figure 6 shows the elements used. At first sight this circuit would appear to be an ordinary im- 'from the antenna apeX as pedance matching circuit of the line shunt type. This, however, is by no means true since the preferred line length for the circuit of the invention is a quarter wavelength. This length is an impossible length for an impedance matching circuit working into any finite resistance load. In this circuit the magnitude and phase of the output Voltage with respect to the input Voltage depend upon the current flowing out, an unknown quantity until the complete circuit including the antenna system is specified. Consequently, the system must be analyzed as a whole including the transformation circuit in order to determine the proper dimensions in ing the desired result. Y Y The curve of Figure 2 shows the relationship of the electrical length D (of Figure 1)v with respect to the power lost in a terminating resistance when that type of termination is used. D may be any integral multiple of 360 degrees minus thevfacter F shown by the curve of Figure 2.

The use of the curve may be clearly understood by outlining the procedure in a practical case. Assume the existence of a` rhombic antenna Vwhose total length is 5 wavelengths and which it is known, by measurement, dissipates 50% of the power fed to it in a resistance load at the far end, It is desired to use the system of Figure l to eliminate this loss in power. It may be further assumed that due to the height of the antenna and the practical way in which the lines must be arranged that the total distance must be at least 11 wavelengths. Then D is taken as equal to 12 wavelengths'minus a factor which must be determined. In other words, in degrees D=12 X 360-F.

F is obtained by reference to Figure 2 from which it is found that for 50% loss in power in a resistance F is equal to 45"; VAssuming the vertical distance in Figure 1 to have been chosen as a 'vquarter wavelength, the distance S must be determinec'l. F and S are the same, as shown in Figure 2. Having made these distances correct, the current distribution would then'be measured,

` in 'a'practical case, by placing'a sliding meter on the return line I4. Any standing wave component on this line may be eliminated by slight'readjustment of the lengths D and S by sliding Athe jumper connections and short-circuiting strip.

Usually a transmission line connected to the far end of a rhombic antenna regardless of its characteristic impedance will not perfectly match the antenna, i. e., some small amount of reected wave usually exists in the far section of the antenna. Thus, a small standing wave component exists in the antenna system regardless of whether there is employed a resistance unit at the end of the transmission line, this resistance unit perfectly matching the line; or a terminating transmission line of resistance Wire long enough to be of innite length inefect; or the feedback system of the invention. This standing wave due to reection at the far apex may be eliminated by the Vuse of Van impedance matching circuit consisting of a line stub l1 shown in Figure 3. When such a circuit is used the phase shift caused by the circuit must be compensated for by a small change in the length of the feedback transmission line. Tests show that about 2%l more energy may be made either to iiow in the resistance load or return to the input transmission line by the use lof the above-mentioned impedance matching syst-em.

at the proper distance Y When such a systemV is used,-

the circuit for obtain- Y surge impedance of although a pure traveling wave exists along most of the length of the far section of the rhombic antenna there is a small standing wave component along the wires of the near section caused by reflection at the side apices H, l2. There is also some reiiection set up on the input transmission line at the input apex 29. These imperfections in the antenna are not of importance in connection With this invention. The antenna when properly adjusted with the feedback system operates in identically the same manner as when loaded with a resistance, the imperfections being the same in either case.

The physical principles involved can be more Vclearly understood by resorting to the mathemat- `age of the antenna then the output voltage Eb is given by, #Elf- EA eXp(`-l`ir1) Where I`i= `x1`+fi;81, of beingthe attenuation constant and the phase constant. q (In actual antenna systems the attenuation is not a constant through the Vlength of the antenna but here fthe interest is only in the total attenuation and any assumption 'may be made which will not falter the final result.) The current IA at the input of the antenna `is while that at the load is -Z?=4 exp (-I1L,)

"i where Zn is the eiective impedance of the antenna system v(also not constant throughout the length).

If, in 'place of the load'irnpedanc'e Z, a generator is 'placed giving a Voltage equal to En in both 'magnitude and phase Vangle the currents in the Vantenna remain undisturbed. 'I'he generator then necessarily becomes a motor.

'Now assume an arrangement 'as in Figure 5 using an ideal transformer witha return line 25 having a propagation constant Taima-juz and a characteristic impedance equal to th'e effective characteristic-impedance of the antenna. If the the return line V24 is ychosen as equal to the eifective surge impedance of the antenna 22, the Voltage ratio of transformer Ta is Vchosen equal to expmui-l-am) and the length a of line 24 is such that 3L1+2=' 1c 360, n being an integentheantenna behaves exactly thesame as when load-ed with the riginalabsorbifng impedance, excepting that allthe power which was in the other connection dissipated is returned to the input end of the antenna'and the radiation is increased by the amount of energy saved.

It is of advantageto use asomewhatdifferent mathematical attack for theconditions here than that commonly taken. As 'is well known, the solutions of the fundamental differential equations fora transmission line'may be written- Determining the constantsA"`and` qin "terms 1.=;ZloiA exp o.) -B expr-nii 2,'2905314 Y3 'of the voltages at the two ends of the line, the The simplest way of determining the proper difollowing relations are obtainedmensions in the system is to assume the antenna El1 h r E h F to be Working as desired, i. e.,'with a traveling Ez= smHgB-L) Y (3) wave forward While the return line has on it a v 5 pure traveling wave toward the input. :l E,Jl cosh [I(L-x)]-E; cosh (Ix) I (4) The currents at the junctions may be assumed x Z0 sinh IL to have the directions shown. The conditions l E E cosh n assumed and desired are stated mathematically (5) by relations 18, 19 and 20 following:

0 sinh IL 10 E E 1 For the condition under consideration there are "2 "1 eXp F" P2M 8) two lines l and 2 connected together at the far E j end so that I1= Za (19) E =E 6 l 2 15 Ea! l Ibl Ibz I2="Z o eXp (-FILI -I`2L2) Combining the expressions for the two lines under At junction J2, these conditions, equations Y I4=I2 r3 (21) sinh (IILQIblZO Eat-E5 cosh Im (8) 20 but sinh (1mm zo E.1 El, @Osh rm (9) Ee 1 2 I@ j'z tan KLB (22) are obtained, which when solved givewhere Eal cosh Pm Ea2 cosh Pm I 25 b1 Z0 SiIlh (Flu +F2L2) Now if the ideal transformer ratio P is chosen E uch that a S E I4=I2I3=Zlexp (I1L1'I12L2)+ a o P= 2=Xp (-05 l. a Lg) E1 l 1 2 eXp (Pili tl-MN (24) there is obtained after Writing the exponential Z" tan K3 expressions for the hyperbolic functions: The voltage at J1 is determined from fundamen- I :Eux eXp (am -i-jzz) -I-eXP (-azz -jzb -eXP(-am -I-jm) -Xp(-2a1L1vl2L24-j1u) 1 l Z0 eXp (aiLi-I-azLz-i-jm-t-jzz) -GXP (-0411 -a2L2-.7'l31L1-2L2) l) E eXP (-04114 .7131L1) [eXp (am 'ljli X2L2 'ijz -i-eXp (am -I-j1t1 am -j22):| I l eXp (a1L1a2L2)-Xp OllLi-Ofzl-z) (12) b1 Zo eXP (am i-Jm +012@ 'tj/3M) EXP ('-aiu aziz '-jili 'jzbz) Now, if, in addition u1+nz=2m wherein n is tal transmission laws by the current I4 and voltan integer, then age Ea2 at J2 and this must of course equal the E E input voltage Eel. Therefore:

a al I1,1=Z l exp (-1L1J1L1)=o eXp (-Im) (13) 50 Ea1=Ea2 COS [Q4 JZL sin KL4 (25) which is the condition for pure traveling wave By substitution: on the antenna `(line l To prove this: Take E E ex F P K Eb from (8) i. e.: a! al P[ M+ 2L2)] COS L4 JEulX I I E=Ea1 cbszh" ISllh F1 (14) 55 {BXP[ (I1L1 FFW-2)] +JeXp' Sm KL4} (26) 1 l For simplicity it may be assumed that the atf I Sstltutmg m (4) the expresslon or b 1 e tenuation on the return line is negligible. Then 1 211 :Ea e I I2='7'2=jKL2 (27) EL :Ea exp 2 F L Xp U1) l I 1 1 since the veloclty 1s equal to light. For practical and purposes the velocity on the antenna wires is also E'a1 cosh [I1(:.-:1;)]exp (-Im) cosh FIX- If- D sinh Fm EXP FKCIHSXP (FILI) IeXP I`1L1 -l-IiXl) *eXp (-Im l-IlXl) eXp I`1L1)] Zo eXp P1, -exp (-Ilq) (16) l equal to the velocity of light. Making this as- Z0 eXP FIX 1) (17) sumption also, l or a pure traveling wave. 70 I'1=a1+y`K (23) For purposes of illustration, a rhombic antenna as shown in Figure 6 may be taken with a return sin Kn line having the same characteristic impedance as COS K 7 Sm Ktll'tan KL3=XP ('111) X (29) the eiective characteristicimpedance of the any Y y 'A tenna and a transformation circuit as illustrated. 75 eXp UK L1+L2 1 cos KLA-ig; j sin (KL.) eXp (am) S (KLl-i-Kz) +2. eXP (Ulu- 1) Sill [KM-H2M Equating real and imaginary parts:

` Lengths 2, Le and 14 are available for adjustment. Within limits L4 may be any length. A length of M4 will always be satisfactory and this length greatly simplifies the relations. Therefore, assuming L4: \/4 Then sin K(L1|-L2):- exp (-anr) (33) and cot (KL3:eXp (11111) cos [Km-H2M (34) from which cot Ka:- cot [Km-H2M (35) Usually it is preferred to make L3 as short as possible. Then 'l1/-(L1|L2)=L31 (36) 1L being an integer while L, +L2= 1% Sin-1 (exp [-amn +m 37) For instance, if the attenuation along the antenna is 40%, exp (-am) :60%. Suppose (ui-l-Lz) is a length in the vicinity of Then (t+1.) =1oi-l Sin-1 (0.6) (1o-0.1025

and the length La of the shunt:0'.1025

Let us further assume the voltage E21 at the junction J1 to be 600 volts and the effective surge impedance of the antenna 11, return line L2, and the two other line sections to be 600 ohms. The vcurrent Io entering the antenna is then `The voltage Eel at junction J1 at the other end of the quarter wave line section must be given by- L'nl: -jLXZO sin 90 -j j1-00 X600 600@ volts which checks the value assumed at the start of the journey around the circuit.

The current I5 being returned to J1 must be given by sin 36o/36.9 =0.60/-53.1 amps.

= 0.64 +3048 (LSD/36.9 amps.

Watts, main 1ine:600 0.80 cos 36.9:384 Watts in antenna=600 1.00=600 Watts returned from antenna:

600x060 cos 53.1:216

The vector diagrams in Figures 'la and '7b show the voltage and current relations at junctions J1 and J2 under these assumptions and are believed self-explanatory.

A system built in accordance with Figure 1 of the invention was tested in the eld and the following results were obtained:

1. Pure traveling wave on the feedback transmission line.

2. Power from transmitter: units.

3. Power into antenna:16'7 units.

4. Power returning to antenna input on feedback line:6'7 units.

The above results were obtained by the usual method of power measurement with a sliding meter on .the transmission lines. The dimensions of the transformation circuit and return transmission line were in accordance with the curves shown in Figune 2 in order for the system to operate in accordance with the invention. Tests made with the same rhombic antenna loaded with a resistance of such a value that the output transmission line from the far end of the antenna is matched indicate that approximately 40% of the power is dissipated in the resistance. These tests proved that the gain of this particular antenna increases to 167% of the value obtained when the antenna is loaded with a resistance by using the system in accordance with this invention.

The invention has been discussed in connection with a rhombic antenna for purposes of illustration only. It may be applied in the same manner to a large number of different types of antennas of the traveling wave type. The particular transformation circuit described is believed to be the best practical circuit under most condtions. Of course, a coil or condenser may be used in place of the stub transmission line for shunting the feedback transmission line in order to obtain the proper transformation of voltage.

In Figure 8 is shown a system wherein a coil 30 is shunted across the feed line TL and the feedback line I4 is tapped at points on this coil such as to result in the propel' voltage ratio. A condenser V3i is also shown paralleling the coil in order to compensate for the leakage reactance.

In Figure 9 is shown a system where the transformation is done bythe quarter wave section of transmission line 40 whose surge impedance is equal to the surge impedance of the feedback transmission line multiplied by eXp am where 1an is equal to the attenuation taking place on the antenna. The other dimensions must be as indicated. I nd that in connection with the average rhombic antenna where #4 wire is used in the antenna and return transmission line that the wire size required in the quarter wave transformer circuit would be equivalent to #16. This circuit may, therefore, be objectionable in some cases from the practical standpoint.

Figure l shows a feedback system as applied to the so-called British series phase antenna and is self-explanatory since the operation of the antenna is well known and my improvement acts in the same way as described above with reference to a rhombic antenna.

Figure 11 shows the system as applied to a socalled fish-bone antenna. The traveling wave along line energizes radiators 52 in its travel. The operation of the feedback loop is still the same as described above.

In Figure 12 is shown an antenna wherein radiators 53, 54 are connected directly to wires of the transmission line 5I, spaced not greater than 1/4 wavelength apart and having a length considerably less than 1A, wavelength in order not to slow down the phase velocity in the system to an excessive extent. The end radiators 54 in this system are shown as being half the length of the other radiators. In this way, the effect of the end radiators is equivalent to a capacity reactance approximately half that of the remaining radiators. The reason for this becomes apparent from an inspection of the equivalent network shown in Figure 13. The eiTect of the antenna units is to add the equivalent of a capacity C and resistance R to each arm of the equivalent K network constituted by the length of transmission line connecting the radiators. By arranging the capacity reactance presented by the first and last radiators to be equal to half that representing the remaining radiators, there is no abrupt change of impedance along the network to cause reflections. The feedback transmission line may then be matched by any of the heretofore discussed means to the equivalent network characteristic impedance of the antenna. This is indicated by network 55. In this manner a traveling wave antenna system is obtained having a smooth network equivalent.

In Figure 14 is shown a feedback system as applied to a wave antenna 60 fed at one end through transformer 6I from a transmitter (not shown) and terminated at the other end by a matching transformer 62. A feedback line 64 connects from the output of transformer 62 to an input matching device, as described with reference to Figure 8. The feedback line 64 is here shown as a balanced transmission line since such a line is necessary in order to avoid excessive losses in the ground.

In Figure l5 is shown a rhombic antenna Il), 20 together with both a feedback transmission line system I4 and a transmission line 65 of resistance wire to act as an absorbing load. Switches 6|, 61 are included so that the antenna may be used with either the feedback system I4 or the power absorbing transmission line i6. In practice an antenna is usually used at one frequency during a larger percentage of the time. During the remaining time it may often be desirable to use an antenna with almost any frequency. Under such conditions the system shown in Figure 15 should be very useful since a high gain in power is obtained at the frequency most used while not losing the advantage of flexibility when it is terminated in a resistance load.

While I have particularly shown and described several modifications of my invention, it is to be particularly understood that my invention is not limited thereto but that modifications may be made within the scope of my invention.

I claim: n

1. A rhombic antenna comprising a pair of conductors having their ends adjacent and the midpoints separated, a transmission line, means for coupling said line to said antenna at one pair of adjacent ends whereby traveling waves are set up in said antennas, a second transmission line having a pair of conductors connected at one en d to the other pair of adjacent ends, the electrical length of said antenna and second transmission line being an integral multiple of 360 degrees, said second transmission line being short-circuited at its other end, and means for coupling said second line to said iirst mentioned means at a predetermined distance from said short-circuited end.

2. A rhombic antenna comprising a pair of conductors having their ends adjacent and the midpoints separated, a transmission line, means for coupling said line to said antenna at one pair of adjacent ends whereby traveling waves are set up in said antennae, a second transmission line having a pair of conductors connected at one end to the other pair of adjacent ends, the electrical length of said antenna and said second transmission line being an integral multiple of 360 degrees, said second transmission line being short-circuited at its other -end and means for coupling said second yline to said first mentioned means at a predetermined distance from said short-circuited end, said last mentioned means having an equivalent electrical length of degrees.

3. A rhombic antenna comprising a pair of conductors having their ends adjacent and the midpoints separated, means for energizing said antenna connected te one pair of adjacent ends, a rst transmission line having one end ter'- minated by a shunt resistance, a second transmission line, means for selectively coupling said lines to the other pair of adjacent endsV of said antenna, means for coupling said second line to said one pair of adjacent ends of said antenna and means for adjusting the phase and amplitude of the currents from said second line appearing at said last mentioned means.

4. A traveling wave antenna comprising a pair of parallel conductors having radiating means coupled thereto along their length, means for energizing said antenna connected to one pair of adjacent ends, a transmission line connected to the other pair of adjacent ends for feeding energy from said second pair of ends back into said rst pair of ends, and means for adjusting the phase and amplitude of currents in said transmission line, the capacity reactance of said radiating members at each end of said parallel conductors being half that of the remainder.

5. In combination, a traveling wave antenna, means for energizing said antenna connected to one end thereof and means connected to the other end of said antenna for preventing reflection of energy from said end, said means comprising a two conductor transmissi-on line connected at one end to said other end and having a length equal to an integral number of wavelengths, said line being short-circuited at its other end and means for connecting the one end of said antenna to said transmission line at a predetermined distance from said short-circuited end.

6. In combination, a traveling wave antenna, means for energizing said antenna connected to one end thereof and means connected to the other end of said antenna for preventing reection of energy from said end, said means comprising a two conductor transmission line connected at one end to said other` end having a length lequal to an integral number of wavelengths, said line being short-,circuited at its other end and means for connecting the one end of said antenna to said transmission line at a distance from said short-circuited end proportional to the amount of power arriving at the other end of said antenna.

7. In combination, a traveling wave antenna, means for energizing said antenna connected to one end thereof and means connected to the other end of said antenna for preventing reflection of energy from said end, said means comprising a two conductor transmission line connected at one end to said other end and having a length equal to an integral number of wavelengths, said line being short-circuited at its other end and means for connecting the one end of said antenna to said transmission line at a predetermined distance from said short-circuited end, said last mentioned means comprising a line having a length equal to an odd multiple, including unity, of a quarter wavelength.

8. In combination, a traveling wave antenna, means for energizing said antenna connected to one end thereof and means connected to the other end of said antenna for preventing reection of energy from said end, said means comprising a two conductor transmission line connected at one end to said other end having a length equal to an integral number of wavelengths, said line being short-circuited at its other end and means for connecting the one end of said antenna to said transmission line at a distance from said short-circuited end proportional to the amount of power arriving at the other end of said antenna, said last mentioned means comprising a line having a length equal to an odd multiple, including unity, of a quarter wavelength.

9. In combination, a traveling wave antenna, means for energizing said antenna connected to one end thereof and means connected to the other end of said antenna for preventing reflection of energy from said end, said means comprising a two conductor transmission line connected at one end to said other end and having a length equal to an integral number of wavelengths, said line being short-circuited at its other end and means for connecting the one end of said antenna to said transmission line at a predetermined distance from said short-circuit-ed end, and means for matching the impedance of said transmission line to the impedance of said antenna at its other end.

10. In combination, a traveling wave antenna, means for energizing said antenna connected to one 'end thereof and means connected to the other end of said antenna for preventing reection of energy from said end, said means comprising a two conductor transmission line connected at one end to said other end having a length equal to an integral number of wavelengths. said line being short-circuited at its other end and means for connecting the one end of said antenna to said transmission line at a distance from said short-circuited end proportional to the amount of power arriving at the other end of said antenna, and means for matching the impedance of said transmission line to the impedance of said antenna at its other end.

1l. In combination, a traveling wave antenna, means for energizing said antenna connected to one end thereof and means connected to the other end of said antenna for preventing reflection of energy from said end, said means comprising a two conductor transmission line connected to said other end and coupled to said rst mentioned means whereby energy arriving at said other end is reapplied to said antenna at said one end, said antenna and transmission line having an overall electrical length equal to an integral number of wavelengths, and a shunt connection across said transmission line at a distance from its coupling to said rst mentioned means equal to a quarter wavelength.

12. A traveling wave antenna having a length at least as long as the length of the operating wave, said antenna having an axis arranged along the line of a desired direction of communication, a first transmission line, means for coupling said transmission line to said antenna at its remote end with reference to said direction line, a second two conductor transmission line coupled at one end to said antenna at its other end and means for coupling said second transmission line to said rst mentioned means at a predetermined distance from the other end of said second transmission line, said second transmission line being short-circuited at said other end.

13. A traveling wave antenna having a length at least as long as the length of the operating wave, said antenna having an axis arranged along the line of a desired direction of communication, a first transmission line, means for coupling said transmission line to said antenna at its remote end with reference to said direction line, a second two conductor transmission line coupled to said antenna at its other end and means for coupling said second transmission line to said rst mentioned means, said last mentioned means comprising a shunt loop connected across said second transmission line of such length as to increase the voltage on said line by a predetermined factor.

14. A traveling wave antenna having a length at least as long as the length of the operating wave, said antenna having an axis arranged along the line of a desired direction of communication, a lrst transmission line, means for coupling said transmission line to said antenna at its remote end with reference to said direction line, a second two conductor transmission line coupled to said antenna at its other end, means for coupling said second transmission line to said rst mentioned means, said last mentioned means including a shunt connected across said second transmission line at a distance and so proportioned as to increase the voltage on said line by a predetermined factor, and means for adjusting the phase of the currents in said second transmission line.

' PHILIP S. CARTER.

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