US2238438A - Electrical network - Google Patents

Description

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F IG. 6 v @hun April 15, 1941. A. ALFoRD 2,238,438 I ELECTRICAL VNETWORKv Filed March 22, 1935 '7 Sheets-Sheet 2 April l5, 1941.

ELECTRICAL NETWORK Filed March 22, 1935 7 Sheets-Sheet 5 l I I j ANTENNA EN I f I ZZ/f N FIG.

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ELECTRICAL NETWORK Filed March 22, 1935 7.Sheets-Sheet 4 7/ 6.? Afa 7 64 FIG. I3

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April l5, 1941. A. ALFORD ELECTRICAL NETWORK 7 Sheets-Sheet 5 ANTENNA Filed March 22, 1955 mYM 2 A A 2 1 z .E E

INVENTUM FIG. I7

April l5, 1941. A ALFQRD l 2,238,438

ELECTRICAL NETWORK Fi`led March 22, 1955 7 Sheets-Sheet 6 N Q La L s 3 Q N Im Q :L L3

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April 15, 1941. A. ALFoRD ELECTRICAL NETWORK Filed March 22, 1935 7 Sheets-Sheet T S +G .n

Nui ER Qn E- m :u n E QEK NH fh QM uw NN mw E INYENTOR: y

ATTWMEY:

Patented Apr. 15, 1941 ELECTRICAL NETWORK Andrew Alford, New York, N'.` Y., ,assigner to MacKay Radio and Telegraph Company,` New York, N. Y., a corporation of Delaware Application March 122, 1935, Serial N0. 12,451

4 Claims.

This invention relates to new and useful improvements in electrical networks and particularly in networks adapted for use with radio antenna circuits.

The main object of the present invention is to provide an impedance matching device for a transmission line interconnecting a high frequency radio transmitter and an antenna operable on more than one-frequency.

In networks of this nature variable condensers and induction coils, or switching devices for selecting a suitable inductance or capacitance are usually employed. By means of such variable or switching devices the electrical constants of the network may be varied to suit the current frequency employed.

Such Variable devices are expensive and are usually located at points remote from the operating personnel which makes their servicing and adjustment inconvenient. Remote control arrangements materially increase the cost of installation and operation.

In accordance with the present invention the use in such networks of variable or switching devices is completely obviated. Fixed impedance l particular frequency and presenting a substan- 1A tially infinite impedance at certain other frequencies.

` tors should, therefore, be determinedr in .accordance with the description and not the drawings. Fig. l illustrates a network associated with a transmission line and capable of matching a load impedance to a surge impedance at one frequency, and presenting a substantially infinite impedance at another predetermined frequency;

Fig. 2 shows two such 'impedances bridged across the transmission line;

Fig. 3 shows a concentric conductor arranged to perform the functions of the network shown in Fig. 1;

Fig 4 shows a modification of Fig. l;

Fig. 5 is the concentric conductor equivalent of Fig. 4;

Fig. 6 shows another modification of Fig. 1;

Fig, is the concentric conductor equivalent of Fig. 6; v

Fig. 8 is a third modication of the arrangement shown in Fig. 1;

Fig, 9 is the concentric conductor equivalent of Fig. 8;

Fig. 10 is a variant of Fig. 2;

Figs. 11 and 11A are two modifications of Fig. 2 in which two transmitters are connected with the transmission line through two branches;

In accordance with one feature of the invention, instead of the customary condensers and induction coils only straight wires are used as in- Fig. 12 illustrates the application of Fig. 1l to a system using three transmitters;

Figs. 13 and '14 illustrate the application of Figs. l and 2 to networks for a system through Awhich currents of three frequencies :are transradio transmitting or receiving station, i. e., at

points` remote from industrial centers, inductances o f the required values can be easily fashioned out of material that is usually in stock and without the necessity of employing other tools or personnel than is available at such stations.

In order to explain the nature of the present invention a few embodiments .thereof will be described and the theory underlying the invention explained Without intending -to limit the invention by such disclosure. As will -be obvious to those skilled in the art the invention may iind various other embodiments without' departing from the spirit thereof as dened in the claims.

The drawings illustrate a fewvof Ythe `many Dosmtted;

Fig. 15 is a modification of Fig. 14 and an extension of Fig. 10 -to a three frequency system;

Fig. 16 is a modiiication of Fig. 1; Fig. 17 showsl the application of Fig. 16 to a v three frequency system;

Fig. 18 shows a four frequency system in which the networks of Figs. 16 and 17 are used; and

Figs. 19 and 20 are graphs for facilitating the calculation of values.

For the sake of simplicity, it is assumed that the spacing ofthe transmission line and other network wires-'is uniform throughout.

Let us assume a network (Fig. 1) consisting of a two-wire transmission line T, with a loop of wire ACEFDB bridged across it, and a loop of wire bridged across this loop at points CD. The distance from AB to CD is Q, from CD to EF, S, and from CD to GH, P. High frequency alternating current is fed to the transmission line at Let ZAB be the impedance at AB at some frequency f. This impedance may be expressed in terms of the surge or wave impedance of the line zo, wavelength )t and the lengths of the various parts of the network shown in Fig, 1.

Let

p-T i T i where c is velocity of light and where P, Q and S are lengths dened in Fig. l. y y

Then the impedance of section CDEF (looking into the section from above) is izo tan s. The m-Y pedance of section CDGH (looking from- CD to-A wards GH) is izo tan p. The impedance of the two sections in parallel is given by l l l l tan s+tan p) .,tans.tanp jzotan s-i-tan p It is known that a section of line of any length L terminated into an impedance, say g at the far end has impedance where k= Applying this formula to the present case, by putting =E and L=Q, we get,

.tarisv tan z3 -yzyt-Vl #man g cot 0 .tan s tan p zu] tan s-i-tan p ZAB= -jzo cot Introducing y we get ZAB:

Z yj s`o that y is the absolute value of Z4?) tan p-l-tan srl-tan s tan p cot 0 (3) tan s tan p- (tan s--tan p) cot 0" For later reference it will be convenient to solve this equation for cot s, thus expressing s in terms of other quantities. This yields y-lcot 0 l-y cot 0 (4) Let us now consider the behaviour of the network shown in Fig. 1 at two frequencies f1 and f2. Let

-cot s=cot p-ibe the wave lengths which correspond to the two frequencies f1 and f2. If we make then ZAB at frequency fz will be infinite regardless of the value of S. Indeed, we have therefore,

=. Laan p-ttan s tan s tan p 2 (Cot P+C s) If we now let P approach then p approaches 1r andcot p approaches iw. Therefore, no matter what the value of S is, y approaches iw, provided that we exclude the special case cots=. When cot p approaches i@ and cot s=, y is indeterminate and therefore the impedance is mathematically indefinite and in practice unstable. Since ZAB=y`zoy, Zas becomes infinite when 'JA does. 'Ihus we see that any section with At any other value of f, however, ZAB is not infinite but has some other value which depends on the value of S. Moreover, S may be adjusted so that ZAB is given any desired value at any prescribed frequency f--vfn Indeed at f=f1 we have S, may be calculated for any value of 1 ZAB il j zo That is, given a desired value of ZAB at f1, We may always calculate from (5) the corresponding value of s1, and therefore S, which will give the prescribed Zins'. The network of Fig. 1, will, therefore, at frequency f1 offer any desired value of reactance, and at the same time with P and Q chosen as above present an infinite impedance at frequency fz.

On account of this property of network, it may, in accordance with the present invention, be used at two different frequencies as an impedance matching means between the transmission line supplying a load having an arbitrary impedance. The network may be precomputed to perform said function without the necessity of varying its constants by switching means or the like when the frequency of the alternating current is changed. It will perform its function when both frequencies are simultaneously impressed on the Itransmission line.

For the purpose of matching a'transmission line to a load in accordance with the present invention two networkslike the one shown in Fig. 1 may be employed. This is illustrated in Fig. 2 in which a high frequency radio transmitter is connected with annantenna by means of the transmission line T. Thel two networks are joined to the transmission line at A1131 and at AzBz.` When the frequency f1 is impressed on the line then, with both networks removed, waves will be reflected from the antenna if the impedance of the latter is not equal to the surge impedance of the line. A standing wave pattern will result from the interference of waves traveling towards and away from the antenna so Vthat at certain points on the line currents of large and at other asesinas poiitsicurrehtsof small amplitude will be observed'.` There willbe placesof maximum and minimum currents. l The location of junction A1B1 will be chosen withY reference toa point at which minimum cur,- renty occurs on the line. Let l1 be the distance between lsaid point and the ratio of Vminimumand the line section between the tenna. If the transmission line is short, q1 will be the ratio of minimum-maximum current all along the line with networks I and II in Fig. '2 removed. If the line is long (of the order of fifty Wave lengths) then the ratio of minimum-maximum currents will varyA along the line, and in this case the' ratio should bel determined inthe neighborhood ofthe proposed locationl of the juncture. If the line were cut at the pointwhere maximum current occurs and the impedance looking into the section of the line connected tothe load is measured, the impedance will be found to be a pure resistance ofsome value R1. If theKline were cut at a pointa quarter wave length away (towards the transmitter), i. e., where the current is minimum, the impedance (looking towards the load) will also appear to be a pure resistance ofmsome value R2.; f .v ,Y In accordance with the. law applicable to quarter wavetransformer R1, R2 and zo (the surge impedance of the line) are related in the following manner:

maximum current .on juncture and the anzg Rz-E (6) Since 'we may assume that the power dissipated in a quartervwave length of the transmission line conductors is very small, equal amounts of power will pass through said points where the transmission line wasfassume'd to be cut. Wemay, therefore, write I2mczR1=I2mnR2 (7) Multiplying Equations 6 and 7 We obtain after reduction Y v Y Y R1=2oq1 n (8) This resistance may, therefore, be substituted for the section of the line between the load-andthe maximum point, without disturbing the'state of standing waves on the line between the maximum point and the transmitter. i

- `Assuming now that the lin'e is cut at a distance Z1 froml R1 towards the transmitter, the impedance looking towards the load is given byEquation l. Replacing by R1 and L by l1 we get z-jzoRl cotgl-1 a z= 2 (9) R z cot--1 1 .'l o )u If a pure imaginary impedance ofvalue jx is bridged across the impedance Z given by (9), the two i'mpedances will resultv in a new impedance Y given by the juncture andl qi be then the problem of impedance matching at one frequency is solved.

Equating separately the real and Aimaginary parts of Equation l0 we derive after replacing R1 by zoqi'v y(8) Equation 13 enables us to find the two possible vplaces vfor the juncture A1-B1 at Which the matching networkI must be connected to the line; vEquation 14 gives the Value of the impedand the network I is to match the .impedance at frequency f1, the length of the free Vend Siv will be given by Mirco, nl

The location of AzBz as well as the length of o the free end S2 of the second network are found in the same manner as explained in connection with network I. However; in this instance the distance from A2132 to the place of minimum cur-lv rent is measured at frequency f2 and in Equation A i2 and ).1 are interchanged. Consequently The formula corresponding to 13A will be 1PZ- tardia/q2) In Equations 13B and 15B q2 is the ratio of minimum-maximum currents at frequency f2.

In practice it willbe convenient to impress on the line rst frequency f1 and then frequency f2 before the networks I and II are bridged across the line. The positions of current maxima should be noted,` and the ratios (q1 and q2) of current minimato maxima at each of the two frequencies should be determined. Then the locations of junctions AiBi and AzBz are determined from 13A and 13B, and theflengths Si and S2 calculated from 15A and 15B. When the two networks thus precomputed are connected to the line, they will be approximately right and will require only minor adjustments to compensate for the disturbances introduced by such factors as insulators, impedance of wires used to short, and other factors secondary in nature and, therefore, not taken into account in the formulae.

Fig. 3 illustrates a modification of the network illustrated in Fig. 1. Instead of the usual twowire transmission line, a concentric tube type of transmission line is used in which a conductor is centrally located within and insulated from a tube 3| of conducting material. ance matching network instead of loops of wire consists of concentric conductors 32, 33 and 34, 35. 32 and 33 are bridged across 36 and 3|, and 34 and 35 across 32 and 33. The free ends are interconnected at 36 and 31. When the two networks like the one shown in Fig. 3 are used in the manner indicated in Fig. 2, they will function in the same manner as networks I and II.

The location of the junctions, as well as the lengths P, Q and S are determined in the same manner as above described provided that the insulation used does not change the propagation velocity of electric waves along the line. Other- The impedwise n and Mmust in the formulae be multiplied by the ratio of the actual velocity to the Velocity of light.

In the network shown in Fig. 4, the closed .loop CDGH-offFiga 1 is replaced by Atwo -parallelwires C4G4 and =DrH4 The length of each wire will be one-half the length of-P (Figui). If used in the combination shown vin Fig. 2, this length will .bc

7i for Ione and 4 for theV secondfnetwork.' If for any reason P (Figi 1) is made longer than half a wave length, the Dim and 'C4G4 mustV be made shorter than in which N is any integral number or zero, and A is the wavelength atwhich the two are equivalent.

When vas in Fig. 4,2D4'C4G4H4-is open Aat the end and is made in length the impedance looking into C4DiH4G4 from terminals C4D4 is not izo tan p, as in the case of closed loop in Fig. 1, but jen cot p. Therefore in this case in Equation '3 for y 'we must replace tan p by -cot p.' Solution of Equation 3,'so modified, yields the following value of cot S:

y+ cot 0 1 -y cot 0 (4A) Thus, when the loop CQD4H4G4` is open as in Fig. 4 Equation 4A and not Equation 4 should be used for calculating the value of s.

Fig.:5 shows a concentric conductor arrangement and bears the same relationship to Fig. 4 as Fig. 3 bears to Fig. `1.

In the arrangementvshown in Fig. 6 the closed loop AEFB of Fig. 1 is Yreplaced by `two parallel open-ended wires -AaEe and BeFs. All values of such networks are .computed in Vthe manner shown vin connection with Figs. 1 and 2 except the length Se.

Fig. 7 is the concentric conductor counterpart of Fig. 6.

Fig. 8 shows a matching network consisting of two open-ended pairs of wires AaEs and BsFa, and CsGa and DaHs. The lengths of these openended pairs of wires P8 and Sa and other values are determinedin accordance with the theory applicable toV Fig.` 4 together with Equation 16.

Fig. 9 shows a network built up'of concentric conductors in the manner of Fig. 8.

Fig. 10 shows a simplified arrangement which can perform the same function as the arrangement shown in Fig. 2. These two arrangements, howevenare not equivalent. While network I in Fig. 2 has innite impedance and therefore does not disturb the line at wave length A2, network I .in Fig, 10 has a nite impedance at A2 and changes the state of standing waves along the portion of the transmission line between the junction A12B12 and the ltransmitter'. Consequently,

-cot s= -tan p+ the length of the free end S10 as well as the location of the junction A11B11 of network II must be determined with reference to the stand-A ing wave pattern at wave length M which results after network I has been connected, and not with reference to the state of standing waves at wave length M which exists when all networks are removed.

In network 11 P10 is made and M and M. Networks III11-V11 are bridged across branches 50 and 5|.

V11 is a loop of wire not? in length and bridged across 50 at any convenient point other than Y NIL? or multiple thereof, .from JJ. At wave length M this loop actsas a'short across the Aline'and prevents the currents of this wave length from distur-hing the normal operation of T1. Similarly 11111 is a loop of wire in length, also bridged across 5| at any convenient point (other than or multiple thereof) for the purpose of preventing currents of wave length M from interfering With T2.

V111 is a loop of wire bridged across 50 between T1 and V11. This network is used to match theY surge impedance of the line coming from fthe transmitter T1 to the impedance of the section of the line disturbedv owing to the presence of V11,

the distance between :acurrent maximum and the juncture of V111 measured towards T1 is given by Equation 13A. VSince the impedance. rlooking into a closed loop is mi 7.20amhk kA13 and B13 to 5| and 52 is where L is the length of the loop,kin View of Equation 14 the length of V111 will be n -1 Ff) 21rtan I ql Loop IV11 performs the same function as V111 and is precomputed in the same manner.

Fig. 11A shows a modication of Fig. 11 in Awhich the transmission line extensions A50, 5| leading to the twotransmitters are bridged each by one impedance so constructed as to block the line extension at one, and so as not to disturbthe line'at the other wave length.

The extension 50'is bridgedrby a horizontal 'l loop at a distance of n from the junction of thev main transmission line.

At a distance of from the junction with 50, the loop is bridged by a vertical loop Y in length. Therefore, the junction of 50 and the loop will be effectively short-circuited at wave lengthl M. The free end S1111 is adjusted so that the impedance of the network -is infinite at wavelength M. U

The construction of the network bridged across line 5| will be obvious from the above and the notations appearing on the drawings.

The arrangement shown in Fig. 11 may be used also when more than two transmitters are associated with the line. .An arrangement for three transmitters is shown in Fig. 12. l The networks 112, 1112 and 11112 bridged across the line T12 function in vthe same manner as'the networks of Figs. 13 and 14. Three transmitters TM, TM and TM arer connected with T14 through branches |00, |0|, |02. Three networks are bridged across each branch.

Loops `|03 and |04 short circuit the line |00 at wavelengths M and M, respectively, and loop |05 matches the surge impedance lof the branch at Wave length M. The positioning values and function of these loops are the same as those discussed in connection with Fig. 11. The fact that we now have two half-wave loops |03 and |04, instead of one, does not interfere with the freedom'of choosing their locations. Y.

The loops |06'|08 and |09|||perform the same Yfunctions with respect to TM and TM as loopA 035| 05 performs with respect Vto TM.

In accordance with the present invention the network can ,be constructed so as to match impedance where currents of more thanltwo fre- ,i quencies are transmitted. Fig. 13 illustrates such network by means of Vwhich lines carrying three frequencies can be accommodated.

A loop of wire 60 is bridged across the transmission line T13 atV AnBia.` The distance from where n can be either zero or any'integral whole number. The choice of n depends upon the valueof M in comparison with M. This section must he long enough to permit the of a loop at 63, 64 at a distance connection frOm A13B13.

At 6I, 62 aloopi) inlengthis connected with loop 6B.

Thev free" end S13 fof loop-60 .is Vmade of such length that the impedance looking into the loop at A13B13 with loop lll removed is innite atV wave length k2. At M it is innite owing to the length of loops 8i! and 66.

Loop 'lil bridged across 56 at 63, Gli is in length from 63, 64 to 1l, 12. Loop l5 bridged across '10,at 1I, 12 is from B13 and A13 as mentioned above) because the impedance at 63, 64 looking towards 6l, 62 had been made zero for M. by the adjustment of S13. On this account, the addition of network 1i), 'I5 does not disturb the functioning. of 60, 80. The impedance at A13B13 is therefore innite at M.

The presence of the whole network bridged across A13B13 will have no e'ect on the transmission line. at M and M, and at the same time the network will have the prescribedvimpedance at k3.

The values of the constants of this network as well as the positioning of junction points A13B13 may be precomputed substantially along the lines discussed in connection with Figs; 1 and 2.

Fig. le illustrates three networks likethe one shown in Fig. 13 connected with a transmission line T14 through which a transmitter is connectedrwith an antenna.

The network 114 is adjusted Yto match the impedance of the line at wave length M and have lnnite impedance at 12 and M. Similarly, H11 matches at M and 11114 at A3 and present innite impedance at the other two frequencies. All considerations applicable to Fig. 2 and. also Fig, 13 apply here also. Y Y

Fig. 15 isl a modification of Fig. Maand the network disclosed therein performs the same function as the one disclosed in the former. By providing a network 1111s in addition to the net-- works 115 and 1115, which correspond to the two balancing networks shown in Fig. 1i), the transmission line T15 may be used for three ldifferent frequencies. .The network 11115 is like the network shown in Figs. 13 and. 14'. It consists ci the length of the loop section from A11-,B15 to junction points 86, 8l of loop 88 is )nl l 4 'l 2 The loop 88 is in length, the loop section from 83,- 84 to 8l, 82 is and the lengthof loop 85 is The free end S15 of 1oop'19 is adjusted in the same manner as the free end S13 of the network shown in-Fig. 13 and the free endfof loopy 8l) is adjusted in the saine manner as the free endvof loop 1E).

rIVhe positioning and proportions of. the three networks will be the same as discussed in connection with Figs. 13 and 14 and Fig. 10.

The sections or networks may also be constructed in the manner illustrated inV Figs. 16-18. The method of generating sections explained in connection with these figures is particularly' applicable to the use of triple, quadruple, etc., sections, as well as to double sections.

The impedance element shown in Fig. 16 is so proportioned that looking into itA at 283, it is of infinite impedance at M aswell as at M. This is accomplished by making loop portion 20D-20| of length loop 2B i-EFSE and loop Nil-263 of a length to be hereinafter stated in greater detail.

Since E61-2M is the loop zee-zes is at i2, effectively short-cir cuited at 291. Section 20a-,201, being aassyis From this it fouows that the length sis of loop 2o 1 203 is equal to 1f the ioop of Fig. 1e is made' longer and the cross connection moved from 203 to 205 then, as shown in Fig. 1'7, by connecting a 2 loop across points 293, the network will not only appear at 200 as an infinite impedance when current of M and x2. wave lengths flow, but the impedance at B may be made to have any prescribed value at wavelength A3 and thus may be employed to match impedances at this wavelength.

By lengthening the free end of loop 2ll0--2il5 and adding a third horizontal loop (see IVi of Fig. 18)

in length, the network may be made to have infinite impedance at M, A2 and )o and match the load to the surge impedance at M.

Further extensions of the system will be obvious from the above.

Fig. 18 illustrates the'use of fixed networks 11s-IVN; constructed as explained in connection with Figs. 16 and 17 and permanently bridged across the transmission line Tia to match the antenna load impedance to the surge impedance when currents of M, Az, as, M wavelengths are impressed upon the line. y

Here as in Figs. 2, 10, l1, 12, 14 and 15, each impedance together with the line conductors between it and the antenna are used as the sole impedance matching means. No adjustable or specially built and protected devices need be provided and the construction and installation may be effected by the usual operating personnel of a radio station. Charts or graphs may be prepared for ready reference in the construction of net-V works herein described. Fig. 19 shows, for example, a curve useful in selecting proper values for a network like the one illustrated in Fig. 16.

The numbers in the first vertical column represent values of S in degrees for the end section 20|-203 short-circuited at 2GB. The abscissa is calibrated for calculate be p 180 M degrees, nd corresponding S from Fig. 19 in degrecs and make loop Z i-2E33 of length with ' one shown in Fig. 4. In order to explain the use ofthis figure let us put y=tan tp in Equation 4A. After this substitution Equation 4A may be trans'- formed 'into the following: cot s=tan p-cot (H-tb) once the value of y has been determined by means of l-q See Equation 14. The auxiliary angle u as well as values of 19:0 may be readily obtained from by means of. Fig. 20. In this figure the straight line oP is provided for this purpose. The actual calculation of p from i consists of looking up iny the right colurn of Fig. 20, marked v Mk2): T

the given value of M vfollowing a horizontal line from this value of to the left to the point where this-line intersects kline oP and, reading o the value of p which is the abscissa of this point.

Since in the network in'Fig.- 16, 0:10, both values are obtained at one time. angle is determined from yby means of curves marked-tan p. The value of y is found inthe left column of this figure and a horizontal line is followed to the point of intersection with the nearest tan p curve. The abscissa of this point of intersection is the desired value of tlf. Once p, and -tl/ are determined, the values of tan p and cot (t9-w) are obtained by means of tan p and cot (t9-1p) curves and values of cot s is deduced by algebraic subtraction. The value of s is read off by means of the cot s curves. Since s=21rS the value of S is then given by S LSV-:EK

Vacrossl the rst connection between the first source and the line effectively short-circuiting the connection at the frequency of the second source, compensating for the short-circuiting action of the impedance devices across the second connection, and preventing the reflection of The auxiliary waves into the first source, and fixed impedance devices permanently bridged across the second connection between the second source and the line eifectively short-circuiting the connection at the frequency of the rst source, compensating for the presence of the impedance devices across the first connection, and preventing the reflec- -tion of waves into the second source.

2. In combination, a transmission line, two sources of alternating current of different frequencies, connections between said sources and one end of the line, a load connected to the other end of the line, fixed impedance devices permanently bridged across various sections of said line, each substantially matching the load impedance to the surge when current of a different frequency is impressed on the line, and presenting `substantially infinite .impedance at other frequencies, impedance devices permanently bridged across the first connection between the iirst source and the line effectively short-circuiting the connection at the frequency of the second source, compensating for the short-circuiting action of impedance devices bridged across the second connection, and preventing the reflection of waves into the first source, and impedance devices permanently bridged .across the connection between the second source and the line effectively shortcircuiting the other connection at the frequency of the first source, compensating for the presence Y of the impedance devices across the first connection, and preventing the reection of waves into the second source.

3. In combination, a transmission line, two sources of high frequency alternating current of different frequencies, connections between said sources and one end of the line, a load connected to the other end of the line, a fixed impedance device permanently bridged across one section of said line substantially matching the load impedance to the surge impedance when current of one frequency is impressed on the line, and presenting substantially infinite impedance at the other frequency, a second fixed impedance device lpermanently bridged across another section of said line substantially matching the load impedance to the ,surge impedance when current of the other frequency is impressed on the line and presenting substantially innite'impedance at said one frequency, a third fixed impedance device permanently bridged across the connection between the rst source and the line effectively short-circuiting the connection at the frequency of the second source, compensating for the short-circuiting action of the fourth impedance device and preventing the reiiection of waves into the rst source, and a fourth fixed impedance device permanently bridged across the connection between the second source and the line effectively short-circuiting the connection at the frequency of the first source, compensating for the presence of the third impedance device, and preventing the reflection of waves into the second source.

4. In combination, a first transmitter of one frequency, a second transmitter of another frequency, a main transmission line, an antenna connected to said main line, a two-conductor line connecting each transmitter to said transmission line, vand an impedance circuit in shunt across each of said two-conductor lines, each of said impedance circuits being constructed and arranged to provide at its point of connection to its twoconductor line, a high impedance to the flow of energy in its associated two-conductor line of the frequency of its associated transmitter and a low impedance to the flow of energy in its associated line of the frequency of the other transmitter, the shunt impedance circuit across one two-conductor line being located from the junction of said one two-conductor line with the main transmission line ardistance equal to an odd multiple of onequarter of the wave generated by the transmitter connected to the other two-wire line, said other shunt impedance circuit being located from the junction of said other two-conductor line with the main transmission line a distance equal to an odd multiple of one-quarter of the wave generated by the transmitter connected to said one two-conductor line, and means connected across said main transmission line between said junction and said antenna for matching the antenna to the characteristic impedance of the main transmission line for the different frequencies of said two transmitters.

ANDREVV ALFORD.

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