USRE22993E - alford - Google Patents

Description

' April 1948- A. ALFORD 3 22,993

MATCHING NETWORK Original Filed Nov. 19, 1936 3 Sheets-Sheet} FIG.2.

FIG.1.

INVENTOR ANDREW ALFORD ATTORNEY April 20, 194 8. ALF oRD Re; 22,993

MATCHING NETWORK Original Filed Nov. 19, 1936 I5 SheetsSheet;2

FIG].

- INVENTO-R ANDREW ALFORD ATTORNEY I April 20, 1948. A. ALFORD 22,993

MATCHING NETWORK Original Filed Nov. 19, 1936 3 Sheets-Sheet 3 K FIG".

0 J I Y L w o v 7- .5 1.0 1.5 I 2.0 [NVENTOR AND/PE W ALI-OED ATTO R N EY Reissuecl Apr. 20, 1948 MAroiiiNG NETWORK Andrew Alford, Cambridge, M as s assignor to Mac" my Radio and Telegraph; Coinjranfi New York, N. Y.,acorporation "ofliehwafe Original No. 2,165,086, dated No. 111,643, Novcn bcr'19, 193G.

cember 21, 1938. Application f reissueiviafen 19, 1945, Serial No. 583,401

13 Claims.

This invention relates to matching'network's and pertains more particularly to networks of this character for interconnecting radio apparatus with two-wire transmission lines;-

It is an object of this invention to provide an electrical network for interconnecting a radio transmitter with a two-wire transmission line so that it will always be insured that the two 0011'- ductors of the transmission linecarry equal cur rents 180 out of phase.

Another object is the provision of an electrical network for interconnecting a radio translating device with a two-wire circuit whereby equal voltages between the two wires of the circuit and the ground will be obtained and at-th'e same time a 180" phase relation between the currents the two wires will result.

Whenever two-wire transmission lines are used tosupply power to the transmitting antennae; it is necessary to'insure' that the two" conductors of the transmission-line carry equal-currents 180 out of phase so that the" transmission line-itself will not-act as an antenna and r'adiate' 'energy in all directions; When the currents in th'e"trans-' mission line are characterized as above men= tinned, the line acts'merely as a conductor to transfer the energy to the antenna; and undesira ble radiation from the transmissionline itself is avoided: v

Transmission-lines can very wellbe balanced at the lower radio frequency ranges by the use of'know'n apparatus and methods with sat-isfac tor'y' results, but as the frequency is increased difficulties are encountered whichhave" not been overcome by the'teachings of the prior art; For example, with frequenciesof the order of nve megacycles the problein' of supplying the line with energy'in a balanced manner-may be" sans: factorily solved by a number ametnoasemploj: in'g air core transformers.

At frequencies of the or 2: stashed at this 'Irequen'cy electrostatic essentials nior''eflic'iiit and exerts agreater distiiittiiii eifct.--Avail higher frequencies the difliculties encounteredare even greater.

daryof the transg sure that the center of t s 661i shall be-at 'Z'eropotential. This con-'- dition is difficult to achieve however, since the ginseng c "o'nnection*- must necessarily have some fihys'iai'liig th and consequently some induster-ice; 'this ind fiance beir'ig' usually high enougn s'o' thatthe droiia'cross the grounding wire at the higher frequencies is sufficient to produce afid'niigeotefiaar a't' 'the' eer'iter of; the coil instead are 6 'dfidteiitial -with the result that capacity coupliiigbtweii the primary and secondary of the transformer results and produces the usual undesirable efiect.

A large number of other arrangements have been suggested for obtain-iiig a balanced condition in transmission lines, but so far as I am aware the various known arrangements are either ex-' tremely complicated and expensive to construct or fail to give a balanced output at the higher frequency ranges.

I have found that it is possible to avoid the difiiculties due-tocafiadity-coupling by the pro-- vision of an electrical network which is relatively simple-and 'hasbeen found'to give very satisfactory results in actual practice.

In accordance with--niyinvention I provide a network of impedanc-es connected across and in series with the line and between line and ground which serve weanlin -the use, causing it to 3 iment of my invention wherein two inductances and a condenser are used.

Figs. 2 and 3 illustrate other networks in accordance with my invention, utilizing two condensers and an inductance.

Fig. 4 illustrates another network in accordance with my invention utilizing two inductances and a condenser.

Figs. 5 and 6 are diagrams used in explaining the operation of the networks shown in Figs. 1 and 2, and in Figs. 3 and 4, respectively.

Figs. '7, 8, 9 and 10 shOW various circuit arrangements utilizing the networks of Figs. 1, 2, 3 and 4.

Figs. 11 and 12 are used in further explanation of the invention.

Referring more particularly to the drawings,

in Fig. 5 the voltage between the wires I4, and

2'l, is V and the voltage between wire 21 and the point 3 is U, the wire 2! being connected to ground in such manner that it is really at ground potential or if not, so that it may be assumed to be at ground or other fixed potential. The impedance impedance 3-5 is C and the impedance 3-4, representing the transmission line impedance is P. Likewise the current through the impedance A is M, and the current through. the impedance P is N.

The power for supplying the currents mentioned, which may for example be derived from a vacuum tube, is assumed to be applied between terminals l and 2 while the transmision line, represented by impedance P, is connected to terminals 3 and 4.

Now the condition desired to be attained is that the voltage U between the point 3 and ground, and the voltage V between the point 4 and ground, shall be equal in magnitude and opposite in phase, that is V=-U. e

This result is secured when the relative'values and signs of the impedances are properly chosen as shown by the following formulae,

expanding and rearranging terms .(6) becomes then Equation 10 becomes 85 is A, the'impedance 5-6 is B, the.

which may be expanded as (12) jbr+jar%jarbcac bsasab+-%as=0 Equating imaginary and real components separately we derive two separate Equations 13 and ,Now substituting (--a) for (b-l-a) in Equation 13 We obtain c=-2b or 0:: 2B

Thus the condition that V=U regardless of the value of P may be satisfied by the two circuits shown in Figs. 1 and 2 provided that elements of the same kind, that is the two inductances in Fig. 1 and the two capacitances in Fig. 2 are equal, and when the third element is such that the impedance across it is equal in magnitudeto one-half of the impedance of either one of the equal elements, and has an opposite sign. To assure this result the two inductances in Fig. 1 must be arranged in such manner as to avoid or at least keep very small, mutual reactance between them. And the third element must be arranged in such manner that the impedance from the junction point of the inductances in Fig. 1 to ground is equal in magnitude to one-half of the impedance of either one of the inductances, and is of-opposite sign. But this impedance need not be necessarily concentrated in the third element itself, that is, it is not necessary that the condenser represented in Fig. 1 should have the exact value of impedance for, for instance, it is entirely possible for part of this impedance to be concentrated in the lead which is necessary to connect this condenser to ground so that in actual practice it may well be that the reactance of the condenser is actually much higher than that which is required; yet a part of it is balanced out by the reactance of the lead which connects the condenser to ground or for that matter partially by the inductance of the lead which connects the other side of the condenser to the junction point of the inductances.

The same, of course, is true of Fig. 2, namely this time the inductance shown as the third element is not necessarily concentrated in the coil itself but partially at least isto be found in the wire which connects this inductance to ground or to the junction point of the two condensers. It is this ieature of the circuit which makes it really practical for with this circuit it is not necessary to find a point in the transmitter that is really at ground potential. Indeed all that is necessary is that the impedance between the junction point of (1:0 or A=C the" similarelements and ground-have the required value.- "It has been found experimentally that'the distributed capacit'y of the inductors to ground utilized in circuit I does not in practice disturb the circuit to anygreat extent. The same is true of the stray capacities to ground of the condensers utilized in circuit 2. This is quite reasonable from the theoretical point of view for the over all effect of such stray" capacities to ground is merely to alter'the impedance between points 3 and 4 respectively to ground, that is, in effect merely to alter the-impedance of the transmission line as seen from terminals '3 and 4. We have already shown above that as long as the fundamental conditions of the circuit are satisfied it is quite immaterial as to what the impedance of the line happens to befor the conditions-to be satisfied by-the three elements are the same irrespective of the valueof this impedance.

The circuits 3 and 4 may best be explained in connection with Fig. 6. This figure differs from Fig. 5 only in that an impedance has been placed between points 4 and 8 rather. than between points-3 and 5. The analysis of this circuit is very similar to the one which has already been carried out in connection with Fig. 5.

Assuming that V represents the voltage between conductors l--8 and 2-4; U the voltage between point 3 and ground, that'is conductor 2-! and X is the voltage between the point 4 and conductor 2-4, then for a condition or perfect balance we have I]: X.

This result is secured when the relative values and signs of the impedances are properly chosen as shown by the following formulae.

20 v V: DM+ E(M+ N) 22- U: (M+ ME adding Equations 22 and 23, we get (24) X+U=NP+2(M+N)E substituting 19 in 24, we get (25) NP=-2ME-2NE or collecting terms (26) N(P+2E) =2ME substituting 21 in 26, we get (27) (P+2E)D=-2E(F+P) expanding, we get Now assuming that D, E and F are pure reactances equal to id, :ie and 9' respectively, and that P is equal to r+:is, then Equating imaginary and real components separately we derive two separate equations 30 and 31, as follows:

New substituting -d for 2c in Equation 30, we obtain Consequently'the reaet anc'esF and" D must be equal in magnitude "and apposite in signal-id reactance E must have one-half "the value of reactanc'e D'and mustequal'insign'rea'ctance F. The two possibilities are" illustrated in "Figs. 3 and 4.

In actual practice it has been found convenient to connect circuits of the type illustrated in Figs. 1, 2, 3 and 4. to the tank circuit. of the last amplifier stage in the transmitter ina manner shown in Figs. 7, 8, 9 and 10. The-method shown in Fig. '7 when employed in connection with the circuit shown in Fig. 1 possesses the advantage that both conductors at the transmission line are connected directly to ground during the operation of the circuit so that any static charges which may accumulate on the antenna during rain or snow can leak off to ground whereby high static potentials may be avoided without the use of any auxiliary resistors or other means of grounding the transmission line. Moreover, the adjustments obtainable from this type of circuit are sufiicient-to take care of a fairly wide range of line impedances so that even when the transmission line isnot flat, that is when it is not perfectly matched to the antenna, the tank circuit of the transmitter may still be properly loaded and the'powcr transferred from the 'last amplifier to the transmission line in an eiiicient way. It has also been found in practice that the circuit shown in Fig. 8 is particuarly well adapted for use in conjunction with the circuit shown in Fig. 2 for in this casethe' impedance which is usually obtained between terminals I and 2 is capacitative so that part'of the inductance IE! is already balanced outand condenser Il may be fairly large and consequently the adjustments are notyery critical and it is found that the tank circuit .may be'loadcd'with reasonable case without the use of unreasonable capacities and inductanccs.

Either one of the circuits shown in Figs. 3 and 4 may be employed inconnection with the arrangements shown in Figs. 7 and 8. The circuit shown in Fig. 9 may also'be used in conjunction with any one of the four circuits shown in Figs. 1, 2, 3 and 4 when the impedance of the line is such that this circuit is more advantageous. Fig. 9 illustrates the connection when the circuit of Fig. 4 is employed, and Fig.110 illustrates the connection whenthe circuit'of Fig. 3 is used. The network components used for obtaining balance are in the circuits of'Figs. 7, 8, 9 and 10 isolated from the high direct current potentials used on the plate of the last tube, by a transformer, usually of the step-down type, and therefore these components andespecially the condensers may be relatively cheap in construction. The use of transformer coupling with grounded secondary reduces the danger of high direct current plate potentials reaching the line.

In order that the losses in the coils employed in any one of the four circuits described above may be kept low it is of course necessary to choose the proper values of inductances and capacitances.

An explanation will now be made as to how the values of inductance and capacitance can be determined for most eflicient operation. This explanation will be made in connection with Figs. 1 and 2 since the method employed in connecare R where L is the inductance of the coil and (:21 times the frequency.

7 The value of Q for ordinary transmitting coils is fairly well known and is usually somewhere around 150 or 200.

Upon the assumption that the Q factor and hence the resistance of the two coils of equal inductance, shown in Fig. l, is the same the total loss H in both Coils may be expressed by the formula On the other hand W the power delivered to the line is Now if we assume that, as is usually the case, the impedance P of the transmission line is a pure resistance (1. e. if we assume that is=0 and r=P) and if we remember that A=C (as shown by Equation 1'7) then We may write Equation 5 as follows:

37 M: N(i+ 1) But since impedance A=R+fiwL and since this impedance is very nearly a pure reactance we may replace A by awL in Equation 37 thus giving T 38 M: NQT r 1) Substituting this in 35, we obtain Hence the loss H is the two coils depends upon the useful power W which in any given case is fixed, the factorQ of the coils which is made as large as possible, and the coefficient K which depends upon I as may be seen from the Equation 41.

Fig. 11 shows how the loss H in the two coils varies with the dimensions of the two coils, or rather, with the value of From this figure is'may be seenthat the smallest value of K, that is the least losses for a given power output and a given Q factor, are obtained when Is equal to .707. The value of K corresponding to such a value of From Fig. 11 it may be seen that the minimum is not at all sharp but there is a whole region where the losses are reasonably low. In actual practice, it is generally possible to design the circuit in such a manner that the circuit operates somewhere within this minimum loss region. It is quite obvious from Fig. 11 that small coils and large condensers are to be avoided in spite of the usual rule which is very often followed, namely, that for minimum loss small coils and large condensers are to be preferred. Fig. 11 definitely shows that this rule does not apply to the circuit shown in Fig. 1.

A similar calculation in connection with the circuit illustrated in Fig. 2 gives the following Then Equation 42 may be written as (42A) H K This result is illustrated in Fig. 12. From this figure again it may be seen that there is a certain best value of condensers and inductances to be used in this second circuit for minimum loss but again the region is fairly wide and in practice it is fairly easy to choose values of condensers and reactances which fall within the minimum loss region.

The smallest value of K1 corresponding to the least losses for a given power output W and a given coil factor Q occurs when The value of K1 for this condition is 2.00. It will be noted that this minimum value of K1 is about 29 percent lower than the minimum value of K for the circuit of Fig. 1. g

It may be pointed out that a circuit employing two condensers and reactor is somewhat more efiicient. from the point of view of loss than circuit l which employed two reactors and only one condenser. However, circuit I possessed other advantages. In practice, of course, the losses in circuit l are quite low so that very often other advantages, of circuit I may out-weigh the low loss property of circuit 2. On the other hand, when the losses are the controlling feature and grounding is provided for in some other manner, circuit 2 may be preferred. The same sort of considerations apply to circuits illustrated in Figs. 3 and 4.

From the preceding description it will be seen eases that 1 on the assumptiomthat the impedancesare pure-reactances the balance or the-line will not be aflfected by a change in the load. Furthermore even when the resistive components of the inductance coils-used are taken into account from the stand-point'of-losses it willbe-noted thatthe power loss isnot greatly "aflected by moderate changesin-theload.

1 While 'I have described certain embodiments of my: invention for the --pur-poses-'-' or illustration, it will be understood that various-modifications and adaptations thereof may be made withi'mt'he spirit oft-he invention as set forth in the appended claims.

What I claim is:

1. An electrical network adapted to insure that currents traversing the two conductors of a two wire transmission line are of substantially equal magnitude and opposite phase comprising three reactances, two of which are of the same sign and the third of which is of the opposite sign, one of said three reactances being connected in series in one of said two wires, another in shunt to said two wires, and the third between one of said two wires and ground.

2. A system in accordance with claim 1 (wherein the two reactances of the same sign are of equal value and the impedance of the third reactance is equal in magnitude to one-half that of either of said two reactances first mentioned.

3. An electrical network having two input terminals and two output terminals and adapted to maintain voltages equal in magnitude and opposite in sign, between each of said output terminals and a. given one of. said input terminals, comprising three reactanoes two of which are of the same sign and the third of which is of the pposite sign, the first of said three reactances being connected in series between an input terminal and an output temiinal, a second of said reactances being connected between the same said input terminal and the other of said output terminals and the third of said reactances being connected between said other output terminal and the other of said input terminals.

4. An electrical network according to claim 3 wherein said first and said second of said reactances are of equal magnitude and opposite sign and said third reactance is of'the opposite sign as said second reactance and of one half its value.

5. An electrical adapted to maintain equal voltages between each of two output temrinals and a given one of two input terminals comprising three reactances, a first and a second of the same sign and a third of the opposite sign, each having a first and a second terminal, all of said first terminals beingconnected together, the second terminals of said first and third reactances constituting said two input terminals and the second terminals of said first and second reactances constitutlngsaid two output terminals.

6. An electrical network in accordance with claim 5 wherein said first and second reactances are of the same sign and of equal magnitude and said third reactance is such that the impedance across it is equal in magnitude to one-half of the impedance of either said first or said second impedance and has an opposite sign.

7. In a radio system, radio translating apparatus including an amplifying tube, a two wire transmission line and means for inter-connecting said apparatus and said line so as to minimize energy transfer between said line and the surrounding space, comprising a network or three reactances, "two-of which are of the same sign and the'third ot-"which is of the; Opposite sign,

to the output circuit of said amplifying tube, and

means connecting said tuned circuit-across two of the reactances of saidnetwork. 8. In a radio system; radio translating apparatu'sincluding an" amplifying tube, atwo wire transmission lineconnected thereto, means i for interconnecting said; apparatus and said 'lineso as-to mini-mizeenergy transfer "between said-line and the surrounding, space, comprising a network of three reactances, two of which are of the same sign and the third of which is of the opposite sign, one of said three reactances being connected inseries inone of said two wires, another in shunt to said two wires, and the third between one of said two wires and ground, a first tuned circuit connected in the plate circuit of said amplifying tube, a second tuned circuit magnetically coupled to said first tuned circuit, means connecting said second tuned circuit across two of the reactances of said network, and means connecting to ground a, point in said second tuned circuit.

9. An electrical network adapted to insure that currents traversing the two conductors of a two wire transmission line are of substantially equal magnitude and opposite phase comprising three reactances, a reactance of one sign being connected in series in one of said two Wires, a reactance of the same sign being connected in shunt to said two wires and a reactance of the opposite sign being connected between said two wires and ground.

10. An electrical network adapted to insure that currents traversing the two conductors of a two wire transmission line are of substantially equal magnitude and opposite phase comprising three reactances, two of which are of the same sign and the third of which is of the opposite sign, a reactance of one sign being connected in series in one of said two wires, a reactance of the opposite sign being connected in shunt to said two wires and a third reactance having the same sign as said reactance of one sign connected between one of said two wires and ground.

11. An electrical network adapted to insure that currents traversing the two conductors of a two wire transmission line interconnecting a source and a load are of substantially equal magnitude and opposite phase comprising three reactances, two of which are inductive and equal in magnitude and the third of which is capacitative and has a magnitude equal to one-half that of either of said inductive reactances, one of the inductive reactances being connected in series in one of said two wires, the capacitative reactance being connected in shunt to said two wires and the other inductive reactance being connected between one of said two wires and ground, the impedance in ohms of each of said inductive reactances being equal to .707 times the resistance of said line together with said load.

12. An electrical network adapted to insure that currents traversing the two conductors of a two wire transmission line interconnecting a source and a load are of substantially equal magnitude and opposite phase comprising three reactances, two of which are capacitative and equal in magnitude and the third of which is inductive and has a magnitude equal to one-half that of and opposite in sign between each terminal of said first pair and a given terminal of said second pair, comprising three reactances two of which are of the same sign and the third of which is of the opposite sign, the first of said'three reactances being connected in series between a terminal of said first pair and a terminal of said second pair, a second of said reactances being connected between the same said terminal of said first pair and the other of said terminals of said second pair, and the third of said reactances being connected between said other terminal of said second pair and the other terminal of said first pair. I

ANDREW ALFORD.

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