US2611895A

US2611895A - Multiplex antenna distribution system

Info

Publication number: US2611895A
Application number: US64063A
Authority: US
Inventors: Lacey Edward
Original assignee: Individual
Current assignee: Individual
Priority date: 1948-12-08
Filing date: 1948-12-08
Publication date: 1952-09-23
Anticipated expiration: 1969-09-23

Description

Sept. 23, 1952 E. LACEY 2,611,895

' MULTIPLEX ANTENNA DISTRIBUTION SYSTEM Filed Dec. 8, 1948 t wa f- 5 u v v 2 4 JNVENTOR.

By EDWARD LACE) ATTORNEY Patented Sept. 23, 1952 Ema UNITED STATES PATENT OFFICE MULTIPLEX ANTENNA DISTRIBUTION SYSTE (Granted under the act of March 3, 1883, as amended April 30, 1928; 370 0. G. 757) 19 Claims.

This invention relates to multiplex circuits, and more specifically to multiplex receiving circuits wherein a plurality of communication channels may be maintained over a common transmission means, such as an antenna, the several channeled signals being separated at the receiving end by a plurality of tuned circuits, each resonant to the particular signal which it is desired that it extract from the antenna.

It is an object of this invention to provide an improved multiplex circuit in which each of the individual channels presents a relatively high impedance to the common transmission means. except when tuned to resonance, under which condition it presents an impedance matching that of the transmission means, whereby optimum energy transfer may be effected. Off resonance, the high impedance prevents an undesirable drain of energy from the common transmission means.

It is another object of this invention to provide a multiplex circuit in which each individual resonant circuit has a very high Q.

In the design of multiplex circuits, either for receiving or transmitting, it is desired to have optimum energy transfer, at resonance, between the transmission means, such as the antenna, and the individual resonant circuit. This is achieved by matching the impedance of each individual circuit at resonance to the impedance of the antenna. It is also desirable that the impedance presented to the transmission means by each circuit when tuned ofi the resonant frequency, be quite high, in order that the particular circuit not drain energy from the antenna except when tuned to its own particular frequency. Briefly, it is desirable that a plot of impedance vs. frequency for each of the several multiplex circuits look somewhat like a V, the impedance above and below resonance bein quite high, and dipping at resonance to the impedance value which is the conjugate of the impedance of the transmission means and the circuit. at which point the transfer of energy will be maximum, by virtue of the impedance match.

It is also desirable in multiplex circuits, in fact in many tuned circuits, that the energy loss in the circuit be kept to a minimum; that is to say, the circuit should have a high Q, or a high ratio: wL/R.

Prior art multiplex circuits have achieved each of these desiderata, but always at the expense of the other.

It is an object of the instant invention to achieve both these desirable features simultaneously in a multiplex system,

It is another object to minimize cross-coupling between the several multiplexed circuits of the system.

It is a further object of the instant invention to provide a multiplex system which is broadband in its overall characteristics; wherein each individual circuit will present, at resonance, an optimum impedance match to the common transmission means to which each circuit is coupled; and wherein each individual circuit will have a high Q.

It is a still further object to attain the above outlined desiderata without the use of vacuum tubes with their consequent vulnerability to overloading.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following description.

The multiplex circuit of the instant invention comprises a transmission means to which each of the several multiplex circuits is coupled in some suitable fashion. For purposes of example and to simplify the disclosure, the multiplex circuit will be shown as a receiving circuit. although it is to be understood that the teachings of the instant invention are equally applicable to a transmitting circuit. Furthermore, the transmission means common to the several individual circuits will be shown as a fixed impedance, broad-band antenna, although it is to be understood that the teachings of the instant invention are equally applicable to other types of transmission means, such as a cable extending between a transmitting and receiving station of a multiplex transmission system.

In accordance with the instant invention. broad-band transformer means are coupled to the broad-band antenna. By the term broadband antenna or broad-band transformer means" is meant a means in which the impedance, or impedance ratio, respectively, is substantially constant over a wide frequency range, so that within that range the reaction of the particular means is the same to all frequencies.

The essence of the instant invention is the combination. with the transformer means de-- scribed above, of a plurality of series-resonant circuits connected to the other side of the transformer means. As a general rule, the impedance of the antenna and of the transmission line to which it is connected is not very low, being in the order of perhaps 50 ohms, which figure will be used hereinafter as exemplary of the transmission means impedance. The impedance of a series-resonant circuit, properly constructed, is

at resonance quite low, being in the order of 1 ohm, which figure will be employed hereinafter as exemplary of the impedance of each series-resonant circuit at resonance. The essence of the instant invention, therefore, is the use of step-down, broad-band transformer means coupling the 50 ohm impedance of the antenna to the 1 ohm resonant impedance of each seriesresonant circuit. The high turns terminals, that is, the terminals on the 50 ohm side of the transformer, are connected to the antenna. The several multiplexed series-resonant circuits are connected to the low turns terminals of the transformer means, that is, the terminals on the low impedance side of the transformer. For the exemplary figures given above, the transformer means would be one having an impedance ratio of 50 to 1 (a turns ratio of about '7 to 1).

Each of the generally similar multiplexed channels, which are fed with energy from the common antenna, includes, in addition to its series-resonant circuit (which is preferably made tunable), a signal amplifying means associated with or connected to a portion, 1. e. one of the elements, of the series-resonant circuit. In the case of a receiving system, the amplifying means consists of suitable means for deriving a signal from the series-resonant circuit, and applying it to a suitable receiver. In the case of a transmitting system, the amplifying means consists of a suitable means for applying a signal of predetermined frequency to the series-resonant circuit, which would, in turn, apply it to the antenna through the transformer means above discussed.

The impedance of each of the several seriesresonant circuits, at resonance, after reflection through the transformer means, is substantially matched to the impedance of the antenna. For the exemplary figures used above, the impedance of the series-resonant circuit, connected to the low turns side of the transmission means, being 1 ohm, is, after reflection thru the transformer, approximately 50 ohms, this being the impedance of the antenna itself. Thus at resonance, optimum impedance match is obtained between the antenna and the particular channel to which the incoming energy is fed. In welldesigned circuits, the impedance of the antenna is substantially resistive, and of course, the impedance of a series-resonant circuit at resonance is also substantially resistive, so that it will be understood that matching of impedances referred to herein neglects reactive components, because in general they are not to be found in circuits of the present sort. It is to be understood, however, that where the antenna, or (more generally) the transmission means, includes a reactive component, it may still be matched to each circuit of the multiplex system by simply including the complement of the antenna reactance in the individual circuit, in accordance with known impedance matching teachings.

There will be described as examples of the instant invention, two major species, each of these species having certain minor modifications which might be desired for some applications. These embodiments of the instant invention will now be described in connection with the accompanying drawing, wherein:

Fig. 1 illustrates the first species of the instant invention, and is characterized by the use of a plurality of individual step-down transformers, as the broad-band transformer means, which is one of the essential features of the instant invention;

Fig. 1A illustrates certain minor modifications that are suitable for the circuit of Fig. 1 for particular applications;

Fig. 2 illustrates the second principal species of the instant invention, wherein the broad-band transformer means constitutes a single, stepdown transformer, to the low turns side of which is connected a low impedance transmission line which feeds the several tunable series-resonant circuits included in the several multiplex channels of the complete system;

Fig. 2A illustrates how the circuit of Fig. 2 may bedmodifled slightly for particular applications; an

Figs. 3 and 4 are included to illustrate the disadvantages of attempting to achieve a satisfactory multiplex system without the use of the broad-band, step-down transformer means constituting one of the essential features of the instant invention.

Referring to Fig. 1, a transmission means H is shown, to which is connected transformer means l2, feeding a plurality of generally similar multiplex channels l3, l4, and It. For exemplary purposes, the transmission means has been shown as an antenna I], which in the instant embodiment is broad-band and of fixed impedance, connected to a transmission line l8. For exemplary purposes, it will be assumed that the fixed impedance of the antenna l1 and of the transmission line I8 is 50 ohms.

In the species of Fig. l, the transformer means l2 assumes the form of a plurality (in this case, three), of step-down, broad-band, powdered-iron core, transformers l9, 2l, and 22, each having a turns ratio of /50 to l. The high turns terminals of the transformer means l2 are connected, as shown, to the transmission line l8.

The multiplex channel It, which is typical of the channels l3, l4, and I6, consists of a seriesresonant circuit 23, connected to the low turns, or 1 ohm, terminals of the transformer IS. The series-resonant circuit 23 comprises an inductor 24, connected in series with a capacitor 26, the two in turn being connected to the secondary, or low turns, winding 21 of the transformer IS. The series-resonant circuit 23 makes no substantial use of the transformer I! for tuning; the transformer serves only as an impedance matcher.

In the example of Fig. l, a receiving circuit is shown. Therefore, radio frequency energy appearing across the capacitor 26 is applied between grid and cathode of a tube 28, which represents the first stage of a suitable receiver amplifier. The capacitor 26 is shown as variable, or adjustable, so that the channel [3 may be tuned to any desired frequency of the band which the receiving system of Fig. 1 is adapted to receive. The channels I and I8 are generally similar to the channel l3 described above.

The operation of the circuit of Fig. 1 will now be briefly considered.

Assume that the antenna I! is receiving energy of three different frequencies, Fl, F2, and F3. Channel l3, i. e. the series-resonant circuit 23. is tuned to Fl; channel I is tuned to F2; and channel I6 is tuned to F3. The impedance of the series-resonant circuit 23, at the resonant frequency Fl, will be very low, approximately 1 ohm. Reflected through the transformer means l2, i.-e. its transformer l9, this 1 ohm impedance appears on the transmission line I! and antenna l! as a 50 ohm impedance, exactly matching that of the antenna l1 and line It. Therefore, for energy of frequency Fl there is optimum coupling and 5 maximum transfer of energy from the antenna II to the channel l3.

To all frequencies other than Fl, channel l3, after reflection through the transformer means l2, presents a very high impedance, so that little energy is diverted into the channel l3, except at frequency Fl. The degree to which frequencies other than Fl are rejected by channel l3 is dependent directly on the character of the frequency response curve of the series-resonant circuit 23, which, in turn, depends directly upon the Q of the circuit. Assuming, purely for example, thatthe inductor 24 has an inductance of about 32 microhenries and that Fl is 1 megacycle, the Q of the series-resonant circuit 23 is equal to:

The term in the numerator is wL;" the term in the denominator is-the total resistance of the circuit, which is the 1 ohm impedance at resonance (pure resistance) of the series-resonant circuit 23, plus the 1 ohm impedance of the low turns winding 21 of the step-down transformer IS: a total of 2 ohms. Carrying forth the above calculations yields a value of 100 for the Q of the resonant circuit feeding the amplifying tube 28. This is a relatively high value, and results in excellent discrimination by the channel l3 against all frequencies except Fl, to which it presents, as outlined above, the 50 ohms required to obtain maximum energy transfer from the antenna H.

The channels I and I6 react to the frequencies F2 and F3 respectively, in a similar manner. It is, therefore, seen that the single antenna l! is able to feed a large number of individual channels, each tuned to its own frequency, with each channel having little or no effect on reception by the other channels and taking energy from the antenna only at its own tuned frequency.

It has been found as a practical matter that it is often desirable to employ, as the transformer means l2, suitable auto-transformer means in place of the regular transformer exemplified at IS in Fig. 1. In this event, the circuit of Fig. 1 may be modified as shown in Fig. 1A, wherein the auto-transformer I9 is connected with its high turns terminals to the antenna H, the tapped capacitor 26' of which is connected in the usual manner across the grid-cathode of the amplifying tube 28'.

While theoretically the transformer l9, or l9, as the case may be, reflects 50 ohms into the primary for every ohm in the secondary, there is as a practical matter a tiny amount of flux leakage which results in a small inductive reactance in the transformer. This may be corrected as shown in Fig. LA by the insertion of a small adjustable capacitor 3|, in series with the high turns windings of the transformer l9. It is to be understood that if desired this reactance-correcting capacitor may also be used with the regular transformer illustrated at l9 in Fig. 1.

A second principal species of the instant invention is illustrated in Fig. 2, wherein l'la represents the broad-band antenna having a fixed impedance of 50 ohms, for example. In this species, the step-down transformer means assumes the form of a single, powdered-iron core, radio frequency transformer 33, having a 0' to 1 turns ratio. The high turns, or 50 ohms, side of the transformer 33 is connected between ground and the antenna l'la. To the low turns winding 6 34 of the transformer 33 is connected 9. 1 ohm transmission line 38, to which are connected the several multiplexed channels l3a, a, and Ida.

Exemplary of the channels is the channel l3a, which comprises a series-resonant circuit 23a, consisting of the inductor 24a and the capacitor 26a, connected in series across the 1 ohm transmission line 36.

Operation of the Fig. 2 species of the instant invention, as a receiving circuit, will now be briefly considered. Assume that three frequencies, Fl, F2, and F3 are being received by the antenna lla. By means of the adjustable or variable capacitor 26a the channel l3a is tuned to Fl. Thus, at frequency Fl, 9. 1 ohm impedance is applied across the 1 ohm transmission line 36, and to the low turns winding 34 of the transformer 33. This 1 ohm impedance is reflected through the transformer 33 at a 50 to 1 ratio, and appears on the high turns winding of the transformer 33 as a 50 ohm impedance. This impedance matches that of the antenna Ho, and of the associated transmission line 31. so that at Fl there is optimum transfer of energy from the antenna l'la to the channel l3a. Frequencies other than Fl are rejected by the series-resonant circuit 23a, because at all other frequencies its impedance is much higher than 1 ohm. Therefore, little or no energy other than that of Fl will be drawn into the channel l3a. As in the first species, described in Fig. 1, the effectiveness of unwanted frequency rejection depends upon the Q of the circuit, which includes the capacitor 26a, the inductor 24a, and the low turns winding 34. The latter having an impedance of approximately 1 ohm, reflected from the 50 ohm antenna l'la through the transformer 33, and the resonant circuit 23a having an impedance of 1 ohm, the total impedance (which is essentially resistive) is 2 ohms. -As in the first example, the wL of the circuit is approximately 200, so that the Q of the circuit is in the order of 100, which is a very high Q, and which results in a sharp resonance curve and attendant excellent rejection of all frequencies except Fl.

In a similar manner, the channels Ila and lie reject all frequencies except F2 and F3, respectively, to which they are responsive, respectively, with an impedance of 1 ohm, reflected through the transformer means l2a as a 50 ohm impedance, matching that of the antenna lla.

It will be readily apparent that the transformer means l2 of Fig. 1 and the transformer means l2a of Fig. 2 are substantially equivalent, the former case using a 50 ohm line l8 to feed a plurality of 50 to 1 impedance ratio transformers, the latter using a 50 to 1 transformer 33 to feed a 1 ohm line 36, which in turn feeds the several channels of the system.

The circuit of Fig. 2 has the practical advantage of requiring only a single, powdered-iron core, broad-band, radio frequency transformer 33, instead of the plurality of transformers l9, 2i, and 22 in the Fig. 1 circuit. It has the disadvantage, however, of requiring a 1 ohm transmission line 36 which, as a practical matter, is somewhat diflicult of construction. In operation, however, the two circuits are obviously the full equivalent of each other.

Like Fig. 1, Fig. 2 also has its variant, shown in Fig. 2A, wherein the transformer 33 is replaced by the auto transformer 33', the other elements of the circuit remaining essentially unchanged. Likewise, as in Fig. 1A, the leakage reactance of the transformer (either 33 or 33') may be comworse on l pensated for by the insertion of a small variable capacitor 3|a in series with the antenna and transformer primary which neutralizes any small inductive component in the transformer.

In order to emphasize the advantages of the instant invention, there will now be hypothesized two different circuits which do not meet the requirements of the instant invention, namely, step-down transformer means, coupling to a plurality of channels including tunable seriesresonant circuits.

Consider first the hypothetical circuit of Fig. 3, which includes an antenna 4|, connected to a transmission line 42, to which are directly connected a plurality of multiplexed channels 43, 44, etc. The channel 43, as an example, includes a series-resonant circuit 46, constituted of an inductor 41 and a variable capacitor 48, connected in series across the line 42. Since the resonance impedance of the circuit 46 is in the order of 1 ohm, it is necessary to connect in series with this circuit a resistive impedance 5| of about 49 ohms value, so that the impedance of the antenna 4|, which is 50 ohms, may be substantially matched.

Consider, however, the effect of such an expedient upon the Q of the circuit. wL, as shown hereinbefore, is approximately 200. The resistance of the circuit, however, is now 1 ohm (circuit 46) plus 49 ohms (the resistor 5|) plus 50 ohms (the antenna 4|), a total of 100 ohms giving a net Q of approximately 2. This results in high energy loss and in lack of resolution, or unsharp tuning, for the channel 43. It is, therefore, evident that the hypothesized circuit of Fig. 3 lacks the high Q marking applicants invention.

Next, hypothesize the circuit of Fig. 4, wherein an antenna 52 is connected to a 50 ohm transmission line 53, across which are coupled the

multiplex channels

54, 56, etc. The channel 54, for example, includes a parallel resonant circuit 51, comprised of a variable capacitor 58 connected in parallel with an inductor 59. Energy is coupled to the inductor 59 by a coil 6| connected across the transmission line 53. -While the coupling between the coils BI and 59 constitutes a transformer in a sense, there is very great fiux leakage between these coils, because the coil 59 must present a relatively high inductive reactance in order to resonate with the capacitor 58. Unlike the instant invention, the circuit 51 makes full use of the winding 59 to form the inductive component of the circuit.

The impedance reflected by the resonant circuit 51 into the transmission line 53 is highest at the tuned resonance point rather than lowest, as it should be, this being characteristic of all parallel resonance circuits. That is to say. off resonance, the impedance reflected into the line 53 by the circuit 51 is relatively low, and hence considerable energy will be drained from the line 53 without doing any good in the channel 54. At resonance, when the channel 54 is tuned to accept the energy, the impedance reflected into the line 53 will be maximum.

It is, therefore, evident that the hypothesized circuit of Fig. 4 has this disadvantage: the untuned channels tend to draw more energy from the line than does the channel which is tuned to the particular frequency of the energy. Thus, much energy is wasted, and the system is relatively low in efficiency.

From the above explanation, it will be seen that there has been disclosed herein (embodied in several species) a multiplexed circuit having simultaneously in combination the following advantages: it is broad-band within the rrequency spectrum to which it may be tuned. Each particular channel has a very high Q, so that the energy loss is minimized. Each channel, when tuned to a particular frequency of incoming energy, has a perfect impedanc match with the antenna or other transmission means feeding the system, so that optimum transfer of energy at the frequency is achieved. Because the coupling impedance (the 1 ohm secondaries) between any two circuits is so low compared to the untuned impedance of each circuit itself, the feedback or cross-coupling between circuits is reduced to a minimum. In this way the local oscillators of the respective receiving circuits do not cross feed appreciably into the other circuits, thru the transmission line. All of the above features are attained without the use of vacuum tubes, with their consequent danger of overloading due to near-by transmitters.

As stated hereinbefore. although the particular examples have illustrated receiving circuits employing fixed impedance antennas, it is obvious that, if desired, the teachings of the instant invention may be employed equally well for transmitting circuits. Likewise, the antennas illustrated herein may be replaced by any suitable transmission lines such as coaxial cables extending between receiving and transmitting stations.

Obviously, many modifications and variations of the present invention are possible in the light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

What is claimed is:

1. A multiplex receiving circuit comprising a fixed impedance, broad-band antenna and a plurality of generally similar, multiplexed channels connected in parallel to said antenna, each channel including a broad-band, step-down transformer having its high turns terminals connected to said antenna, a tunable, series-resonant circuit connected across the low turns terminals of said transformer, the impedance of said circuit at resonance reflected thru its transformer being substantially matched to said fixed impedance of said antenna and being much higher 011' resonance; and a means for deriving a signal from one of the elements of said circuit.

2. A multiplex receiving circuit comprising broad-band transmission means; and a plurality 'of generally similar, multiplexed channels connected tosaid transmission means, each channel including a broad-band, step-down transformer having its high turns terminals connected to said transmission means, a tunable, series-resonant circuit connected across .the low turns terminals of said transformer, the impedance of said circuit at resonance reflected thru its transformer being substantially matched to the impedance of said transmission means and being much higher 015 resonance, and a means for deriving a signal from one of the elements of said circuit.

3. A multiplex circuit comprising a fixed impedance, broad-band antenna; and a plurality of generally similar, multiplexed channels connected to said antenna, each channel including a broad-band, stepdown transformer having its high turns terminals connected to said antenna,

mum

a series-resonant circuit connected across the low turns terminals of said transformer, the impedance of said circuit at resonance reflected thru its transformer being substantially matched to said fixed impedance of said antenna, and a signal amplifying means connected across a portion of said circuit.

4. A multiplex circuit comprising: broad-band transmission means; and a plurality of generally similar, multiplexed channels connected to said transmission means, each channel including a broad-band, step-down transformer having its high turns terminals connected to said transmission means, a series-resonant circuit connected across the low turns terminals of said transformer, the impedance of said circuit at resonance reflected thru its transformer being substantially matched to the impedance of said transmission means and being much higher oil resonance, and a single amplifying means connected across a portion of said circuit.

5. A multiplex circuit comprising a fixed impedance, broad-band antenna; a plurality of generally similar broad-band, step-down transformers having their high turns terminals connected to said. antenna; and an equal plurality of generally similar, series-resonant circuits connected, respectively, across the low turns terminals of said transformers, the impedance of each said circuit at resonance reflected thru its transformer being substantially matched to said fixed impedance of said antenna and being much higher off resonance.

6. A multiplex circuit comprising broad-band transmission means; a plurality of generally similar, broad-band, step-down transformers having their high turns terminals connected to said transmission means; and an equal plurality of generally similar, series-resonant circuits connected, respectively, across the low turns terminals of said transformers, the impedance of each said circuit at resonance reflected thru its transformer being substantially matched to the impedance of said transmission means and being much higher off resonance.

'7. A multiplex circuit comprising a fixed impedance, broad-band antenna, a plurality of step-down broad-band transformers connected in parallel to said antenna, a plurality of tunable, series-resonant circuits connected, respectively, to the low turns terminals of said transformers, and a plurality of signal amplifying means connected, respectively, one across a portion of each said circuit.

8. A multiplex receiving circuit comprising a fixed impedance, broad-band antenna; a stepdown, broad-band transformer having its high turns terminals connected to said antenna; a plurality of tunable, series-resonant circuits connected to the low turns terminals of said transformer, and a plurality of means for deriving signals from a portion of each said circuit.

9. A multiplex receiving circuit comprising broad-band transmission means; a step-down, broad-band transformer having its high turns terminals connected to said transmission means; a plurality of tunable, series-resonant circuits connected to the low turns terminals of said transformer, and a plurality of means for deriving signals from a portion of each said circuit.

10. A multiplex circuit comprising a fixed impedance, broad-band antenna; a step-down, broad-band transformer having its high turns terminals connected to said antenna; a plurality of series-resonant circuits connected to the low I a i 10 turns terminals of said transformer, and a plurality of signal amplifying means connected, respectively, across a portion of each said circuit.

11. A multiplex circuit comprising broad-band transmission means; a step-down, broad-band transformer having its high turns terminals connected to said transmission means; a plurality of series-resonant circuits connected to the low turns terminals of said transformer, and a plurality of signal amplifying means connected, re-

spectively, across a portion of each said circuit.

12. A multiplex circuit comprising a fixed impedance, broad-band antenna; a step-down, broad-band transformer having its high turns terminals connected to said antenna; and a plurality of series-resonant circuits connected to the low turns terminals of said transformer.

13. A multiplex circuit comprising broad-band transmission means; a step-down, broad-band transformer having its high turns terminals connected to said transmission means; and a plurality of series-resonant circuits connected to the low turns terminals of said transformer.

14. A multiplex receiving circuit comprising: a fixed impedance, broad-band antenna; stepdown, broad-band transformer means having the high turns terminals thereof connected to said antenna; a plurality of generally similar multiplexed channels, each channel including a tunable, series-resonant circuit connected to the low turns terminals of said transformer means, the impedance of said circuit at resonance reflected thru said transformer means being substantially matched to said fixed impedance of said antenna and being much higher off resonance, and means for deriving a signal from a portion of said circuit.

15. A multiplex receiving circuit comprising: broad-band transmission means; step-down, broad-band transformer means having the high turns terminals thereof connected to said transmission means; a plurality of generally similar multiplexed channels, each channel including a tunable, series-resonant circuit connected to the low turns terminals of said transformer means,

the impedance of said circuit at resonance refiected thru said transformer means bein substantially matched to the impedance of said transmission means and being much higher off resonance, and means for deriving a signal from a. portion of said circuit.

16. A multiplex circuit comprising: a fixed impedance, broad-band antenna; step-down, broad- .band transformer means having the high turns terminals thereof connected to said antenna; a

plurality of generally similar multiplexed channels, each channel including a series-resonant circuit connected to the low turns terminals of said transformer means, the impedance of said circuit at resonance reflected thru said transformer means being substantially matched to said fixed impedance of said antenna and being much higher off resonance, and signal amplifying means connected across a portion of said circuit. 1'7. A multiplex circuit comprising: broadband transmission means; step-down, broadband transformer means having the high turns terminals thereof connected to said transmission means; and a plurality of generally similar multiplexed channels, each channel including a, seriesresonant circuit connected to the low turns terminals of said transformer, the impedance of said circuit at resonance reflected thru said transformer means being substantially matched to the impedance of said transmission means and being much higher off resonance, and signal amplifying means connected across a portion of said circuit.

18. A multiplex circuit comprising: a fixed impedance, broad-band antenna; step-down, broadband transformer means having the high turns terminals thereof connected to said antenna; and a plurality of generally similar series-resonant circuits connected to the low turns terminals of said transformer means, the impedance of each said circuit at resonance reflected thru said transformer means being substantially matched to said fixed impedance of said antenna and being much higher 011 resonance.

19. A multiplex circuit comprising: broadband transmission means; step-down, broad-band transformer means having the high turns terminals thereof connected to said transmission means; and a plurality of generally similar seriesresonant circuits connected to the low turns ter- REFERENCES CITED The following references are of record in the file of this patent:

'UNI'IED STATES PATENTS Number Name Date 1,688,036 Clement Oct. 16, 1928 1,979,315 Cotter Nov. 6, 1934 2,085,434 Loftis et a1 June 29, 1937 2,145,548 Landon Jan. 31, 1939 2,223,084 Weissner et al Nov. 26, 1940 2,229,043 Butler Jan. 21, 1941