US3508272A - Tunable coupler for electrically short monopole antennas - Google Patents

Description

P 1970 v P. J. KHAN ET AL 3,508,272

' TUNABLE COUPLER FpR ELECTkICALLY SHORT MONOPOLE ANTENNAS Filed July 29, '1968 OUT FIG. 3 (a) I FIG. 30:)

[BAND l- Me. 5 5 1 0 1 5 2 0 2Y5 s o 3': 4'0

CIR cu IT 1 i RjSONATE 'S CIRCUIT 2 RESONATES MONOPOLE I ANTENNA INVENTORS,

PETER J. KHAN GARY A. VANDER HAAGEN DAVID E. OLIVER. BY Mihzn. W, AGENI W 12 *wjh/ ATTOK'NEY3 United States Patent US. Cl. 343-745 4 Claims ABSTRACT OF THE DISCLOSURE A wide-range electronically tunable antenna coupling system for use with electrically short monopole antennae. Impedance matching is realized by the use of a unique varactor diode circuit which provides tunable impedance matching over a very wide range of frequencies.

BACKGROUND OF THE INVENTION This invention relates to coupling systems for antennae and more particularly to a wide-range, electronicallytunable power matching antenna coupling system for use with electrically short monopole antennae.

Prior art attempts to provide wide range tuning and impedance matching between an electrically short monopole antenna and a receiver have consisted primarily of electromechanical switching techniques, biased ferrite antenna networks or mechanical tuning circuits. Conventional antenna coupling circuits have been severely limited in their operating frequency range due to the essential requirement of circuit gain and reliability which could not be sacrificed in lieu of a broad operating frequency range. At best, such circuits were capable of tuning over a 2.5/1 frequency range.

The present invention overcomes the disadvantages of the prior art by the utilization of a unique varactor diode circuit exhibiting wide range electrical tuning capability in excess of a 6 to 1 frequency range ratio, while simultaneously maintaining the desired circuit gain, simplicity and reliability.

The general purpose of this invention is to provide a wide-range, electrically-tunable, power-matching, receiving antenna coupling system for use with electrically short monopole antennae.

BRIEF DESCRIPTION OF THE DRAWINGS The exact nature of this invention will be readily apparent from consideration of the following specification relating to the annexed drawings wherein:

FIGURE 1 shows an equivalent circuit for a short monopole antenna;

FIGURE 2 shows an L-type matching network for the equivalent circuit of the monopole antenna shown in FIGURE 1;

FIGURE 3(1)) shows a frequency diagram of the coupling circuit shown in FIGURE 3(a) and FIGURE 4 shows one arrangement of a monopole antenna matching network as envisioned by the instant disclosure.

DESCRIPTION OF THE INVENTION In order to render the theoretical aspects of the coupling network more discernible, a basic equivalent circuit of a short monopole antenna is shown in FIGURE 1. Looking back into the output terminals of a monopole an tenna, as represented by terminals and 11 of FIGURE 1, a capacitive reactance 12 would be presented in series with a parallel arrangement of an inductive reactance 13 and a resistance 14.

As seen in FIGURE 1, an electrically short monopole antenna is characterized by a highly capacitive reactive impedance, such that the direct connection of such an antenna to a coaxial cable for transmission of an input signal to a receiver would result in a large mismatch or signal loss. This problem of mismatch was eliminated by the use of a variable reactance coupling network schematically shown in its simplest form in FIGURE 2.

The basic coupler 20, shown in FIGURE 2, is essentially an inductive type matching network including a variable series inductor 15, which is adjustable to such a value as to produce a series resonant circuit with the antenna capacitance 12. A variable capacitive device 16, is chosen to be parallel-resonated with antenna inductance 13. At resonance, the resistance appearing between terminals 17 and 18 is the antenna resistance 14. A broadband transformer 19 having a resistive characteristic equal to the antenna resistance at resonance is connected across terminals 17 and 18 as a terminating load device. The primary winding of broadband transformer 19 is matched to the coupler impedance at resonance and the secondary winding of the transformer matches the impedance of the coaxial transmission line connecting the coupler to the receiving unit.

Monopole antennas are generally required to operate over a wide range of frequencies, therefore, the inductive and capacitive elements of the coupler must be tunable by a ratio equal to or greater than (Af) where Variable inductive and capacitive elements currently available are either conventional mechanical devices or saturable reactors and varactor diodes.

As a typical example of the invention, a 5 to 30 me. antenna coupler with tuning provisions was developed. The circuit features which best represent the spirit of the invention will be contained therein. The procedure employed in the development of a variable inductance for resonating with the monopole capacitance was as follows, with the circuit shown in FIGURE 3(a). The capacitors 30 and 40 were chosen to be varactors having a capacitance range of 10 pf. to pf. The inductive element 31 was chosen to resonate at a maximum value of capacitance 30 at a frequency just above 5 mc., here chosen to be 5.4 mc. Inductance 41 was chosen to resonate with the maximum value of capacitance 40 at 10.4- me. The 5 me. to 30 me. frequency range was divided into two bands shown in FIGURE 3'(b), such that inductance 31 and capacitance 30 principally determine the impedance on band 1, and inductance 41 and capacitance 40 principally determine the band 2 impedance.

The variable capacitor 40 was adjusted to have a value of 120 pf. for the tuning of frequencies in band 1, and the exact value of inductance required was obtained by variation of capacitor 30. Capacitor 30 was maintained at a value of 120 pf. for the tuning of band 2 frequencies. The procedure provided a guide to the adjustment of capacitors 30- and 40', thus providing a basis for the choice of varactors, to give the appropriate inductance over the frequency range. However, more detailed computation was required to determine the capacitance values of capacitors 30 and 40 at some frequencies in band 1 and the lower end of band 2. The adjustment was carried out experimentally. Therefore, the values of capacitors 30 and 40 were adjusted such that at the frequency of operation, the two circuits were operating at a point below self resonance, at which point the inductive reactance of the circuit equaled the capacitive reactance of the antenna. The amount of inductive reactance was controlled by the deviation from resonance.

Referring now to FIGURE 4, wherein a complete antenna coupler is symbolically shown, the two resonant circuits developed above and shown .in FIGURE 3(a) are used in conjunction with a third resonant circuit which appears capacitive to the monopole antenna and thus matches the inductive reactance of the antenna for extending the efiective bandwidth of the coupler as evidenced by FIGURE 3(1)).

The variable capacitive device 16 shown in FIGURE 2 may be replaced with the variable resonant circuit of FIGURE 4 which comprises an inductive device 52 in series with a variable capacitor 51 having a second variable capacitor 50 connected in a parallel arrangement with the series inductance 52 and capacitor 51. A similar procedure to that outlined above for the series resonant circuits may be used for determining the value and type of elements required in the parallel resonant circuit. Here, however, operation of the series inductor 52 and capacitor 51 combination is operated above resonance such that the parallel resonant circuit of capacitor 50 and inductor 52 with capacitor 51, always appears capacitive. In actual practice, the variable capacitive devices 50 and 51 are replaced with a pair of varactors exhibiting the desired characteristics. The net variation in inductance and capacitance of the parallel capacitive arrangement, realized by utilizing the above described technique, approaches the product of the ratios of the change in capacitance of the two varactor diodes employed. For example, if the ratios of the two varactors used are approximately 3 to 1 and 2 to 1 respectively, the total range would be the product of the two ratios or approximately 6 to 1.

This technique allows multiple diodes to act as either variable capacitors or inductors at low loss with the realization of wide range electrical tuning capability in excess of a 6 to 1 frequency range.

The coupling network herein described may be used in various radio receiving systems and is in no way limited to the use described in the specification. It should be understood that the coupler may take many mounting forms which would suggest to those skilled in the art various alternative supporting and coupling arrangements.

We claim:

1. A wide-range electronically turnable antenna coupling network for coupling an electrically short monopole antenna to a load termination device, comprising:

first and second parallel resonant circuits connected in series between one terminal of a two-terminal monopole antenna and one terminal of a twoterminal load termination device, said resonant circuits exhibiting an inductive reactance which is equal 4 to the absolute value of the characterstic capacitive reactance of the monopole antenna when operated at a frequency slightly below self resonance; and

a third parallel resonant circuit connected across the terminals of the load termination device and in parallel therewith;

one terminal of said load termination device provides a common connecting point be'twen the series connected parallel resonant circuits and the parallel connected parallel resonant circuit;

a second terminal of said load termination device provides a common connecting point for the other terminal of said parallel connected resonant circuit and the other terminal of said monopole antenna, said third resonant circuit exhibiting a capacitive reactance which is equal to the absolute value of the characteristic inductive reactance of the monopole antenna when operated at a frequency slightly above self resonance;

whereby the coupler exhibits an output impedance equal to the characteristic resistance of the monopole antenna.

2. The coupling network as set forth in claim 1, wherein said third resonant circuit comprises a first varactor in parallel arrangement with a series combination of a second varactor and an inductive element.

3. The coupling network as set forth in claim 1, wherein each of said first and second parallel resonant circuits consists of a parallel arrangement of a varactor and an inductive element.

4. The coupling network as set forth in claim 3, wherein said third resonant circuit comprises a first varactor in parallel arrangement with a series combination of a second varactor and an inductive element.

References Cited UNITED STATES PATENTS 2,745,067 5/1956 True et a1. 33317 2,920,323 1/1960 Dunson 343--850 3,098,231 7/1963 Vrain et a1. 343-745 3,110,004 11/1963 Pope 307-320 XR ELI LIEBERMAN, Primary Examiner M. NUSSBAUM, Assistant Examiner US. Cl. X.R. 343-861

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