Capacitive-Type, Electrically Short, Broadband Antenna and Coupling Systems

The present invention relates to an improved antenna for transmitting and receiving radiation, and more particularly to a simplified and highly efficient antenna having a physical length which is short relative to the wavelength of the radiation and which is broadband. It also relates to automotive and other mobile system use of a single short antenna which can be coupled (1) in one way as the transmitting-receiving antenna for a Citizens Band radio or (2) in another way as a electrically-short efficient antenna to receive signals on the bands from AM to and including FM.

In my previous patent U.S. Pat. No. 4,675,691, incorporated herein by reference, I described an arrangement of electrodes which showed how an electrically-short antenna can be constructed by using an electrostatic capacitor such a split-cylindrical capacitor, as the radiating member of a resonant circuit. In this patent, the conductors forming the capacitors are concave surfaces.

As described herein, a short length antenna is defined as one which has a length equal to or less than one quarter of a wavelength (.lambda./4) of its resonant frequency. Usually, such short antennas typically exhibited a high Q or a rather sharp tuning peak.

In the present invention, we describe how we have subsequently found the new forms of capacitors can be made to operate as broad-band, efficient antennas.

To understand the previous theoretical appraisals of these type of antennas we refer to the following references, incorporated herein by reference.

Kraus, John D., "Antennas" 2nd Ed., McGraw-Hill, N.Y., 1988, especially pp. 711-714.

Hansen, R. C., "Fundamental Limitations of Antennas," Proc. IEEE, 69, 170-182, February, 1981.

Wheeler, H. A., "Fundamental limitations of small antennas," Proc. IRE, vol 35 pp. 1479-1484, Dec. 1947.

Ramo, Simon, and J. R. Whinnery, "Fields and Waves in Modern Radio" John Wiley & Sons, Inc , New York, N.Y., 1944, pp 432 and 458-459.

Professor Kraus, widely recognized as one of the foremost authorities on antennas, devotes a section of his recent book to the properties of electrically-short antennas. He relies on the work of R. C. Hansen and Wheeler, to conclude that the radiation resistance decreases with increasing wavelength, and that therefore no electrically small, efficient antenna is possible. This result is understandable since the treatment of antenna radiation for short antenna structures have assumed that the radiation takes place by means of dipole radiation formed by wires connected to the antenna structures.

It is the object of the present invention to provide an antenna with broad bandwidth, which becomes more efficient as the wavelength of the radiation increases.

It is a further object of the present invention to demonstrate the coupling of the herein described antenna structures and those described in my prior patent U.S. Pat. No. 4,675,691, to an AM-FM radio set to receive both AM and FM signals. Because of the increase of radiation resistance with increasing wavelength for either antenna the use of a wide-band inductor in series with (either) one of them provides good signals at both AM and FM frequencies.

It is a further object of the present invention to provide an improved capability of receiving or transmitting vertically polarized radiation by virtue of the physical arrangements of the electrodes.

The capacitive-type antenna described herein are connected in series with an inductor by means of a non-radiating twisted pair of wires. Because of the geometry, these wires have a minimum of length in which they are open to free-space. This length is the distance from the shielding provided by the electrodes of the capacitors, to the shielding of the electrical circuit box. This length is too short to provide the source of radiation. Rather, the source of radiation (and reception) is from the electric fields between the electrodes of the capacitor plates of the capacitive-type antenna themselves, i.e., it is derived from the fluctuations of charge on the capacitor plates.

The invention may be characterized as an antenna for transmitting or receiving radiation having a wavelength .lambda.. The invention comprises:

a first electrode forming a first surface of a capacitor radiator,

a second electrode, spaced from the first electrode by a gap, and forming a second surface of the capacitor radiator,

the sum of the gap dimension and the dimensions of the first and second electrodes not exceeding .lambda./4,

an inductor having one end thereof coupled to one of the first and second electrodes,

a wire connecting the one end of the inductor to the one of the first and second conductors, and an additional wire connecting the other of the electrodes to ground, and

a structure for inhibiting transmission or reception of electromagnetic energy of wavelength .lambda. from the wire means and the additional wire means so that transmission or reception of electromagnetic energy primarily emanates from the electrode surfaces.

FIG. 1 is a perspective view of an antenna according to a first embodiment of the invention with annular electrodes mounted, with a gap between them, on a non-conducting annulus.

FIG. 2 is a cross-section view of a second embodiment of the invention with annular electrodes with one end being open, the other covered with a cap. These electrodes are mounted on a non-conducting annulus with a gap between the opposing open ends.

FIG. 3 is a perspective view of a third embodiment of the invention with plane electrodes mounted on the ends of a non-conducting annulus.

FIG. 4 is a diagram of an electrical coupling circuit which may be used for the antenna structures of FIGS. 1-3 when coupled to the input of a radio transmitter or receiver through a balun.

FIG. 5 is a diagram of an electrical coupling circuit which may be used with the antennas of FIG. 1-3 when coupled to the input of an automobile AM-FM radio receiver.

Reference will now be made to the various drawings to describe the presently preferred embodiments of the invention. FIG. 1 shows an antenna, 1, which is formed in a cylindrical shape composed of an annular tube 2 made of dielectric material. The diameter of the tube 2 may, for example be about 7/32", and its length may be on the order of 2". Mounted on the surface of tube 2 are two electrodes, 3 and 4 each composed of an annulus of a good conductor such as aluminum. The electrodes are separated by a gap, 5, of, for example 1/8" which prevents direct electrical conduction. Thus the assembly forms a capacitor.

A twisted wire pair 18 is shown entering the distal end of electrode 4. This twisted wire comes from the circuitry of FIGS. 4 or 5. The use of the twisted wire inhibits radiation of electromagnetic waves therefrom. One wire form the twisted pair is coupled to electrode 4 at point 30 and the other wire is fed across the gap 5 to contact electrode 3 at a point 32 as illustrated. Points 30 and 32 are positioned near the gap, although the electrodes 3 and 4 will themselves provide shielding for the wire 18 so that the point of contact with the electrodes need not necessarily be adjacent the gap as illustrated. For additional shielding, the portion 34 of the twisted pair 18 extending across the gap 5 may be spacedly covered with an electrically conductive shield to further inhibit radiation therefrom.

FIG. 2 is a cross-section view of a second embodiment of the invention. In FIG. 2, annular electrodes 7 and 8 each have one end thereof open and the other end covered with a cap 7a and 8a respectively. Electrodes 7 and 8 are mounted on a non-conducting annulus 10 and are spaced on the annulus 10 by a distance 9 so as to form a gap between the opposing open ends of the electrodes 7 and 8. The resulting structure likewise forms a capacitive structure. Again, twisted pair 18 may be fed into the antenna structure of FIG. 2 through an end electrode thereof. In this case, twisted pair 18 passes through an aperture in electrode 8 and connects to electrode 8 at point 36. Electrically conductive portion 40, extends across the gap 9 and connects to electrode 7 at point 38. Again, portion 40 may be electrically shielded. It is understood that in the twisted pairs used herein contain two conductors each insulated from one another and each twisted around the other so that each shields the other from radiating.

FIG. 3 illustrates a drum antenna structure fabricated in accordance with the principles of the invention. Electrically conductive drum surfaces or electrodes 23 (only one being shown) are positioned on the end of an insulating support member 24. The twisted pair 18 is fed through an aperture in the support member 24 and each wire thereof is separated and fed to the respective drum electrode 23. Portions 46 and 48 within the support member may be shielded as shown at 50 to prevent e.m. radiation. Shield 50 may be in the form of a conductive cylindrical sleeve spaced from the wire portions 46 and 48 as, for example, by means of an cylindrical insulator coextensive with the sleeve 50.

In FIGS. 1-3, it is understood that the antenna structures illustrated are dimensioned to be considered "short length" antenna which means that the length of the antenna is .ltoreq..lambda./4 at its resonant frequency. In the case of the antenna of FIG. 1, the length of the antenna refers to the overall length of electrodes 3 and 4 including the gap dimension 5. Likewise in the case of FIG. 2, the antenna length is taken as the combined length of electrodes 7 and 8 and the gap dimension. In the case of FIG. 3, since the thickness of the drum electrodes may be taken as negligible the length is taken to be the length of the insulating support member 24. In general, the gap dimension may be defined as the shortest straight line path between the spaced electrode surfaces and the antenna length defined as the sum of the gap dimension and the electrode length extending along this straight line path.

FIG. 4 illustrates the chassis and circuit design for the antenna of FIG. 1 in a CB antenna application. It is understood, however, that the same circuit may equally well be used for the antenna embodiments of FIGS. 2 and 3. In reference to FIG. 4, one end, of one of the twisted pair of wires, 18a, contained in the tube 2 of the dielectric material, is connected to the electrode 3. The other end of wire 18a is connected to one terminal of balun 15. The second wire 18b has one end thereof connected to the electrode 4, and its other end connected to one terminal of a mechanically tunable, resonating inductor, 14. The other terminal of the inductor 14, is connected to a terminal of the balun 15. Thus, the design of FIG. 4 connects an LC circuit (composed of inductor 14 and capacitive antenna 1) in series with the balun 15. Balun 15 may, for example, be a 300 Ohm to 300 Ohm standard balun. The measured radiation resistance of the antenna circuit was approximately 25 ohms at resonance of 28 MHz.

The other terminals of the balun 15, are connected in the usual fashion. One end to ground, the chassis 13, the other to the central terminal of the coaxial receptacle, 16 of a 50 ohm transmission line. The transmission line in turn was connected to a Radio-Shack CB, TRC 415, Catalogue number 21 1509A. Transmission and reception was successful on all channels.

Using a Kraco, AM-FM-Cassette radio receiver, the antenna of FIG. 1 was mounted to a circuit box as shown in FIG. 5 for use in an AM-FM circuit arrangement. As seen in this figure, an LC circuit is connected in series with the coaxial line.

In reference to FIG. 5, one end, of one of the twisted pair of wires 18a is connected to the electrode, 3, while the other end is connected to the grounded chassis 19. Further, one end of the second wire 18b is connected to the electrode 4, with its other end connected directly to a terminal of a resonating inductor 20. The other terminal of the inductor 20, is connected directly to a central conductor of a coaxial receptacle 21 without coupling through a balun as in the embodiment of FIG. 4. The ground of the chassis forms the ground shield of the coaxial receptacle 21. Receptacle 21 thus forms connectors which are coupled to a receiver.

In place of twisted pairs of wires, separately and individually shielded wires may also be used. Alternately, the wires, or unshielded parts thereof may simply be made short enough so that the radiation emitted or received therefrom is relatively small as compared with that emitted/received from the electrodes which form the capacitive plates of the antenna structures of FIGS. 1-3. The primary requirement for the circuits of both FIGS. 4 and 5 is that the radiation emitted/received by the wires connecting the circuits to the electrodes be minimized while the radiation emitted/received from the capacitive electrode plates be maximized.

The radio was placed inside an automobile and connected it to the car battery through the cigarette lighter socket. The antenna chassis box, 19 with the antenna of FIG. 1 connected thereto was placed on the roof of the automobile and coupled to the receiver by a standard cable. Radio signals were heard throughout both the AM and FM bands demonstrating the wide-band nature of this antenna system.

A cylindrical split-curved plate antenna (such as illustrated in U.S. Pat. No. 4,675,691) was also mounted vertically in place of the antenna of FIG. 1 and this split-curved plate antenna demonstrated the same bandwidth as the antenna design of FIG. 1 herein.

The antenna radiation resistance was measured in the same manner as described in my prior U.S. Pat. No. 4,675,691, as shown therein in FIG. 2. The following tables set forth the results of the measurements.

As may be seen from the above tables, in accordance with the principles of the invention, the radiation resistance of the antenna structures varies inversely with frequency. This is precisely the opposite relationship as exist in conventional dipole or whip antennas.

In general, the antenna includes some mechanism for inhibiting transmission or reception of electromagnetic energy of wavelength .lambda. from the wires which connect the capacitive electrodes to the circuits illustrated in FIGS. 4 and 5. This mechanism may be the use of relatively short wires 18a and 18b which, because of their relatively short wires 18a and 18b which, because of their relatively short length, do not effectively radiate or receive electromagnetic radiation. In such a case, the short wires radiate very little, and the major contributor to the circuit radiation resistance would be the electrodes defining the capacitive plates. In another embodiment, the mechanism of inhibiting the transmission or reception of electromagnetic energy comprises the shielding of the first and second wires which is effective to minimize radiation and reception therefrom. Clearly, a combination of both short wires and shielding is also within the scope of the invention. Other mechanisms may also be apparent to those of skill in the art to minimize the radiation/reception of electromagnetic from the wires and maximize the energy radiated/received from the electrodes forming the capacitive plates of the antenna.

The invention has been described in terms of preferred embodiments of the invention. However, modifications and improvements of the invention will be apparent to persons of ordinary skill in the art and the invention is intended to cover all such modifications and improvements which fall within the scope of the appended claims.