WO1996029758A1

WO1996029758A1 - Variable capacitance antenna with constant impedance matching system for multi frequency reception and transmission

Info

Publication number: WO1996029758A1
Application number: PCT/US1996/003420
Authority: WO
Inventors: Art H. Unwin
Original assignee: Unwin Art H
Priority date: 1995-03-20
Filing date: 1996-03-13
Publication date: 1996-09-26
Also published as: AU5363396A; EP0815614A1

Abstract

A variable capacitance antenna allows individual adjustment of linear resonators on a beam antenna. A linear resonator is associated with each dipole element on a common support boom. A variable capacitance is positioned inside the boom and created using an arrangement of two coaxial conductive tubes as capacitive plates. One of the conductive tubes may be axially moved by a motor using a remote drive control. The movement of one conductive tube relative to the other can vary the capacitance from 0 to 100 picofarads, for example. The variable capacitance antenna can be used for both horizontal and vertical signal polarization applications.

Description

VARIABLE CAPACITANCE ANTENNA WITH

CONSTANT IMPEDANCE MATCHING SYSTEM

FOR MULTI FREQUENCY RECEPTION AND TRANSMISSION

BACKGROUND OF THE INVENTION 1. Field of the Invention

This invention relates to a system for efficient transfer of radio frequency

(RF) energy from an energy source to a radiating system or vice versa. More specifically, this invention relates to antennas that are capable of operating on more than one frequency band using remote tuning. Also, this invention relates to a matching system for efficient transfer of RF energy to and from antennas having at least one driven element. This invention is particularly useful for expanding the usable frequency span of an antenna at high efficiencies for amateur radio, commercial radio, and military applications.

2. Discussion of the Related Technology The operating bandwidth of any directional antenna may be specified in terms of standing wave ratio (SWR) on the feed line, pattern degradation, or loss of gain. The effective bandwidth of an antenna is commonly specified as a maximum value of SWR and is usually limited to 2:1 or 3:1. A low SWR is desirable to increase antenna efficiency. Operation of a high SWR on the effective bandwidth will result in a high SWR on the transmission line and a degradation of forward gain and front-to-back gain ratio. In most instances, bandwidth is limited by the matching device between the antenna and the signal feed line, rather than by the antenna characteristics. For example, when adjusted for maximum gain, the bandwidth of a typical three-element Yagi antenna is about 2.5 percent of the design frequency, due to SWR limitations. This means that an antenna array cut to 14.15 MHz would have a bandwidth of only about 350 kHz, centered on the design frequency, between the 2:1 SWR points on a transmission line. In like fashion, for an antenna beam designed for ten-meter operation at 28.5 MHz, the antenna array should be cut for low or high frequency operation in the band. Conventional beam antennas, such as a Yagi antenna, include at least one driver element tuned to resonate at a desired receive/transmit frequency band and positioned at right angles to a support boom. To increase the directivity of such an antenna, a parasitic reflector element, usually tuned to a frequency slightly higher than the driver resonant frequency, can be placed parallel to the driver element along the boom. For further increased directivity, one or more director elements, usually tuned to frequencies slightly lower than the driver resonant frequency, can be placed at various distances along the boom on the other side of the driver element and parallel to the driver element. The driver and other elements are electromag- netically coupled for maximum gain and directivity and are usually of approximately the same length. In these antennas, the driver and the other elements are basically dipoles which in combination are resonant for a particular frequency band.

Trapped dipole antennas, which are variations of the Yagi antenna, can accommodate up to three transmit/receive frequency bands. Trapped dipole antennas have elements of approximately the same length positioned on a common support boom similar to Yagi antennas. In addition, however, trapped dipole antennas have electrical circuits consisting of wound inductance and capacitance arrangements, commonly called traps, placed near the ends of each element to force each element to resonate at a desired frequency band. Wound inductances, however, have several drawbacks, including high loss and heat generation. Trapped dipole antennas are often used in the amateur radio field where a series of bands are available, because one antenna can be used for several selected frequency bands. In order to use all the frequencies available for amateur radio transmission, however, more than one trapped dipole antenna would be required to obtain maximum efficiency of the transmitted signal.

As with all antennas, efficient transfer of radio frequency (RF) energy from the energy source is of paramount importance, since any mismatch at this point determines in large part the possible frequency span that the antenna can be used for and also the amount of energy available for radiation. In relation to the above antenna, a constant impedance matching system is provided to expand the characteristics of the system for constant matching at the frequency of use. A theoretical variation on the trapped dipole antenna was described in Les

Moxon, HF Antennas for All Locations 122-43 (2d ed. 1992). This variation is similar to the trapped dipole antenna, except that a non-wound inductance/capacitance circuit is placed at the center of each element instead of at the ends of each element. The advantage of this linear resonator variation is that the antenna is electrically two antennas side by side in what is commonly known as a "double Zepp" arrangement. An element of this design exhibits more gain than an element of the trapped dipole design. This antenna has a higher efficiency than a trapped dipole antenna in multi-element form, but it is difficult to construct without increasing weight wind load and using specialized components.

Various matching methods and devices are discussed in The ARRL Handbook for Radio Amateurs 17-1 to 17-22 (The American Radio League 1992). This text also discusses in depth the relationships between matching devices and bandwidth.

SUMMARY OF THE INVENTION With the recent assignment of more bands for amateur radio use, the need for multiband antennas has increased. Multiband antennas are needed to transmit on more than one amateur radio frequency band, receive public short wave transmissions, and receive and transmit on frequencies between bands.

The variable capacitance antenna provides a beam antenna arrangement with multiband reception and transmission capabilities. In addition to multiband capabilities, the variable capacitance antenna provides fine multifrequency tuning capabilities. According to one embodiment, a variable capacitance arrangement associated with each antenna dipole element is installed inside the boom of an antenna. Adjusting the capacitance of each dipole element alters the gain, efficiency, and directivity of the antenna as a whole. The antenna capacitance may be remotely tuned to a selected frequency and the antenna remotely rotated to maximize gain and directivity and minimize surface area to reduce wind loading.

Adjusting the dimensions of the variable capacitance antenna makes it applicable for any frequency where resonant dipole elements can be used. Also, the variable capacitance antenna can be used to receive or transmit either horizontally polarized signals, such as amateur radio signals, or vertically polarized signals, such as commercial radio signals.

An advantage of this antenna is that it enables both the receiving and the transmitting of signals in a large number of frequency bands. Another advantage of this antenna is that it has minimal weight and wind load. Another advantage of this antenna is that it supplies high efficiency and high gain yet has a minimal number of "unused" elements. Yet another advantage of this antenna is that it is capable of remote tuning to maximize efficiency for each and every frequency within a frequency band or series of frequency bands. Yet another advantage of this antenna is that it provides tuning such that a desired frequency having a weak signal may be received clearly even if signals on nearby frequencies are strong. Yet another advantage of this antenna is that, by remotely tuning the antenna during broadcast of signals, it minimizes or removes interference with received television signals.

The constant impedance matching system is similar to the popular delta matching system, but instead of a point contact from a transmission feed line to a radiating element, the constant impedance matching system uses capacitive coupling. Capacitive coupling is achieved by placing capacitive coupling elements proximal to and in parallel with the driven element. Additionally, capacitive coupling elements may be extended by winding a conductive extension around the driven element but having the extension not directly in contact with the driven element. These capacitive coupling elements may be in various forms such as metal rods, metal wire, or even conductive adhesive tape. These capacitive coupling elements, with or without extensions, allow RF energy to flow to the radiating element at the point of best impedance match. This point changes with frequency, the placement of the antenna, and the working height of the antenna, but it will transfer RF energy at the best matching point regardless of the height of the antenna and the antenna's environment. By following this method of matching, present delta match driven arrays may be modified to have a wider operating bandwidth and lower SWR curve, and the antenna arrays themselves may be cut and tuned for better gain and directive pattern arrangement. When using capacitive coupling elements, the effective bandwidth of an antenna array is limited only by the antenna characteristics and not the matching system. A switch may be provided to directly connect (i.e., short) and disconnect the capacitive coupling elements from the driven element and allow a choice between the broader frequency response with a flatter SWR curve and a focused frequency response with a sharper SWR curve. Also, use of capacitive coupling elements reduces some frequency sensitivities of an antenna and allows radiating phasing lines to connect a driven element to a parasitic element to drive the parasitic element in phase or out of phase with the driven element. A shunt capacitor (or capacitors) may be used with the capacitive coupling elements to provide increased frequency coverage compared to the capacitive coupling elements alone. A capacitor electrically connected to the driven element, but placed at an appropriate distance from the driven element to prevent intercomponent capacitive coupling, promotes phase coherence on both sides of the transmission feed point. A shunt capacitance allows the antenna to have broader gain characteristics and flattens the SWR curve.

Replacing the conventional delta match broadens the frequency response of the system. If one or more motor-driven variable capacitors of the Variable Capacitance Antenna is exchanged for a more commercially available fixed value capacitor, which is small and does not have to be protected from the environment to the same extent as moving parts require, the broad frequency response of the antenna can be retained at the small expense of less focused tuning.

Other advantages of the constant impedance matching system for vertical antennas is that the feed point can be moved higher than the conventional feed point at the center or the base of a radiating element, which will provide different gain at a lower radiation angle, by taking advantage of the height of the feed point. This higher feed point location also decreases cosmic noise reception, thus lowering the noise floor. Another advantage is that a vertical all-band antenna can be used as an environmentally-friendly flag pole or other support by placing the transmission cable within a hollow radiating element. The capacitive coupling elements could be outside the pole but have a low profile.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a variable capacitance antenna having three dipole elements.

Figure 2 shows a cross section of the support boom of a variable capacitance antenna detailing the structure of the variable capacitor portion and the remote tuning portion. Figure 3 shows a rack and pinion movement for the remote tuning portion of a variable capacitance antenna.

Figure 4 shows a variable capacitor portion for use with two elements. Figure 5 shows a first embodiment of a constant impedance matching system having capacitive coupling elements in the form of coupling rods.

Figure 5A is a cross section along line A-A of Figure 5 that details the important dimensions that can affect the degree of coupling capacitance or impedance matching.

Figure 6 shows a second embodiment of a constant impedance matching system having a dielectric material interface to capacitive coupling elements.

Figures 7A and 7B show a prior art delta matching system and a delta matching system with capacitive coupling elements. Figures 8A and 8B show a prior art balanced-to-unbalanced delta matching system and a balanced-to-unbalanced delta matching system with capacitive coupling elements.

Figure 9 shows a third embodiment of a constant impedance matching system having a shunt capacitance. Figure 10 shows a fourth embodiment of a constant impedance matching system having multiple shunt capacitances.

Figure 11 shows a fifth embodiment of a constant impedance matching system having a transmission feed line inside a driven element.

Figure 12 shows how a current feed searches for a good impedance match to make an efficient transition point.

Figure 13 shows how the embodiments of the constant impedance matching system can provide additional band coverage and gain in addition to the broadening effect supplied by the capacitive coupling elements.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 1 shows an embodiment of the variable capacitance antenna having three dipole elements. The director element 1, driver element 2, and reflector element 3 are shown mounted on a common support boom 4. The variable capacitance antenna requires at least one driver element 2, however, there can be any number of directors including zero. In a preferred embodiment, the variable capacitance antenna contains one director element 1 and one reflector element 3, however, reflector elements may also be any number including zero. Preferably, the three dipole elements are made of light-weight, electrically conductive material, such as aluminum alloy. Each of the dipole elements is approximately of length L, which preferably is approximately half the wavelength of the lowest frequency of interest. If the wavelength of the lowest frequency of interest is between 10 and 20 meters, then each dipole element length L would be approximately 10 meters, or 34 feet. Each element can have any diameter, however, a diameter of 1 1/2 inches is suggested, with a tapered design for minimal wind load. Each dipole element should be electrically isolated from the boom if the support boom 4 is made of a conductive material. To provide sufficient strength to support the dipole elements yet weigh a minimal amount, the support boom 4 is preferably a thin wall aluminum alloy tube with an outside diameter of 2 to 3 inches. A fiberglass or fiberglass and aluminum boom, however, is also acceptable. For a four element variable capacitance antenna designed to receive/transmit wavelengths of 10 to 20 meters, the boom length N is preferably 8 to 10 feet. For a two element antenna, the boom length N can be considerably shorter.

Connection of a transmission line to the variable capacitance antenna can be completed by the traditional delta match to the driver element 2 of the antenna at points P and Q shown in Figure 1. The distance between points P and Q depends on the impedance of the transmission line, but it is expected to be five feet for a 34 foot driver element.

In the center portion of each element is a non-wound inductor/capacitor arrangement, commonly known as a linear resonator. A capacitor portion 20 of a linear resonator is inside the boom 4 and connected to a dipole element using conductive connecting wires 5, 6 such as wire braiding or tubing. A non-wound inductance portion 21 of the linear resonator may be formed at the center of each dipole element, between the connection points of connecting wires 5, 6 on the dipole element. In one embodiment, the inductor portion 21 of the linear resonator is of length M, which can be approximately 7 feet for a dipole element 34 feet long. The structure of an existing Yagi antenna can be modified to include a linear resonator with a variable capacitance portion inside the Yagi antenna boom. For a single element antenna, alternatively, capacitor portion 20 may be placed inside the element. Connecting screws 7, 8 should be spaced 4 inches or more away from inductor portion 21 of the linear resonator to prevent capacitive coupling between the connecting wires and their associated connecting screws 5, 7 and 6, 8 and the inductor portion 21. Connecting screws 7, 8 attach the connecting wires 5, 6 from the inductor portion 21 of a dipole element to the capacitor portion 20 inside the boom 4. Connecting screws 7, 8 are described in detail in Figure 2.

Figure 2 shows a cross section of the boom of the variable capacitance antenna detailing the capacitor portion 20 of a linear resonator which includes conductive tubes 9, 13. Tubes 9, 13 are effectively the conductive plates of a variable coaxial cylinder capacitor. Advantageously, tubes 9, 13 are constructed of an aluminum alloy to provide a strong, lightweight, variable capacitor. Preferably, tube 13 has a length of 18 inches. Tube 13 may be slide fit onto a 1/4 inch inner diameter nonconductive tube 12 approximately 20 inches shorter than the length of boom 4. Tube 13 can be secured to tube 12 by a detente 16 made by a center punch. Tube 13 may be electrically isolated from tube 9 by nonconductor tube 11 having a 3/4 inch inner diameter inside the length of boom 4. For an antenna with a short boom length, however, if nonconducting tube 11 is of sufficient strength and length, then it can take the place of boom 4, i.e., boom 4 is not required (as shown in Figure 4). Tube 9 should be approximately 2 inches shorter than tube 13, or approximately 16 inches, to enable electrical contact to connecting wire 5. In a preferred embodiment, tubes 9 and 11 are stationary while tube 12 (and associated tube 13) is moveable. Advantageously, nonconductive tubes 11, 12 are constructed of lightweight plastic. The electrical isolation of conductive tubes 9, 13 by nonconductive tube 11 prevents high frequency voltage breakdown and arcing. Capacitance is measured between electrically conductive connecting screws 7 and 8. Screws 7, 8 are electrically isolated from the boom 4 to prevent stray electrical coupling. Screw 7 makes an electrical connection between connecting wire 5 and conductive tube 9, and screw 8 makes an electrical connection between connecting wire 6 and conductive tube 13. Screw 7 directly connects electrically and mechanically to tubes 9 and 11. Screw 7 provides mechanical coupling between conductive tube 9 and nonconductive tube 11 to prevent movement of either tube 9 or tube 11 inside the boom 4. Screw 8 connects to conductive tube 13 through tube 10 and contact 15. Preferably, contact 15 is made using a flexible, insulated steel wire spring approximately 1/4 inch wide and .010 inches thick in the shape of half a coil. A opening at 45° can be made in nonconducting tube 11 that allows insertion of contact 15 through tube 11 to electrically connect with tube 13. Additionally, screw 8 prevents movement between tube 10 and tube 11. Alternatively, the contact can be made of linear bearings 15A as shown in Figure 3. Linear bearings provide multiple points of contact to conductive tube 13 and an increased voltage rating for the variable capacitor due to the air gap between tubes 13 and 9 in addition to nonconductive tube 11, however, linear bearings do not provide the desirable scrubbing action that wire contact 15 provides.

Antenna capacitance varies when conductive tube 13 moves relative to conductive tube 9 along the axis of the boom 4. Nonconductive tubes 11, 12 provide a "track" for the movement of conductive tube 13. Positioning the tubes so that stationary conductive tube 9 and moveable conductive tube 13 are maximally coupling (fully overlapping) provides, for example, a capacitance of approximately 100 picofarads. Moving tube 13 so that the conductive tubes 9, 13 are completely decoupled (nonoverlapping) produces zero capacitance and decouples the dipole element associated with the variable capacitor. Figure 2 shows conductive tubes 9 and 13 completely decoupled. The exact amount of capacitance provided by this arrangement is determined by the surface area of conductive tubes 9, 13, the separation distance between the conductive tubes 9, 13, and the distance of the coupled length thereof. By varying the diameter of tube 9 along its length, capacitance change may be made nonlinear, which can be advantageous for multi- element antennas used for multifrequency purposes.

Figure 2 represents capacitor portion 20 inside the boom which can be duplicated for each dipole element. Moving the inner conductive tubes along the axis of the boom relative to the outer conductive tubes provides a change of capacitance for each dipole element. This variable capacitance arrangement makes possible fine adjustments to the capacitance of the dipole elements. This change in capacitance varies the resonance frequency of the dipole elements and provides an antenna with high efficiency radiation transfer and gain with directivity. Additionally, the variable capacitor portion 20 enables the antenna to have a sharp frequency focus so that the antenna can receive weak signals on one frequency and reject strong signals on nearby frequencies.

The maximum coupling length of the conductive tubes of the capacitor portion in the reflector element 3 and director element 1 can be varied by +10% and -10%, respectively, compared to the maximum coupling length of the conductive tubes in the capacitor portion of driver element 2. Because the dipole elements are variably tunable, this variable capacitance antenna is capable of receiving and transmitting in more frequency bands with a higher gain for any particular frequency than previous antennas. Each dipole element can also be independently tunable by providing separate, shorter, nonconductive tubes for each dipole element (rather than a single long nonconductive tube 12 for all the dipole elements) as a track for each inner conductive tube 13.

In a preferred embodiment, controlling the movement of tube 13 is accomplished by using a reversible low-speed motor or step motor 17. Motor 17 may be connected near the center of capacitor portion 20 as shown in Figure 2, or it may be connected near an end of capacitor portion 20 as shown in Figure 3. As shown in Figure 2, movement of tube 13 can be attained by winding a cord 19 secured to tube 12 in two places and wrapping the cord 19 around the motor shaft 18A of the motor 17. Tube 11 may be cut away at appropriate points to prevent tube 11 from impeding the movement of the cord 19. Since there is not much friction between tube 11 and tube 13, pulling forces (torque) on the cord 19 can be as low as 15 lbs/in for a 40 foot boom. Reversal of the direction of motor shaft 18A may be accomplished by reversing the electrical connection to the motor 17, which can be controlled remotely along with fine adjustments of the motor speed. Preferably, nonconductive spacers 14 prevent tubes 9, 11, 12, 13 of the capacitor portion 20 from bumping into the inner walls of the boom 4. Advantageously, tube 11 has a low friction interface such as linear bearings 22 on its inner diameter to reduce friction between tube 11 and tube 12 and prevent side thrust forces from slowing or stopping the motor.

High friction assemblies can use more positive methods of movement such as rack and pinion movement between motor 17 and tube 12 as shown in Figure 3. Rack portion 25 may be attached to or integrated into nonconductive tube 12. The rack portion 25 interacts with pinion motor shaft 18B to provide force to move nonconductive tube 12 and associated conductive tube 13 along the axis of boom 4. Note that Figure 3 shows conductive tube 13 partially coupled with conductive tube 9.

For a single element vertical antenna, motor 17 may be located at the bottom of the element to allow easy access and provide for nut and screw rotation methods or mobile antenna retraction drive systems to provide axial movement of tube 13 within the driven element. Varying the capacitance of each dipole element, as opposed to having the same capacitance for each dipole element, will vary the footprint of radiation greatly. If each dipole element is of the same length, making the reflector element 3 (shown in Figure 1) slightly more capacitive than the driver element 2 (shown in Figure 1) and the director element 1 (shown in Figure 1) slightly less capacitive than the driver element 2 produces maximum gain from the direction of the driver element 2 to the director element 1 as per a conventional Yagi antenna design. By varying dipole element lengths, the variable capacitance antenna can provide a reversal of maximum gain direction for some frequencies within the design of the antenna. For additional variability in directivity, the entire antenna structure may be rotatable about its horizontal axis for vertical polarization applications.

Figure 4 shows a variable capacitor portion without a boom for use with two elements. In this embodiment, two outer conductive tubes 9a, 9b share one inner conductive tube 13. Figure 4 shows the variable capacitor completely decoupled. However, moving conductive tube 13 toward the right will couple conductive tube 9a and its associated dipole element through connecting wires 5a, 6a. Then, moving conductive tube 13 back toward the left will decouple conductive tube 9a and its associated dipole element and couple conductive tube 9b and its associated dipole element through connecting wires 5b, 6b. This feature is advantageous in that it allows tailoring of the spacing between coupled elements in a multi-element antenna. The spacing of the coupled elements are important in determining the gain, establishing the front to back ratio, and ensuring that element gains or phases are additive rather than subtractive. Figure 5 shows a first embodiment of a constant impedance matching system having capacitive coupling elements in the form of coupling rods. Element 2 is a driven element of an antenna preferably made of a light-weight, electrically conductive material, such as aluminum. Element 2 may be part of an antenna array having parasitic element 1, and element 2 and can be any length depending on the frequencies of interest. Capacitive coupling elements may be in the form of conductive coupling rods or coupling wires. Coupling rods 501a, 501b can be placed in a parallel fashion alongside element 2, but with an optional direct electrical or direct physical connection between the rods 501a, 501b and the element 2. If element 2 is approximately thirty-four feet in length, coupling rods 501a, 501b, may each be approximately two feet in length, with a spacing 503 of approximately four inches between the rods. Note that coupling rods 501a, 501b do not necessarily have the same length, nor do they have to be placed symmetrically about the center of the radiating element. Note also that conductive wire can easily be substituted for conductive rods as capacitive coupling elements.

Preferably, connecting wires 505a, 505b attached to coupling rods 501a, 501b are made of aluminum wire at least one-tenth of an inch in diameter, each approximately two feet long. The connecting wires may be attached at opposite ends of the coupling rods 501a, 501b or at any other point along the coupling rods. Connecting wires 505a, 505b provide an electrical connection between the coupling rods 501a, 501b and an impedance transformer 506 which may have a 4:1 ratio and provide a balanced match to a fifty ohm coaxial cable 507, which is termed an unbalanced transmission line.

Gaps 504a, 504b between element 2 and coupling rods 501a, 501b should be as small as possible to ensure optimal capacitive coupling. Gaps 504a, 504b of .03 inches, however, generally provide acceptable impedance matching. Note that gaps 504a, 504b do not have to be identical. If high voltages are present, a dielectric air gap could be replaced by a suitable dielectric material as shown in Figure 6.

Switches 508a, 508b can be installed to directly connect connecting wires 505a, 505b to the radiating element 2 via the capacitive coupling elements 501a, 501b as per conventional matching systems (shown in Figures 7 A and 8A). Closed switches short the capacitive coupling elements directly to the radiating element. Closing switches 508a, 508b makes a fixed point connection from the radiating element to the transmission cable and produces the narrow focused frequency response with sharp SWR curve of conventional matching systems. Opening the switches produces a broadened frequency response with a flattened SWR curve. One or more radiating phasing connections 509a, 509b may connect driven element 2 to parasitic element 1 in an antenna array when capacitive coupling elements are used. These radiating connections 509a, 509b may be used to drive parasitic element 1 in phase or out of phase with respect to the driven element, because the capacitive coupling elements allow the radiating element to be less frequency and wavelength conscious. Although radiating connections 509a, 509b are shown as convergent connections, the radiating connections may alternatively be divergent, parallel, or asymmetrical. Note that these radiating phasing connections 509a, 509b are direct, radiating connections; they are not non-radiating transmission line connections of a specific length, such as quarterwave transmission lines. Also in contrast to quarterwave transmission lines, the lengths of the radiating connections are not as critical.

Figure 5A is a cross section along line A-A of Figure 5 that details the important dimensions that can affect the degree of coupling capacitance or impedance matching. For an element 2 of thirty-four feet in length, O_t could be approximately one-half inch in outside diameter and D₂ could be approximately 1-1/4 inches in outside diameter. The spacing S between the centers of element 2 and rod 501b could be one inch if the dielectric gap 404a is one-eighth of an inch. A small gap is desirable to improve capacitive coupling and reduce the antenna's profile.

A conductive tape or strip may be used along with dielectric tape, instead of coupling rods or coupling wire, to create other forms of capacitive coupling elements.

Figure 6 shows a second embodiment of the constant impedance matching system having a dielectric material interface to the capacitive coupling elements 501a in the form of a coupling rod with dielectric material interface 602 and the other capacitive coupling element 603 is in the form of conductive adhesive tape with dielectric material interface 601. A dielectric material, such as Teflon™ tape 601, is wrapped around driven element to create a suitable dielectric material interface between coupling rod 501a and radiating element 2. This figure also shows conductive extension 604 electrically connected to capacitive coupling element 501a. A conductive extension could be used to increase the capacitive coupling available to the system. Preferably, conductive extension 604 is an insulated wire at least one-tenth of an inch in diameter helically wrapped around driven element 2. Alternatively, conductive extension 604 could be a uninsulated wire, and dielectric material interface 602 could be extended to provide an interface for the uninsulated wire. In one embodiment with a capacitive coupling rod of four feet in length, the conductive extension was approximately thirteen feet in length with ten turns along thirteen feet of the driven element. Preferably, the turns are "loose" in order to prevent inductance along the conductive extension.

For the other capacitive coupling element, another dielectric interface 601 is created (or the first dielectric interface could be extended), and conductive tape 603 is wrapped outside of the dielectric interface to achieve capacitive coupling of the coaxial cable 507 through impedance transformer 506 via connecting wire 505a, 505b. Note that conductive tape 603 may be easily replaced with a conductive sheet of aluminum or other conductive material. Also, the conductive material need not wrap completely around the radiating element.

Note that in any embodiment, any form of capacitive coupling element or dielectric interface may be substituted for another form. For example, coupling rods may be substituted for coupling wires or conductive tape and vice versa. Note that a capacitive coupling element made of wire and a conductive extension made of wire may be a single length of wire loosely wrapped around a length of a driven element. As another example, an air dielectric interface could be substituted for a dielectric material interface such as tape or insulation around a wire. Figures 7A and 7B show a prior art delta matching system and a delta matching system with capacitive coupling elements in the form of coupling rods. A typical delta matching system as shown in Figure 7A has balanced lines (or coaxial baluns) 505a, 505b attached to element 2 at fixed points of best impedance match for the frequency of interest. Replacing the fixed points with coupling rods 501a, 501b as shown in Figure 7B broadens the frequency response of the system by supplying an impedance match for more than one frequency and flattening the SWR curve. Figures 8A and 8B show a prior art balanced-to-unbalanced delta matching system and a balanced-to-unbalanced delta matching system with capacitive coupling elements in the form of coupling rods. Figure 8A shows a delta match with lines 505a, 505b attached to a balanced-to-unbalanced transformer 506 connected to a coaxial cable 507. In this situation, the frequency response and SWR of the system may be improved by replacing the fixed-point connections of the prior art matching system with coupling rods 508a, 508b as shown in Figure 8B.

Figure 9 shows a third embodiment of a constant impedance matching system having a shunt capacitance. In addition to the elements shown in Figure 5, Figure 9 includes a high voltage capacitor 94a electrically connected to and positioned an appropriate distance from element 2. For a frequency range of 7-155 MHz, this capacitor may have a fixed value of approximately 10-100 pf and 4 Kv with-stand voltage. A variable capacitor, of course, may be used instead of a fixed capacitor. A shunt capacitance may mounted on any unsevered radiating element. Capacitor 94a may be electrically connected to stand-off arms 91a, 91b by aluminum wire 93a, 93b or other conductive material. Stand-off arms 91a, 91b may be made of aluminum rod of one-quarter inch diameter and bent in a fashion that enables them to be clamped to element 2 for electrical connection. Clamps 92a, 92b may be common pipe clamps that hold capacitor 94a and wires 93a, 93b at a certain distance 92 away from element 2 to prevent intercomponent capacitive coupling. With an element 2 of thirty-four feet in length and 1-1/4 inches in outside diameter, distance 92 is preferably six inches. Of course, other methods and elements may be used to position capacitor 94a an appropriate distance 92 from element 2.

The distance 95, 96 of stand-off arms 91a, 91b from coupling rods 508a, 508b can be approximately six inches. Note, however, that the shunt capacitor does not have to be positioned directly centered across from the coupling rods. Instead, the shunt capacitor may be offset from the center of the coupling rods. Additionally, stand-off arms 91a, 91b do not have to be positioned symmetrically around coupling rods 501a, 501b. Instead, stand-off arms may be positioned asymmetrically with respect to the coupling rods, or both stand-off arms may even be on the same side of the coupling rods. This embodiment provides a radiating system with increased gain compared to the embodiments without a shunt capacitance. A shunt capacitor (or capacitors) in conjunction with capacitive coupling elements provides for increased frequency coverage when compared to the capacitive coupling elements alone as shown in Figures 7 and 8. Notably, this embodiment allows usage of all amateur radio frequency bands between 7 MHz and 30 MHz and even 144 MHz, all with an acceptable SWR in both the horizontal and vertical planes.

Figure 10 shows a fourth embodiment of the constant impedance matching system having multiple shunt capacitances. As noted before, the position of a shunt capacitance with respect to the capacitive coupling elements is not critical. In fact, several individual capacitors 94a, 94b and 94c may be placed along driven element 2 to improve the electrical characteristics of the antenna.

If desired, one or more of the variable capacitor portions 94 may be replaced by an inexpensive fixed-value capacitor. Although the sharpness of frequency tuning will be reduced by the removal of a variable capacitor portion, the capacitive coupling elements allow the antenna to retain a broad frequency response and high gain while contributing an improved SWR curve.

Figure 11 shows a fifth embodiment of the constant impedance matching system having a transmission feed line inside a driven element. This embodiment is preferably for use in a vertical all-band antenna. Capacitive coupling elements 1104, 1105 in the form of strips of conductive adhesive tape are attached to the outside of driven element 1 using a dielectric interface 1103, such as Teflon™ tape. Connecting wires 505a, 505b travel through insulated holes in the radiating element, which are hidden and electrically shielded, and connect the capacitive coupling elements 1104, 1105 to coaxial cable 507 located inside the driven element 2. Shunt capacitance 94a may also be placed inside the driven element and connected to the outer surface of the driven element through electrically shielded openings 1101, 1102. Note that shunt capacitance can be placed anywhere along the length of the driven element, and the shunt capacitance could also be attached to the outside of the driven element if desired. A shunt capacitance used with this embodiment can transform a vertical driven element of approximately forty feet in height to a multi¬ band antenna for frequencies from as low as 7 MHz to high frequency bands up to 30 MHz, and it could also used in the very high frequency range of 144 MHz and above.

Figure 12 shows how a current feed searches for an impedance match to make an efficient transition point. Graph C with points C., C₂, C₃, C₄, . . ., C_N graphically represent the changing impedance amplitude points on driven element 2 with respect to frequency f . Depending on the impedance amplitude at a given frequency, current I will capacitively couple to radiating element 2 at point I,, I₂, 1₃, I„, . . ., or I_N on coupling rods 501a, 501b. This optimal impedance matching provides a broader frequency response than conventional matching techniques. Figure 13 shows how the third embodiment of the constant impedance matching system can provide additional bandwidth coverage and gain in addition to the broader frequency response effect supplied by the capacitive coupling elements alone. Capacitor 94a in conjunction with coupling rods 501a, 501b creates a current flow I that is in phase on both sides of the feed point. This phase coherence allows the antenna to have broader gain characteristics and flattens the SWR curve to create a desirable lower SWR.

Although the present invention and its advantages has been described in detail, it should be understood that various changes, substitutions, and alterations can be made without departing from the spirit and scope of the invention as defined by the appended claims. Therefore, the presented embodiments should be considered in all respects as illustrative and not restrictive and all modifications falling within the meaning and equivalency range of the appended claims are intended to be embraced therein.

Claims

CLAIMS We claim:

1. A variable capacitance antenna comprising: a support boom exhibiting an elongated cavity; a dipole element mounted on the support boom; and a variable capacitor positioned in the elongated cavity and electrically connected to the dipole element.

2. A variable capacitance antenna according to claim 1, further comprising: an inductor connected in parallel with the variable capacitor.

3. A variable capacitance antenna according to claim 1, wherein the variable capacitor comprises: a first conductive surface electrically connected to the dipole element; and a second conductive surface located proximally to the first conductive surface and electrically connected to the dipole element.

4. A variable capacitance antenna according to claim 3, wherein the first conductive surface is a first conductive tube; and the second conductive surface is a second conductive tube arranged coaxially with the first conductive tube, and the second conductive tube diameter is greater than the first conductive tube diameter.

5. A variable capacitance antenna according to claim 4, wherein at least one of the conductive tubes is axially displaceable.

6. A variable capacitance antenna according to claim 5, wherein the diameter of at least one of the conductive tubes varies along its length.

7. A variable capacitance antenna according to claim 5, wherein the second conductive tube is fixed.

8. A variable capacitance antenna according to claim 5 further comprising: a sliding contact electrically connected to the first conductive tube, wherein the sliding contact is connected to the dipole element.

9. A variable capacitance antenna according to claim 7 further comprising: a drive connected to the first conductive tube.

10. A variable capacitance antenna according to claim 9, wherein the drive is a reversible motor.

11. A variable capacitance antenna according to claim 9, further comprising: a remotely located drive control.

12. A variable capacitance antenna according to claim 3, wherein the variable capacitor further comprises: a coaxially aligned nonconductive tube arranged between the first conductive tube and the second conductive tube.

13. A variable capacitance antenna according to claim 12, wherein an interface between the nonconductive tube and the first conductive tube is a low friction interface.

14. A variable capacitance antenna comprising: a dipole element exhibiting an elongated cavity; and a variable capacitor positioned in the elongated cavity and electrically connected to the dipole element.

15. A variable capacitance antenna according to claim 14 further comprising: a drive connected to the variable capacitor.

16. A variable capacitance antenna comprising: a support boom exhibiting an elongated cavity; at least two dipole elements mounted on the support boom; and at least one variable capacitor positioned in the elongated cavity and electrically connected to at least one dipole element.

17. A variable capacitance antenna according to claim 16 comprising: two dipole elements; and two variable capacitors, wherein each dipole element is electrically associated with a single variable capacitor.

18. A variable capacitance antenna according to claim 16 comprising: three dipole elements; and three variable capacitors, wherein each dipole element is electrically associated with a single variable capacitor.

19. A variable capacitance antenna according to claim 16, wherein the variable capacitor comprises: an axially displaceable first conductive tube; a coaxially aligned second conductive tube, wherein the second conductive tube diameter is greater than the first conductive tube diameter and the second conductive tube is electrically connected to a first dipole element; and a coaxially aligned third conductive tube axially displaced from the second conductive tube, wherein the third conductive tube diameter is greater than the first conductive tube diameter and the third conductive tube is electrically connected to a second dipole element.

20. A variable capacitance antenna according to claim 16, wherein one of the at least two dipole elements is shorter than another of the at least two dipole elements.

21. A variable capacitance antenna comprising: a dipole element; and an insulated variable capacitor, wherein the dipole element is mounted on the insulated variable capacitor.

22. A constant impedance matching system comprising: a radiating element; and a capacitive coupling element placed proximal to the radiating element and electrically connected to a transmission line for capacitively coupling the transmission line to the radiating element.

23. A constant impedance matching system according to claim 22, further comprising: a switch connected between the radiating element and the capacitive coupling element.

24. A constant impedance matching system according to claim 22, further comprising: a dielectric interface between the radiating element and the capacitive coupling element.

25. A constant impedance matching system according to claim 24, wherein the dielectric interface is air.

26. A constant impedance matching system according to claim 24, wherein the dielectric interface is dielectric material.

27. A constant impedance matching system according to claim 22, further comprising: a parasitic element; a radiating line directly connecting the radiating element to the parasitic element for driving the parasitic element.

28. A constant impedance matching system according to claim 22, further comprising: a shunt capacitance connected to the radiating element, wherein the shunt capacitance is at a distance from the radiating element so as to prevent intercomponent capacitive coupling between the shunt capacitance and the radiating element.

29. A constant impedance matching system according to claim 22, further comprising: a variable capacitor electrically connected to the radiating element for focused frequency tuning.

30. A constant impedance matching system according to claim 22, further comprising: a conductive extension electrically connected to the capacitive coupling element for increasing the capacitive coupling of the transmission line to the radiating element.

31. A constant impedance matching system according to claim 30, wherein the conductive extension is a wire.

32. A constant impedance matching system according to claim 31, wherein the wire is insulated.

33. A constant impedance matching system comprising: a radiating element exhibiting an elongated structure; and a conductive element exhibiting an elongated structure placed proximal to and parallel to the radiating element and electrically connected to a transmission line for capacitively coupling the transmission line to the radiating element.

34. A constant impedance matching system comprising: a radiating element; a first capacitive coupling element located proximal to the radiating element and electrically connected to a transmission cable for capacitively coupling the first transmission line to the radiating element; and a second capacitive coupling element located proximate to the radiating element and electrically connected to the transmission cable for capacitively coupling the second transmission line to the radiating element.

35. A constant impedance matching system according to claim 34, wherein the first capacitive coupling element comprises a conductive rod.

36. A constant impedance matching system according to claim 34, wherein the first capacitive coupling element comprises conductive wire.

37. A constant impedance matching system according to claim 36, wherein the conductive wire is insulated.

38. A constant impedance matching system according to claim 34, wherein the first capacitive coupling element comprises conductive adhesive tape.

39. A constant impedance matching system according to claim 34, further comprising: at least one shunt capacitance electrically connected to and located proximate to the radiating element.

40. A constant impedance matching system according to claim 34, further comprising: a first switch connected between the radiating element and the first capacitive coupling element; a second switch connected between the radiating element and the second capacitive coupling element.

41. A constant impedance matching system according to claim 34, further comprising: a variable capacitor electrically connected to the radiating element for focused frequency tuning.

42. A constant impedance matching system comprising: a radiating element exhibiting an elongated cavity; a capacitive coupling element located proximal to the radiating element and electrically connected to a transmission line for capacitively coupling the transmission line to the radiating element; and a transmission line located inside of the elongated cavity.

43. A constant impedance matching system according to claim 42, wherein the capacitive coupling element is located outside of the elongated cavity.

44. A constant impedance matching system according to claim 42, further comprising: a shunt capacitance located inside of the elongated cavity.

45. A method for improved matching of a transmission line to a radiating element comprising: directly electrically connecting the transmission line to a capacitive coupling element; capacitively coupling the transmission line to the radiating element through the capacitive coupling element.

46. A method for improved matching of a transmission line to a radiating element according to claim 45, further comprising: directly connecting a parasitic element to the radiating element using a radiating line.

47. A method for improved matching of a transmission line to a radiating element according to claim 45, further comprising: shorting the capacitive coupling element to the radiating element.