US3582952A

US3582952A - Short high-frequency antenna and feed system therefor

Info

Publication number: US3582952A
Application number: US789644A
Authority: US
Inventors: John L Heins
Original assignee: Aero Systems Inc
Current assignee: Aero Systems Inc
Priority date: 1968-12-11
Filing date: 1968-12-11
Publication date: 1971-06-01
Anticipated expiration: 1988-06-01

Abstract

An antenna system is described wherein a short antenna of about 0.1 lambda or less is employed with a coaxial cable feed. The coaxial cable is directly connected to ground at but one end thereof and is coupled to ground at the other end via a capacitor. The capacitor is selected to provide good matching of a transceiver to the antenna with or without a coupler. In an alternate antenna system a loading network is employed which is mounted within the antenna to provide a stably performing antenna throughout adverse weather conditions such as rain, snow or ice.

Description

United States Patent [72] Inventor John L. l-leins Massapequa Park, Long Island, N.Y. [21] Appl. No 789,644 [22] Filed Dec. 11, 1968 [45] Patented June I, 1971 [73] Assignee Aero Systems, Inc.

Miami, Fla.

[54] SHORT HIGH-FREQUENCY ANTENNA AND FEED SYSTEMTHEREFOR ISCIaim SDraWingFigs. s2 U.S.Cl. 343/750, 343/830, 343/831, 343/850 511 1111.01. H0lg9/30 so FieldofSearch 343/s2s- [56] References Cited UNITED STATES PATENTS 2,615,131 10/1952 Lindenblad 343/831X 2,913,722 11/1959 Brueckmann 343/830X 3,358,286 12/1967 Heins 343/750 Primary Examiner-Herman Karl Saalbach Assistant Examiner-Marvin Nussbaum Attorney-Hopgood and Calimafde /4 RAD/0 Mull/981 or J- J CWPZER 4 FIG.4

? INVENTOR.

JOHN z. l/f/NS PATENIED JUN 1 I97! FIGS FIGS

SHORT MIGlH-FREQUIEIWIY ANTENNA AND FEED SYSTEM THEREFOR this invention relates to an antenna system. More specifically, it relates to anantenna system for aircraft communication generally in and below the high frequency range.

In the communication field, antennas are employed which, for practical considerations, must have a very small electrical length. For instance, a typical short length antenna usable in the high frequency communication band from 2 to 30 MHz, is described in my Pat. No. MHZ. Short antennas are needed when one considers the dimensions of an antenna of one or several wavelengths long in comparison with the aircraft or building which is to support such antenna. The wavelength at 30 MHz. is of the order of 32%feet and the wavelength at 2 MHZ. is about 500 feet. Since it is generally desirable for efficiency and impedance matching considerations to communicate with antennas whose length is of the order of a quarter, a half or a full wavelength of the frequency at which communication is sought, it is clear that such antennas are impractical for aircraft applications. The aircraft itself is generally not longer than the wavelength at the low end of the high frequency communication band, i.e. 2 MHz. and even if future aircraft were to have such dimensions, it is impractical to consider employing an externally mounted antenna of such length when one considers the high operating speeds of jet aircraft.

When one employs antennas such as those used in the 2 to 30 MHZ. band, the radiation resistance of the antenna decreases. Practically, this means that if one were to use, for instance, a 4-foot-long antenna, the effective electrical length of the antenna at 30 MHz is approximately 0.125 A and at 2 MHz the electric length equals approximately 0.008 A. The corresponding decreases of the effective radiation resistance of the antenna becomes quite noticeable so that the antenna is not easily matched and the transmitter power is not efficiently radiated by the antenna. In addition, very short antennas tend to become very-reactive, requiring a variable loading capacitor and an antenna coupler. As described in my above-mentioned patent, an antenna coupler generally and preferably is placed adjacent the short antenna to tune out the various reactances throughout the range of desired operating frequencies. In this manner a substantially resistive impedance is presented to the radio transmitter or receiver connected to the coupler at the other end thereof.

In practice, however, it quite frequently occurs that the an tenna coupler cannot be physically located near the antenna. One may envisage such location problems which it is realized that an antenna coupler which includes a variable inductance in series with the line and a parallel variable capacitor weighs approximately 12 lbs. and occupies a space of about half of a cubic foot. In aircraft, such spacial requirement cannot easily be provided directly adjacent the antenna. Since an antenna frequently is located at aircraft extremities, the adjacent location of a heavy coupler may appreciably affect the location of the crafts center of gravity. For a tail-mounted antenna, the room available can be too small for the coupler so that an intermediate network such as a coaxial cable must be employed. To one skilled in the art, it can be appreciated that the insertion of a coaxial cable of constant characteristic impedance between a coupler and a highly reactive antenna effectively transforms the impedance of the antenna to a high virtually purely reactive component. This means that when the coupler tunes out the reactive components, the net resistive component presented to the transmitter or receiver tends to be exceedingly low.

Another factor that has been generally detrimental to a remotely located coupler is that the coaxial cable loss will vary throughout the high frequency spectrum from 2 to 30 MHz. with peaks and valleys separated from one another by as much as 30 to 40 db. The precise frequency location of such valleys and peaks will of course very with different lengths of coaxial cable. Since the 30 to 40 db. variation figure just quoted includes optimum adjustment of the antenna load capacitor and the coupler, it can be seen that any improvement that would reduce such large variations would greatly and reliably improve communication in the high'frequency passband.

It is therefore an object of this invention to provide an antenna system which presents a low cable loss with a short antenna.

It is a further object of this invention to provide an antenna system which includes a coupler that may be remotely located from a short, highly reactive antenna.

It is still further an object of this invention to provide an antenna system for an aircraft or alike small size vehicle wherein the antenna employed has a small effective electrical length yet performs with great efficiency in the transmission of systems and with little loss of received signal.

It is yet another object of this invention to provide an antenna system for the high frequency band or below, wherein a short antenna is employed of an effective electrical length generally less than about 0.1 of the operating wavelength.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will best be understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, the description of which follows.

FIG. 1 is a diagrammatic view of an antenna system as employed with an aircraft in accordance with my invention;

FIG. 2 is a generalized diagrammatic view of the embodiment of FIG. 1;

FIG. 3 is a diagrammatic view of another embodiment of the invention;

FIG. 4 is a diagrammatic view of still another embodiment in accordance with the invention; and

FIG. 5 is a diagrammatic view of a rearrangement of the embodiment of FIG. 3.

Briefly stated, my invention contemplates a short antenna of an effective electrical length generally less than about onetenth of the operating wavelength and wherein an antenna feed system is employed of the coaxial cable type. The coaxial cable outer conductor is selectively connected to ground at a first location by a direct connection and at a second location with a capacitor selected to 'provide a low loss antenna system.

In F IG. 1 is shown the rear fuselage of a plane 10 to which is mounted at the rear end thereof an antenna I2 which protrudes therefrom and is generally called a tail probe. The antenna is driven by a transmitter 14 via an antenna coupler 16 which is remotely located from the antenna and interconnected thereto by a coaxial cable 18. The antenna 12 is made generally in accordance with my Pat. No. 3,358,286 and com prises a hollow tube 20 of a length La which is generally less than about one-tenth of the operating wavelength. The hollow tube 20 has an inner conductor 22 which electrically and mechanically connects to the tube at the remote end 24 thereof. The other end of the inner conductor 22 is connected to ground via a capacitive loading network such as capacitor 26. The antenna tube 20 is insulatively mounted to the fuselage as is illustrated by the circumferential gap 28 and is fed by a cable system comprising the coaxial cable 18 from the coupler. The coaxial cable 18 is of the standard SO-ohm or 75- ohm type and includes an inner conductor 30 and an outer conductor 32. The outer conductor generally is grounded and in this instance is left floating at the antenna end where it is connected or coupled to ground through a capacitor 34, hereinafter referred to as the cable outer conductor grounding capacitor. The other end of the coaxial cable 18 near the coupler I6 is grounded by a direct connection to ground. For il- Iustration purposes, the length of the coaxial cable is indicated at Le and the spacing from the end of the outer conductor 32 to the fuselage end of the antenna is indicated as S.

The loading capacitor 26 generally is selected to provide adequate tuning capability of the antenna. The cable grounding capacitor 34, however, is selected commensurate with the length of the coaxial cable in order to provide the proper im-' pedance transformation needed to present the proper electrical resistance to the transmitter or receiver. Although it would be considered convenient to provide a generalized formula for determining the size of the loading capacitor and the cable grounding capacitor, as well as the length of the coaxial cable, in practice such installations do not lend themselves to rigorous mathematical analysis. Part of the reason resides in the fact that aircraft configurations vary as widely as can be imagined and their sizes affect the performance of an antenna.

Typically, in one aircraft the tail probe will be the most efficient and effective antenna for the high frequency communication band, whereas for another craft the bellymounted probe will produce better results. One can appreciate such variations when the wavelength of the communication frequency is taken into account. Typically, an aircraft total wing span length may be of the order of the wavelength employed. Thus, the aircraft is likely to affect the antenna pattern as well as the radiation resistance, and the reactance of the antenna. Since predictability is not a practical method for determining the size of the capacitors, other methods are employed. It is preferred that the capacitors are selected as follows.

First, the antenna is mounted on the craft at a suitable location and tests are performed to determine first the optimum loading capacitor and cable grounding capacitor in conjunction with the coupler tuning effect. One may perform such tests by either transmitting a known signal of known intensity level through the coupler to the antenna and monitoring the output as a function of the various capacitor values, or by measuring the input impedance of the antenna and coupler at the transmitter. By recording the capacitive values for the optimum or largest signal strength (or highest radiation resistance) over various frequencies throughout the band of interest, the 2 to 30 MHz. band, a set of data is obtained to select the capacitor 34 value. Such procedure for determining the various elements is well known in the art and further detailed description is not necessary. Suffice it to say here, however, that even where some compromises must be made, the improvement obtained by the placement of a cable ground capacitor 34 at the antenna end is so significant that the 30 to 40 db. variations referred to in the introduction are reduced to a ripple which has a peak value of about 4 db. Such small 4 db. fluctuation is considered barely noticeable since the aircraft orientation usually introduces variations in the received signal well in excess of such fluctuation.

In FIG. 2 I show a generalized version of the embodiment of FIG. 1 wherein a radio transceiver 14 is employed supplying and receiving a signal through the coupler 16 which is remotely located from the tubular antenna and connected thereto via a coaxial cable 18. The cable adjacent its antenna end is provided with a cable ground capacitor 34. Note that the load capacitor 26 is indicated as variable but the cable ground capacitor is indicated as a fixed value. For some aircraft configurations it is entirely possible that for optimum performance capability, both the load and the cable ground capacitor are variable. This may be easily accomplished by applying a pair of motors (not shown) in operative relationship with the cable ground and

loading capacitors

34, 26. The motors are controlled by the coupler 16 to establish the desired capacitive values.

In some installations such as of the fixed frequency type, a coupler for tuning of the short antenna is not needed and a fixed tuning capacitor may be employed. FIG. 3 is illustrative of such an embodiment wherein a radio transceiver is employed that is connected to the antenna via a coaxial cable 18 which, near the antenna end thereof, has its outer conductor connected to ground through capacitor 34. At the radio transceiver end, the outer conductor 32 is directly coupled to ground. By again selectively choosing the capacitor both for loading and cable grounding, a highly efficient transmitting system is obtained.

FIG. 4 is illustrative of an antenna system which is especially useful for low frequency (LF; 200 to 415 kHz.) transmissions such as in the 200 kHz. range. Again I employ a tubular antenna but within the antenna I mount a loading network 36 so that it may emerge from the bottom end of the antenna 12 at ground potential. In the particular embodiment of FIG. 4, the loading network comprises a coil 38 and a capacitor 26' tuned to provide the desired loading at the 200 to 415 kHz. frequency. The antenna is either fed by a coaxial cable 18 from a radio transceiver device 14 or from a coupler 16 closely mounted thereto. The cable 18 at the antenna end has its outer conductor connected to ground via coaxial cable grounding capacitor 34.

A significant advantage of the embodiment of FIG. 4 resides in the fact that the center conductor after passing through the loading network 36 emerges from the antenna at electrically ground potential. Thus, the accummulation of dirt, snow or rain does not have a strong effect on the performance. Typically, the length of the antenna of FIG. 4 may be approximately 60 feet which, at the wavelength of 1500 meters, provides an effective electrical length of approximately 0.0l225 of the operating wavelength. Yet, I have found that by employing an antenna system such as shown in FIG. 4, a relatively small ground plane may be used without a significant variation in the power radiated from the transmitter due to environmental effects.

In FIG. 5 I show a similar antenna system to that shown in FIG. 3, but the coaxial cable '18 in FIG. 5 has its antenna end directly coupled to ground and its coupler or radio transceiver end connected to ground through a cable grounding capacitor 34. The embodiment of FIG. 5 would be especially useful where both the cable ground capacitor 34 and the loading capacitor such as shown in FIG. 2 have to be variable. The variability of the cable grounding capacitor in the configura tion of FIG. 5 may be simply accomplished by merely attaching a wafer to a rotary switch that usually is employed within the coupler.

The hollow antenna used with this invention operates with very short electrical lengths. For instance, I have found that an antenna tube length of about 14 inches operates satisfactorily over the frequency range from to 1600 kHz. The electrical length of the antenna thus may be as small as 0.0001 of the operating wavelength. Such short antenna has particular utility on an aircraft.

While the principles of the invention have been described in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of the invention as set forth in the objects thereof and in the accompanying claims.

I claim:

I. An antenna system comprising an antenna including a hollow conducting tube having an electrical wavelength of less than 0.1 of the operating wavelength of the antenna, a coaxial cable feed connected to the antenna and having an inner and an outer conductor, said outer conductor being connected to ground at a preselected location thereof, a cable grounding capacitor electrically coupling a predetermined location of the cable outer conductor to ground, said hollow conducting tube further including an inner conductor electrically and mechanically connected to a tube end, and a loading network coupling the other end of the tube inner conductor to ground.

2. The antenna system as recited in claim 1 wherein said capacitor connects the outer conductor at its antenna end to ground.

3. The antenna system as recited in claim 1 where said capacitor connects the outer conductor to ground at an end thereof away from the antenna.

4. The antenna system as recited in claim ll wherein said loading network is mounted within the tube and emerges from said tube at substantially electric ground level.

5. The device as recited in claim 4 wherein the loading network comprises an inductance and a capacitance in series connection with one another.

6. The device as recited in claim 1 wherein the electrical length of the antenna is of the order of between 0.0001 to 0.00l of the antenna operating wavelength.

7. An antenna system comprising an antenna formed of a hollow conducting tube with an inner conductor-having an end electrically and mechanically connected to the tube at one end thereof, said tube having a length generally less than about 0.1 of its operating wavelength.

3 load capacitive reactance coupling the other end of the inner conductor to ground,

a coupler for coupling transmitting RF signals to the antenna and received RF signals from the antenna, said coupler being remotely located from said antenna,

a coaxial cable electrically interconnecting the remotely located coupler to the antenna and having a center conductor enclosed by an outer conductor,

said outer conductor being directly connected to ground and a cable outer conductor grounding capacitor coupling a predetermined location of the outer conductor to ground. 8. The device 'as recited in claim 7 wherein said cable grounding capacitor is selected commensurate with the length of the coaxial cable'to provide optimum transmission capability of the antenna over its operating frequency.

9. The device as recited in claim 7 wherein both said load capacitive reactance and said cable capacitor are variable for optimum performance of the antenna over its operating frequency.

10. The device as recited in claim 7 wherein said grounding capacitor is selected commensurate with the minimum optimum loss of signal in the cable over the operating frequency of the antenna.

11. The device as recited in claim 7 wherein said cable grounded capacitor is selected to provide optimum performance of the antenna at the low end of the operating frequency.

12. The device as recited in claim 7 wherein said cable grounded capacitor connects the antenna end of the outer conductor to ground.

13. The device as recited in claim 7 wherein said cable grounded capacitor connects the coupler end of the outer conductor to ground.

Claims

1. An antenna system comprising an antenna including a hollow conducting tube having an electrical wavelength of less than 0.1 of the operating wavelength of the antenna, a coaxial cable feed connected to the antenna and having an inner and an outer conductor, said outer conductor being connected to ground at a preselected location thereof, a cable grounding capacitor electrically coupling a predetermined location of the cable outer conductor to ground, said hollow conducting tube further including an inner conductor electrically and mechanically connected to a tube end, and a loading network coupling the other end of the tube inner conductor to ground.

4. The antenna system as recited in claim 1 wherein said loading network is mounted within the tube and emerges from said tube at substantially electric ground level.

6. The device as recited in claim 1 wherein the electrical length of the antenna is of the order of between 0.0001 to 0.001 of the antenna operating wavelength.

7. An antenna system comprising an antenna formed of a hollow conducting tube with an inner conductor having an end electrically and mechanically connected to the tube at one end thereof, said tube having a length generally less than about 0.1 of its operating wavelength. a load capacitive reactance coupling the other end of the inner conductor to ground, a coupler for coupling transmitting RF signals to the antenna and received RF signals from the antenna, said coupler being remotely located from said antenna, a coaxial cable electrically interconnecting the remotely located coupler to the antenna and having a center conductor enclosed by an outer conductor, said outer conductor being directly connected to ground and a cable outer conductor grounding capacitor coupling a predetermined location of the outer conductor to ground.

8. The device as recited in claim 7 wherein said cable grounding capacitor is selected commensurate with the length of the coaxial cable to provide optimum transmission capability of the antenna over its operating frequency.