Tactical high frequency antenna

BACKGROUND OF THE INVENTION

The present invention relates to antennas of electromagnetic radiation and more particularly to a transportable antenna system which is adapted to radiate electromagnetic energy efficiently from a very short radiator compared to the wavelength of the radiated energyin military applications, the dimensions of portable ground based or vehicular high frequency antennas are necessarily a small fraction of the wavelength. It is well known that ground based antennas which are substantially smaller than λ/4 are usually inefficient electromagnetic radiators. In addition the required wide frequency range of interest in the present invention (2 to 30MHz) imposes serious practical limitations in tuning and effecting efficient power transfer from the transmitter to the antenna. For example, known prior art apparatus heretofore utilized a series whip antenna being in the order of 30 feet in length and generally mounted on a plate over the ground or on the side of a vehicle or other supporting means. This type of antenna although being portable, is physically not compatible for certain use, e.g. jungle environment because of the nature of the terrain. Secondly, this type of antenna by nature presents difficulty in being resonated over the desired operating frequency range.

One known type of transportable high frequency antenna system attempting to overcome the inherent difficulties in obtaining an electrically short antenna for ³⁵operation in the range from 2 to 30MHz is taught for example in U.S. Pat. No. 3,510,872, J. H. Mulhaney, which utilizes a collapsible helical antenna element. A second type of electrically short antenna is disclosed in U.S. Pat. No. 3,209,358, R. A. Felsenheld, which discloses, inter alia, a linear antenna having electronically controlled tuning means. In addition to ferrite core antennas for obtaining compactness, electrically short radiating antennas have also been achieved in the prior art by the use of inductively loaded folded monopole antennas such as taught in U.S. Pat. No. 3,103,01 1, E. W. Seeley.

SUMMARY OF THE INVENTION

The subject invention is directed to an improvement in high frequency antennas and briefly comprises a short inductively loaded vertical radiating element having a capacitive top termination coupled to one end, while the opposite end is coupled to a ground system through the series combination of remotely controlled reactance tuning means and energy feeding means. A tuning detector coupled to the feeding means is adapted to provide an output signal indicative of the resonant or non-resonant state of the antenna. This signal is then coupled to a control unit which is adapted to vary the reactance tuning means until resonance is achieved. When the antenna consists of a ground based system the ground system consists of a counterpoise comprising a number of radially oriented wires lying in the horizontal plane and suspended above the ground. In a vehicular configuration, the radially oriented wires acting as the counterpoise are deleted and the ground- ing system comprises the vehicle body itself. Also, in the ground based configuration, cable choke means are additionally included in the feeding means for suppressing radiation in the RF feed lines.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an electrical schematic diagram illustrative of the land based embodiment of the subject invention;

FIG. 2 is a diagram illustrative of a typical plan view 1θ of the capacitive top terminations at one end of the radiating element shown in FIG. 1, as well as the counterpoise consisting of a number of radially oriented wires located at the other end;

FIG. 3 is an electrical schematic diagram of a vehicular mounted embodiment of the subject invention; and

FIG. 4 is a partial electrical schematic diagram illustrative of yet another embodiment of the subject invention.

DESCRIPTIONS OF THE PREFERRED EMBODIMENTS

Referring now to the drawings and more particularly to FIG. 1, the radiating means consists of a relatively short inductively loaded vertical radiator element 10 having a capacitive top termination 12 connected to the upper extremity thereof and a counterpoise ground system 14 coupled to the other extremity by means of the series of combination of a frequency band select inductive reactance 16, a variable fine tuning capacitance 18, and antenna feed point means comprising s broadband transformer 20 having its secondary winding connected between the tuning capacitor 18 and the counterpoise 14.

The vertical radiating element 10 comprises a metal mast or wire supported by plastic or fiber glass tubing and typically having a height /1 = 6 feet for operation in the high frequency range (HF) of the electromagnetic spectrum of from 2 to 30MHz. The top termina40 tion 12 is comprised of a plurality of radially oriented wires or metal tubes 22 extending radially outwardly in a horizontal plane from the vertical central axis of the radiating element 10. The effect of the capacitive top termination is to increase radiation efficiency and re45 duce antenna reactance. A means for maintaining the radially oriented elements 22 in the horizontal plane is one of choice, depending upon the intended use. For example, if the elements 22 are comprised of wires, some form of insulating support, not shown, would be 50 required. In vehicular installations, the top capacitance elements may also be required to be arranged to reduce wind resistance. In FIG. 2, the top capacitance elements 22 are shown arranged in a regularly spaced configuration 45° apart from one another. This is shown ⁵⁵ merely for sake of illustration and is not meant to be interpreted in a limiting sense, however.

Since the physical dimension of the radiating element 10 is only a small fraction of the wavelength, it acts over a broad frequency range as though it were a capacitor insofar as its circuit properties are concerned. Accordingly, a loading inductance 16 is required to achieve quarter wavelength (λ/4) resonance at a given operating frequency. Since the inductance varies in6«. versely, as the square of the frequency, for a frequency range of 2 to 30MHz the inductor 16 must have a range variation of 1:225. Accordingly, the inductance 16 is comprised of a multi-tapped inductor having selected

3,909,830 portions of the winding adapted to be shorted out by means of a shorting switch 24.

The operation, the inductive reactance 16 operates substantially as a fixed inductance for a selected frequency band in a plurality of overlapping plurality of in the high frequency (HF) range. Accordingly, the inductor 16 is adapted to be adjusted in steps to provide coarse tuning of the radiating element to a predetermined frequency band within the HF range. The variable capacitance 18 on the other hand adapted to provide a fine tuning adjustment. Thus resonance is obtained by means of the variable capacitance 18 in series with a substantially fixed inductor 16.

the counterpoise 14 is used to reduce ground losses in the ground based embodiment shown in FIG. 1, and consists of a pluralityof radially oriented wires or other type elements 26 disposed in a horizontal plane just above the ground 28. The elements 24 are also shown regularly disposed in a radial fashion in FIG. 2 for purposes of illustration only. Experimentation has shown that in the counterpoise 14 preferably comprises at least sixteen wire elements 26, each approximately 1 3 feet long and suspended 12 inches above the ground 28 in order to effectively shield the antenna element 10 and stabilize its resistance to a relatively low value.

The antenna system resulting from the configuration shown in FIG. 1 exhibits a relatively high Q which makes remote controlled fine tuning of antenna resonance necessary due to the fact that the antenna reactance is sensitive to the presence of nearby objects. Accordingly, the band select inductance value of the inductor 16 is provided by means of an electrically controlled rotary selector 30 mechanically coupled to the switch 24 while the variable fine tuning capacitor 18 is controlled by means of an electrically operated reversible motor 32. Both the rotary selector 30 and the reversible motor 32 are operated by respective control signals applied thereto from a control unit 34 with the motor 32, at least operating in response to an electrical output signal from an antenna tuning state detector 36. This will be considered in greater detail subsequently.

Fine tuning of the antenna is preferably accomplished by means of the variable capacitor 18 in view of the following considerations. The antenna tuning range, within a given band, can be calculated from the approximate formula:

<img file="US3909830A_D0001.tif" />

wherein ft and fi are the upper and lower operating frequencies, respectively, C_m„_F and C_mm are the maximum and minimum capacitance of the variable fine tuning capacitor 18, and C_mt denotes the antenna capacitance. Typically C„„i — 50pF, C,„i„ = 1 OpF, and C„,„._r — 300pF which then results in a ratio of

<img file="US3909830A_D0002.tif" />

Variable vacuum capacitors having the recited values for C„_lnj. and C„„„ are commercially available.

If on the other hand fine tuning by means of variable inductance were to be used, a relatively narrower tuning range would be achieved in most bands as exhibited by the formula:

A / G + 6,,,,,, A ~ / Lt + where ft and f, are the upper and lower operating frequencies, L„_mj. and L_mi„ is the inductance of the variable inductance utilized as the fine tuning element, and Lf denotes the fixed inductance, i.e. inductance of the element 16 shown in FIG. 1. For typical values of L_F — ΙΟΟμΗ, L„,„_F = 12μΗ and T„„„ = ΙμΗ, a ratio of

<img file="US3909830A_D0003.tif" />

would be obtained, which is substantially less than that obtainable with capacitance tuning. From the foregoing, the ratio (f₂/fi) utilizing both capacitative and inductive fine tuning in20 dicates that fewer bands are required to cover the entire HF range (2 to 30MHz) when the variable capicator tuning method is employed. Obviously, since fewer bands are needed to cover the frequency range, and since the required capacitor type components are com25 mercially available, capacitor fine tuning is highly preferable.

In the land based embodiment shown in FIG. 1, an RF sourse such as a transceiver 38 is coupled to the antenna feed point 40 by means of the matching trans³⁰ former 20 by way of RF transmission line segments 42 and 44 and the tuning detector 36. The transmission line 44 is additionally configured to include a cable choke portion 46 which is shown in FIG. 1 in block diagrammatic form, but in actuality, consists in having a ³⁵ portion of the transmission line 44 simply formed into a coil. Matching of the antenna to the feed line by means of the broad band transformer 20 is relatively easy, first since resonance is always established at the operating frequency, the corresponding antenna feed ⁴⁰ point impedance is resistive even though for example with 50ohm radio transceivers, the feed point impedance is usually less than 50ohms when the λ/4 resonant mode is used. Secondly, in conjunction with the counterpoise 14, the antenna input impedance is rela⁴⁵ lively constant over a broad frequency range, e.g. 2 to 13MHz. At upper frequencies above 13MHz, when preferable, the matching transformer 20 may be unnecessary and therefore deleted. In such a case, the transmission line 42 would be connected directly to the an⁵⁰ tenna feed point 40 without incurring a substantial mismatch. Alternatively, the transformer 20 could be designed so as to achieve minimum VSWR over the entire HF range from 2 to 30MHz.

The impedance match between the antenna and ⁵⁵ transmission line 42 from the transceiver 38 is determined by the electrical characteristics of the broadband transformer 20 and the tuning state of the antenna radiating element 10. Assuming that the transformer introduces no reactances of its own, a “match ⁶⁰ is obtained only when the antenna is non-reactive, i.e.

in its resonant state. The tuning detector 36 which for example preferably comprises a radio frequency phase sensor, well known to those skilled in the art, is coupled to the antenna feed point by means of the broadband ' matching transformer 20 and the RF transmission line segment 42. A phase sensor is a device which is adapted to measure the phase relationship between the

3,909,830 radiating element’s excitation voltage and the current and produces a DC output voltage which is proportional to the phase difference therebetween, with zero output from the detector indicating antenna resonance. An output signal from the sensor of a positive polarity for example, indicates that the antenna is in a nonresonant state and has a corresponding reactive impedance which is inductive while a negative polarity output is indicative of a non-resonant state wherein the corresponding reactive impedance is capacitive. The phase sensor accordingly must accurately monitor the tuning state of the antenna over the entire frequency range of from 2 to 30MHz.

The DC output signal from the phase sensor 36 is coupled to the control unit 34 by means of a shielded cable 48 which also includes a portion thereof configured into a cable choke portion 50 in the same manner as the cable choke portion 46. The remote control unit contains electrical circuitry e.g. a servo-system necessary to effect control of coarse and fine tuning, and includes the necessary voltage and power supplies as well as an antenna tuning state indicator or meter, manual controls and a source of primary power. The DC signal derived from the phase sensor 36 for example, is at least used to drive the reversible motor 32 mechanically coupled to the variable capacitor 18. When desirable this signal can also be used to initiate operation of the rotary selector. This is provided by means of a multiwire cable 52. The cable 52 also includes a portion thereof which is configured into a cable choke portion 54 in the same manner as the cable chokes 46 and 50. It should be pointed out that when desirable the separate cable chokes could be combined. The purpose of the cable chokes 46, 50 and 54 is to suppress feed line radiation and reduce ground losses. The cable choke 46 in the RF transmission line 44 is the most important and is located relatively close the the antenna for maximum effectiveness and provides a high impedance across its extremities, e.g. 5kilohms at frequencies where feed line excitation is most troublesome. In the case of the subject antenna system, feed line excitation appears to be of concern in the frequency range of from 2 to 10MHz rather than at higher frequencies. This is explained by the fact that the frequencies above approximately 10MHz, the counterpoise 14 acts as a better ground. The cable chokes 46, 50 and 54 moreover become increasingly essential as the dimensions of the counterpoise 14 decreases.

Referring now to a vehicular installation, attention is directed to the FIG. 3. In such embodiment, the elements are exactly the same as shown and described with respect to the ground based embodiment shown in FIG. 1 with the exception that the cable chokes 46, 50 and 54 and the counterpoise 14 illustrated in FIG. 1 are now eliminated. This is due to the fact that the radiating element 10 is terminated in the vehicle body 56 which effectively acts as the counterpoise. In all other respects, the two embodiments are exactly alike.

Another embodiment of the subject invention adapted for either land or vehicular use is shown in FIG. 4. A land based configuration is shown for purposes of illustration since it discloses utilizing a counterpoise 14 as the “ground system”. Whereas in the earlier embodiments shown in FIGS. 1 and 3, the coarse or band tuning is accomplished by means of a multiple tap single inductor 16 having a selected portion of the winding shorted by means of the rotary se lector driven switch 24, the present embodiment discloses the use of a plurality of inductors 58 selectively coupled individually in series with the fine tuning capacitor 18 and the vertical radiator element 10. The selective interconnection is provided by means of a multiple position two section band selector switch including one section 60 which connects one end of the selected inductor to the variable capacitance 18 while the other section 62 connects the opposite end of the same selected inductor to the antenna feed point 40. As in the other embodiments the switch position is mechanically controlled by the roatry selector 30.

Also in the embodiment shown in FIG. 4, the tuning detector 36 is located between the antenna and the broadband radio frequency transformer 20. In other words their relative positions are reversed with respect to the antenna feedpoint 40 and RF source, i.e. the transceiver 38.

The embodiment disclosed in FIG. 4 has several advantages over the two previous embodiments in that it is possible in some instances with this configurations to obtain more than one frequency at which resonance is indicated by the phase sensing type tuning detector 36 for a given setting of the fine tuning capacitor 18 and the band select setting of the inductor 16. Although only one of the resonances constitutes the desired resonance frequency associated with the antenna, the “extraneous resonances” results from the relatively complicated capacitive and inductive coupling occuring between the various sections of the coarse tuning (band select) inductor 16. In principle the extraneous resonances are not harmful as long as they can be distinguished from the desired antenna resonance and are somewhat separated in frequency. From a practical standpoint, however, it is desirable and in some applications essential to eliminate all extraneous resonances within the particular band of operation in order that the antenna can be resonated without ambiguity.

By utilizing separate tuning inductors for each selected band within the HF range, only one resonance frequency was observed in each band corresponding to the desired antenna resonance. All extraneous resonances occured at frequencies outside the operating band.

Secondly the stray reactances of the transformer 20 can in some instances cause faulty tuning when the transformer is connected between the phase sensing tuning detector 36 and the antenna. The problem is obviated when the tuning detector 36 is installed between the antenna and the transformer.

The advantage obtained by the configuration shown in FIG. 4 is that separate coarse tuning inductors facilitate the selection or setting of the various bands. Separate inductors also yield a lower power loss than a single tapped inductor, and more importantly, since only the desired antenna resonance is obtained in each band the use of separate inductors facilitates automatic tuning.

Under normal circumstances, the antenna radiating element 10 will be initially in the non-resonant state and thus will be badly matched to the transmitter portion of the transceiver 38. Consequently, little power will be supplied to the non-resonant antenna and the DC error signal produced by the phase sensor 36 will be correspondingly small. The polarity of the phase sensor output signal determines of course whether to increase or decrease the tuning reactor. As the fine tun3,909,830 fore but which requires sliding or rolling contacts which are a source of erratic operation, noise and power loss. For the ground based installation, the isolated counterpoise in combination with the cable choke 5 effectively shields the antenna against strong variations in the ground characteristics and decreases power losses. Even though its dimensions are modest, the counterpoise is usually a more efficient “ground system than one of comparable size composed of wire ra10 dials lying on the earth’s surface or buried. In addition, there is theoretical evidence indicating that up to 98% of the total ground loss occurs within a radial distance of 0.05X from the antenna radiator axis in the low HF range which provides justification for the use of the 15 counterpoise as set forth in the present specification.

Accordingly, having thus set forth what is at present considered to be the preferred embodiments of the subject invention,