US3325814A

US3325814A - Broadband receiving antenna

Info

Publication number: US3325814A
Application number: US466206A
Authority: US
Inventors: Stromswold Chester
Original assignee: Sanders Associates Inc
Current assignee: Lockheed Corp
Priority date: 1965-06-23
Filing date: 1965-06-23
Publication date: 1967-06-13
Anticipated expiration: 1984-06-13
Also published as: DE1516841A1

Description

June 13, 1967 c. STROMSWOLD $325,814

BROADBAND RECEIVING ANTENNA Filed June 23, 1965 lNVENi'OA CHESTER STROMSWOLD GL-am A TOR/V r United States Patent 3 325 814 BROADBAND R ECE IWHNG ANTENNA Chester stromswold, Nashua, N.H., assignor to Sanders Associates, Inc, Nashua, N.H., a corporation of Dela- Ware Filed June as, 1965, Ser. No. 466,206 12 Claims. c1. 343]l18) This invention relates to a radio direction finder and to broadband radio frequency wave-intercepting devices particularly suited for use in a direction finder.

A direction finder of the present type is used to determine the arrival direction of signals within a wide frequency range. In the prior art, however, as the frequency range is extended, the antennas and other wave intercepting devices of the direction finder increasingly react with intercepted energy. At certain frequencies, they reradiate the intercepted energy and thereby mask the original source. As a result, erroneous direction indications are produced.

Accordingly, it is an object of the present invention to provide an improved direction finder suited for wideband operation.

Another object is to provide a wideband direction finder in which the wave-intercepting devices operate with relatively uniform efficiency over the entire frequency range.

A further object of the invention is to provide compact and efiicient radio frequency intercepting devices such as antennas and shields having relatively uniform characteristics over a wide frequency range including frequencies at which the dimensions of each device are commensurate with the operating wavelength.

Another object of the invention is to provide a Faraday shield that is highly efficient at relatively low frequencies and essentially free of resonance effects at markedly higher frequencies.

A more particular object is to provide a Faraday shield that effectively blocks electric field lines of force at relatively low frequencies and remains relatively transparent to magnetic fields at frequencies ten and fifteen times higher.

It also is an object of the invention to provide a sense antenna for use in a direction finder and characterized by relatively high effective height at low frequencies and relatively uniform operation up through frequencies many times higher than the lowest frequency of operation.

A further object of the invention is to provide a broadband array of closely spaced receiving antennas having substantially uniform operation and relative freedom from mutual impedance coupling and shadowing within the array.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawing of a direction finder embodying the invention.

In general, the invention .provides a broadband direction finder, operating for example from below megacycles to in excess of 500 megacycles. This broadband operation is made possible by the use of antennas and shields incorporating electrically dissipative elements, principally fixed resistance elements or frequency varying resistive elements such as a ferrite material, in series in the intercepting devices. The resistive elements are employed in such manner that the wave intercepting devices operate 3,325,8l4l Patented June 13, 1967 with no appreciable reradiation or mutual coupling with adjacent conductors over frequency ranges in excess of 10: 1. As a result, the direction finder is able to detect the true source direction of intercepted signals and thus produce correct output indications.

A direction finder sense antenna incorporating resistive elements in series with it according to the invention has only relatively small changes in operating characteristics even at frequencies where its physical dimensions are commensurate with the wavelength. The sense antenna exhibits relatively little scattering. The resistive elements do not effectively degrade the low frequency performance of the sense antenna so that at low frequencies it has substantially the high performance characteristic of a conventional antenna designed for optimum operation at the low frequencies.

The invention also provides a Faraday shield that screens electric field from its interior with high efficiency at low radio frequencies and yet does not interfere with the operation of magnetic field sensing devices in its interior at frequencies several times greater than the lower operating frequencies.

More particularly, turning to the drawing, the illustrated direction finder is constructed with a directive loop antenna 10 arranged to be rotated in the vertical plane by a motor 12. The signal developed by the antenna 10 is applied through a transmission line 14, incorporating a rotary joint indicated at 16, to an input port of a receiving and indiciating unit indicated generally at 18. The unit 18 is also connected with the drive motor 12 to receive a signal indicating the instantaneous rotation orientation of the loop antenna.

The loop antenna 10 employs conventional construction and, as is well know for such antennas, has a directinoal response to the magnetic field of of an electromagnetic wave such that the antenna output sign-a1 is at a null when the direction incoming signal is transverse to the plane of the antenna. The output signal from the loop antenna 10 thus drops to its null valve and rises to its peak valve twice during each full rotation of the loop antenna, with the two peak signals having opposite polarity. The loop antenna can utilize several loops of different sizes to efiiciently cover wide frequency ranges.

A stationary sense antenna indicated generally at 20 is also connected to the receiving and indicating unit 18. The sense antenna develops a signal having the same polarity regardless of the arrival direction of the intercepted wave. Thus, the signals from the loop and sense antennas are in phase to reinforce each other with one orientation of the loop antenna, and, when the loop tantenna is reversed, the two signals have opposite phase and subtract. The receiving and indicating unit 18 combines the sense antenna signal and the loop antenna signal to unambiguously indicate the direction from which the loop antenna 10 has intercepted an incoming signal.

Although the sense antenna 20 could be a single monopole disposed above the loop antenna 10, the present direction finder employs an array of monopole antennas 22 spaced in a circular path around the loop antenna 10 and substantially horizontally coextensive with it. This arrangement is considerably more compact than having the sense antenna above the loop antenna and, since the monopoles are disposed on all sides of the loop antenna, most of them will be outside its electrical shadow. Dielectric supports 23 support the monopole antennas on a conductive base plate 28.

To shield the loop antenna 10 from electric field lines of force, which would interfere with and detract from its direction finding operation, a Faraday shield indicated generally at 24 is disposed around the sides and top of the loop antenna 10 and inside the array of monopoles.

The shield has a hat-like superstructure of conductive bands 26 disposed around the top and sides of the loop antenna. Each band 26 is contained substantially within a vertical plane and connects to the base plate 28.

A direction finder such as shown in the drawing and having a Faraday shield that is approximately 12 inches high and approximately the same diameter and in which each monopole is approximately inches high, operates over a bandwidth extending from below 30 megacycles to more than 500 megacycles. The monopoles are made this long to have sufficient effective height at the low end of the frequency range. The shield is large enough to enclose a loop 10 operating efficiently below 30 megacycles. Nevertheless, the sense antenna and shield dimensions are a relatively insignificant part of a wavelength at the lower end of the frequency range. More particularly, an eight-wavelength at 30 megacycles is ap proximately 50 inches. At the upper end of the frequency range, however, the linear dimensions of the monopoles 22 and of the conducting bands 26 of the Faraday shield are commensurate with a fraction of the wavelength. For example, both elements are more than a quarter-wavelength in size at 300 megacycles. As a result, the monopoles and the bands 26 of the shield tend to exhibit strongresonances. However, as will now be described, they are so constructed that they do not resonate; instead, they have substantially uniform operation throughout a broad frequency range such as the one described above.

As shown in detail for the monopale 22a, each monopole 22 is constructed with resistive segments 30 and conductive segments '32 connected in an alternate series succession. The resistive segments can be of ferrite material to provide a frequency varying resistance or, more simply, be of conventional carbon or like resistive material. The electrically resistive material is thus essentially distributed throughout the portion of the monopole that intercepts incoming radio waves. The resistance of the segments 30 can thus be considered as a distributed parameter of the monopole and part of its internal im edance. The resistive segments 30 are distributed throughout the monopole in the sense that no conductive segment 32 approaches its resonant frequency, or alternatively, is an appreciable part of a wavelength long, throughout the range of operating frequencies. Each monopole 22 can, alternatively, be fabricated with a continuous piece of resistive material. For ease in connecting the monopoles 22 to the receiving unit 18, each

group

20a, 20b, 20c, and 20d of the three adjacent monopoles is connected in parallel by means of resistive jumpers 34.

Transmission lines

35a, 35b, 35c and 35d connect the groups of antennas to the unit 18.

The total resistance of each monopole, due to the resistive segments 30 therein, is intermediate the values of impedance at resonance and way below resonance for the same monopole having no resistive segments. For example, an allustrative monopole entirely of conductive material such as copper has substantially zero ohmic resistance, a radiation resistance of approximately 40 ohms at resonance and a non-resonant impedance of the order of 10,000 ohms. The same monopole constructed according to the invention has resistive segments 30 with a total resistance of around 500 ohms. Each jumper 34 has a resistance of around 50 ohms.

With this arrangement, the resistance of each monopole is sufficiently small to have substantially no effect on the total impedance at frequencies Where the monopole does not tend to resonate. The added resistance thus has a negligible effect on antenna performance at these frequencies. At frequencies where the monopole structure tends to reasonate, however, the monopole resistance has considerable effect, being, for example, 10 times greater than the resonant impedance of the same size nonresistive monopole. The added resistance substantially degrades the performance of the monopole at frequencies in the neighborhood of resonance and thus essentially eliminates standing waves from the monopole.

As a result, each monopole 22 can have sufficient height to be effective at low frequencies and yet at much higher frequencies it does not tend to re-radiate energy or to couple with an adjacent monopole or with the Faraday shield 24.

A further feature of the novel structure for the sense antenna 20 is that the amplitude of the signal input to the receiving and indicating unit 18 does not exhibit sharp fluctuations as would be the case where the sense antenna develops sharp resonances. Accordingly, the radio frequency input circuit of the unit 18 can be constructed with relatively high sensitivity and without the need for burnout protection devices that would diminish the sensitivity.

Turning now to the Faraday shield 24, when it is constructed with conduction bands 26 of highly conductive material such as copper or silver, it effectively shields electric fields from its interior at relatively low frequencies and over a considerable range. However, at higher frequencies, resonances develop in the shield and it reradiates energy in the same manner as a resonant antenna. Again, energy reaches the loop antenna 10 from a number of directions and the direction of the ultimate source of the energy cannot be ascertained. These unwanted resonance phenomena are substantially eliminated according to the invention by constructing the Faraday shield with bands 26 that are electrically resistive, as by making them with resistive strands so that each band has a resistance, for example, of 1,500 ohms for the 30-500 megacycle direction finder discussed above.

It has been found that with this arrangement the Faraday shield has relatively negligible magnetic shielding at loW frequencies so that the loop antenna provides substantially the same operation as when enclosed in a Faraday shield having highly conductive bands. At the higher frequencies where the highly conductive bands tend to resonate, the lossy bands 26 in the present Faraday shield remain free of resonances and remain essentially transparent to magnetic fields, thereby enabling the loop antenna to operate with high efiiciency over the entire frequency range desired for the direction finder.

As with the monopole antennas 22, the bands 26 of the shield thus having resistive segments distributed along their lengths. The resistance of each band is intermediate the magnitudes of its impedance at resonance and off resonance.

The invention thus provides lossy radio frequency Wave-intercepting devices of sufficient size to be effective at low frequencies. The resistance of each device is relatively small compared to the magnitude of its reactance at the low frequencies in its operating range. Hence, the added resistance has little effect at these frequencies. At the higher frequencies of operation where the reactive impedances of the devices tend to decrease, the internal series resistances become increasingly effective and maintain the operating characteristics substantially uniform. That is, at frequencies where the wave-intercepting device normally tends to be resonant, the resistance therein dissipates the resonant currents that tend to develop, thereby enabling the device to continue operating as a nonresonant device much the same as it does at the low frequencies. When incorporated in the present direction finder, these features of the sense antenna and Faraday shield make it possible for a single direction finding structure to operate over a remarkably wide frequency range that heretofore required separate antenna structures that involved considerably more bulk and cost than the present direction finder.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efiiciently attained and, since certain changes may be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.

Having described the invention, what is claimed as new and secured by Letters Patent is:

What is claimed is:

1. Broadband radio reception apparatus comprising in combination (A) receiving means operable over a range of frequencies extending from a first frequency to a second frequency that is higher than said first frequency,

(B) antenna means (1) having electrically dissipative means distributed in series therein,

(2) being a quarter-Wavelength long at a third frequency intermediate said first and second frequencies,

(3) being less than an eighth-wavelength long of said first frequency,

(4) having a resistance that (a) is less than the magnitude of the antenna impedance at said first frequency, and

(b) is greater than the magnitude of the antenna impedance at said third frequency.

2. Radio reception apparatus according to claim 1 in which said dissipative means includes at least one electrically resistive segment in series in said antenna means.

3. Radio reception apparatus according to claim 1 in which said dissipative means is of ferrite material having a frequency dependent resistance.

4. Radio reception apparatus according to claim 1 in which said antenna means is constructed with alternate series-connected portions of electrically resistive material and portions of substantially non-resistive electrically conductive material.

5. Radio reception apparatus according to claim 1 in which said antenna means has a wave-intercepting structure of electrically-resistive material.

6. A radio direction finder comprising in combination (A) radio receiving means responding to antenna signals within a range of radio frequency extending between a first frequency and a sec-nd frequency higher than said first frequency,

(B) means for controlling the orientation of antenna means and providing an indication corresponding to said orientation,

(C) directive antenna means connected with said orientation controlling means and with said receiving means, and

(D) sense antenna means including at least one antenna element, said sense antenna means (1) being connected with said receiving means,

(2) having electrically resistive means distributed in series in each element thereof,

(3) each antenna element being a quarter-wavelength long at a third frequency intermediate said first and second frequencies, and being less than an eighth-wavelength long at said first frequency,

(4) each antenna element having a resistance that (a) is substantially less than the magnitude of the antenna element impedance at said first frequency, and

(b) is substantially greater than the magnitude of the antenna element impedance at said third frequency.

7. A radio direction finder according to claim 6 in which said sense antenna comprises a plurality of antenna elements disposed along a closed path encircling and closely spaced from said directive antenna means.

8. A radio direction finder according to claim 7 (A) in which said directive antenna means is responsive to magnetic field lines of force, and

(B) further comprising a multiple-conductor electric field shield, said shield being disposed around said directive antenna means and within said closed path and being constructed with electrically dissipative means distributed in series in each conductor thereof so as to be substantially non-resonant throughout said frequency range between said first and second frequencies.

9. A radio direction finder comprising in combination (A) radio receiving means responding to antenna signals within a range of radio frequencies extending between a first frequency and a second frequency higher than said first frequency,

(B) directive antenna means connected with said receiving means and being responsive to magnetic field of lines of force, and

(C) an electric field shield disposed around said directive antenna means and constructed with conductive members at least some of which (1) are a quarter-wavelength long at a frequency intermediate said first and second frequencies and less than an eight-Wavelength long at said first frequency, and

(2) have electrically dissipative means distributed in series therein to have a resistance that is greater than the magnitude of the reactance of the conductive member at said frequency at which it is a quarter-wavelength long.

10. A radio direction finder according to claim 9 in which said shield is a Faraday type shield composed of a plurality of conductive bands each of which are electrically resistive means in series therein to be substantially nonresonant throughout said frequency range between said first and second frequencies.

11. An antenna system for a radio direction finder, said antenna system comprising in combination (A) directive loop antenna means arranged for rotation about an axis,

(B) a Faraday shield substantially shielding said directive antenna means from electric field lines of force,

(1) said Faraday shield being composed of conductive bands each of which has electrically resistive means distributed in series therein.

12. An antenna system according to claim 11 further comprising a plurality of monopole sense antennas disposed along a circular path around said Faraday shield, each monopole antenna having electrically resistive means distributed in series therein.

References Cited UNITED STATES PATENTS 2,656,536 10/1953 Lockhart 343-118 2,712,602 7/1955 Hallen 343-828 X 2,917,744 12/1959 Gray 343842 X RODNEY D. BENNETT, Primary Examiner. R. E. BERGER, Assistant Examiner.

Claims

6. A RADIO DIRECTION FINDER COMPRISING IN COMBINATION (A) RADIO RECEIVING MEANS RESPONDING TO ANTENNA SIGNALS WITHIN A RANGE OF RADIO FREQUENCY EXTENDING BETWEEN A FIRST FREQUENCY AND A SECOND FREQUENCY HIGHER THAN SAID FIRST FREQUENCY, (B) MEANS FOR CONTROLLING THE ORIENTATION OF ANTENNA MEANS AND PROVIDING AN INDICATION CORRESPONDING TO SAID ORIENTATION, (C) DIRECTIVE ANTENNA MEANS CONNECTED WITH SAID ORIENTATION CONTROLLING MEANS AND WITH SAID RECEIVING MEANS, AND (D) SENSE ANTENNA MEANS INCLUDING AT LEAST ONE ANTENNA ELEMENT, SAID SENSE ANTENNA MEANS (1) BEING CONNECTED WITH SAID RECEIVING MEANS, (2) HAVING ELECTRICALLY RESISTIVE MEANS DISTRIBUTED IN SERIES IN EACH ELEMENT THEREOF, (3) EACH ANTENNA ELEMENT BEING A QUARTER-WAVELENGTH LONG AT A THIRD FREQUENCY INTERMEDIATE SAID FIRST AND SECOND FREQUENCIES, AND BEING LESS THAN AN EIGHT-WAVELENGTH LONG AT SAID FIRST FREQUENCY, (4) EACH ANTENNA ELEMENT HAVING A RESISTANCE THAT (A) IS SUBSTANTIALLY LESS THAN THE MAGNITUDE OF THE ANTENNA ELEMENT IMPEDANCE AT SAID FIRST FREQUENCY, AND (B) IS SUBSTANTIALLY GREATER THAN THE MAGNITUDE OF THE ANTENNA ELEMENT IMPEDANCE AT SAID THIRD FREQUENCY.