US3153787A - Scanning antenna with power modulation - Google Patents

Description

Oct. 20, 1964 LA VERGNE E. WILLIAMS SCANNING ANTENNA WITH POWER MODULATION Filed April 10, 1961 INVENTOR. L AVEIZGNE E..WILL.\AMS

ATTO 2 N EYS United States Patent "ice 3,153,787 SCANNING ANTENNA WITH POWER MODULATION La Vergne E. Williams, Indialantic, Fla, assignor to Radiation, Inc., Melbourne, Fla., a corporation of Florida Filed Apr. 10, 1961, Ser. No. 102,061 '8 (Ilaims. (Cl. 343-100) The present invention relates generally to antenna systems and more particularly to power modulating tracking antenna systems with conical scan.

Power modulation is a method whereby the gain of an antenna is varied as a function of its vertical scan so its gain is considerably reduced when the beam is pointed at low angles to the horizon. In angular tracking applications, this is important because transmission and reception of low angle signals is degraded due to ground reflections. Reflections result in inaccurate target location indications because the ground reflections do not come from the true target direction.

Prior power modulation systems have usually employed variable attenuators synchronized with the antenna conical scan to introduce large amounts of signal attenuation when the antenna beam is pointed at low angles to the horizon and relatively small amounts of attenuation when the antenna is at higher vertical angles. These prior systems have proven undesirable because they are expensive or they utilize complicated moving parts requiring maintenance, in addition to a synchronized mechanical following system. Also, the prior devices have proven unfavorable because they introduce signal attenuation (insertion loss) when none is desired thereby necessitating increased transmitter power.

Consequently, it is an object of the present invention to provide a new and improved power modulation antenna system which requires no moving parts by itself.

It is a further object of the present invention to provide a power modulation antenna system that absorbs little or no power when the antenna beam is directed above the ground (maximum vertical angle) in its scan cycle.

Another object is to provide a power modulating antenna system that is easily adaptable to existing apparatus, is relatively inexpensive, easy to manufacture and service.

Basically, the present invention employs an antenna system having a conical beam pattern. With conical scan the pattern position is cyclically varied in the vertical plane to achieve vertical scan. An absorber is disposed adjacent the antenna for absorbing substantial amounts of beam power only when the beam is at low vertical angles, i.e., low angles with respect to the horizon, thereby minimizing the signal received from reflections to and from the target.

The method of power modulation described herein can be applied to radar (two way) systems or receiving antennas that track an external signal such as a beacon, telemetry transmitter, or radio sonde.

The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of one specific embodiment thereof, especially when taken in conjunction with the accompanying drawings, wherein:

FIGURE 1 is a schematic diagram of one preferred form of the present invention; and

FIGURE 2 is an illustration of the manner in which the nutating cup of FIGURE 1 is rotated.

FIGURE 1, the schematic diagram of the preferred form of the present invention, comprises a stationary dipole antenna 11, connected to a conventional coax line 12, in turn connected to receiver or radar apparatus 13. Dipole antenna 11 is positioned within nutating metal 3,153,787 Patented Oct. 20, 1964 reflecting cup 14, having a plastic support member 15 located at its open end. Member 15 is a plastic or other suitable material, transparent to electromagnetic energy of the frequency in which the present system operates. Plastic tube 16, within which coax line 12 is positioned, is connected to support 15 at an oif center position. Tube 16 is rotated by motor 17 which drives it through shaft 18 and gear 19 so cup 14 is an off-center vertically rotating or nutating member. Counter weight 27 is disposed on the surface of cup 14 adjacent the cup end closest to the point where tube 16 engages support 15 to maintain the mechanical load driven by motor 17 balanced. Stationary absorber 21 made of some suitable material, such as hairflex, is secured to the upper surface of spherical, electromagnetically transparent, plastic radome 22 which protects the structure enclosed therein from the weather and other influences. The absorber receives none of the antenna energy when cup 14 is in its lower position but receives a substantial portion of antenna energy when cup 14 is in its upper position.

This is best shown in FIGURE 2 wherein the upper position of nutating cup 14 is illustrated by circle 23 and the lower .cup position is illustrated by circle 24. With cup 14 in position 23, absorber 21 is in front of approximately one-half of the reflecting cup and consequently, the effective energy received by or transmitted from antenna 11 is reduced approximately in half. In position 24, no portion of absorber 21 is in front of cup 14 and the transmitted or received beam is not affected by the absorber.

Referring again to FIGURE 1, parabolic reflector 26 is positioned to affect a pencil beam pattern for primary feed antenna 11 and reverses the direction and changes the horizontal angle of the signal impinging thereon. Radome 22 is positioned approximately at the focal point of reflector 26 and secured to the reflector by struts 28 and 29. The vertical movement of cup 14 provides vertical scan for the pencil beam transmitted and received by dipole 11 by controlling the direction which most of the energy is directed towards or received from dish 26. With the cup in its lowest position 24 (FIGURE 2), the secondary pattern of the antenna, i.e. that part directed toward cup 14, is directed above the axis of the antenna and the ab sorber does not absorb signal energy. When cup 14 is rotated to its upper position 23, the secondary pattern is directed below the axis of the antenna and a substantial portion of the signal is absorbed by absorber 21. Since energy is either obtained from or directed towards low angles with respect to the horizon, the antenna gain is a minimum for low horizontal angles. Thereby, ground reflections in the transmitted and received signals are sub stantially reduced. In this manner, power modulation is obtained with relatively simple structure requiring no additional moving parts and with a minimum of cost.

Of course, it is to be understood that radar receiver and transmitter 13 is positioned at a remote location as is motor 17 and any suitable driving apparatus associated therewith. The invention can be applied to other conical scan tracking antenna systems such as those employing a nutation horn. The disclosed illustration being for schematic purposes only.

While I have described and illustrated one specific embodiment of my invention, it will be clear that variations of the details of construction which are specifically illustrated and described may be resorted to without departing from the true spirit and scope of the invention as defined in the appended claims.

I claim:

1. An antenna system comprising antenna means having a scanned pencil beam pattern of R.F. electromagnetic energy, means for cyclically varying the vertical position of said pattern and means for absorbing substancomprises a n'utating primary feed.

3. A power modulation antenna system comprising an antenna feed system for transmitting and receiving a beam of RF. electromagnetic energy having a secondary pattern, means for reflecting the beam emanating from said antenna feed system, means for cyclically varying the vertical position between said antenna feed system and said reflector, means for absorbing substantial energy from said beam only when the secondary pattern is directed at low angles relative to the horizon.

4. A power modulation antenna system comprising an antenna feed for a beam of R.F. electromagnetic energy, a reflector for said beam, means for cyclically varying the vertical position of said beam, and means for absorbing substantial amounts of energy from said beam only A} when said cup is in an upper position, and a parabolic reflector cooperating with said cup for reflecting said energy impinging thereon. I

6. The system of claim 5 further including means for rotating said cup by the reflector axis, said cup being rotated about a point removed from its longitudinal axis, said antenna being located at said point, said absorber being positioned completely above said cup when said cup is in the lowest position. i

7. A power modulation antenna system comprising means having a secondary scanned pencil beam pattern of RP. electromagnetic energy, means forZyclical ly varying the vertical position of said pattern, and means for when said beam is at low angles with respect to the absorbing substantial amounts of power from said beam only when it is in the minimum elevation portion of its scan cycle.

,8. The system of claim 4 wherein said means for ab sorbing is positioned remotely from said reflector and proximately to said feed.

References Cited in the file of this patent UNITED STATES PATENTS 2,407,250 Busignies t Sept. 10, 1946 2,419,556 Feldman Apr. 29, 1947 2,597,313 Gardner May 20, 1947

Claims (1)

1. AN ANTENNA SYSTEM COMPRISING ANTENNA MEANS HAVING A SCANNED PENCIL BEAM PATTERN OF R.F. ELECTROMAGNETIC ENERGY, MEANS FOR CYLICALLY VARYING THE VERTICAL POSITION OF SAID PATTERN AND MEANS FOR ABSORBING SUBSTANTIAL AMOUNTS OF POWER FROM SAID PATTERN ONLY WHEN SAID PATTERN IS DIRECTED AT LOW ANGLES WITH RESPECT TO THE HORIZON.

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