US 3364492 A
Description (OCR text may contain errors)
Jan. 16, 1968 1.. v. GRIFFEE SINGLE ELEMENT HOMING ANTENNA Filed Jan. 21, 1965 FIG RECEIVER CARDIOD PATTERN WITH SWITCH I? IN POSITION SHOWN CARDIOD PATTERN WHEN SWTICH /7 IS REVERSED INVENTOR.
LESLIE v GRIFFEE I w f FIG 2 ATTORNEYS United States PatentOfilice 3,364,492 Patented Jan. 16, 1968 3,364,492 SINGLE ELEMENT HOMING ANTENNA Leslie V. Griffee, Dallas, Tex., assignor to Collins Radio Company, Cedar Rapids, Iowa, a corporation of Iowa Filed Jan. 21, 1965, Ser. No. 426,911 2 Claims. (Cl. 343832) ABSTRACT OF THE DISCLOSURE This invention describes a single element antenna which is capable of producing a reversible cardioid pattern. The antenna is comprised of two monopoles which are arranged in a parallel relationship and both of which are fed against a ground plane. The monopoles are connected by a transmission line. A receiver is connected to the monopoles through a double pole double throw switch such that the monopoles are alternately connected to the receiver by reversing the position of the switch. Each reversal of the switch causes the cardioid pattern to reverse around the transmission line.
This invention relates to a single element antenna for use with helicopters and other aircraft and vehicles.
Many modern day aircraft are equipped with radio receivers which are used for homing purposes. One of the requirements of a homing system is an antenna that has the correct radiation pattern. Most homing receivers require an antenna system that has a reversible cardioid pattern such that it can he switched alternately to the right and left. The usual antenna used to supply such a radiation pattern is a pair of phased dipoles with the phasing line switched from one side to the other at a rate of 100 c.p.s. for example.
Theoretically a monopole fed against ground operates just as Well as the dipole array. However, the dipole array has heretofore been used because the balanced dipole array has a tendency to isolate itself from the skin currents set up in the aircraft skin. This is important because irregular skin configurations cause irregular currents which unpredictably distort the radiation pattern. A monopole antenna fed against the aircraft skin as ground is, of course, subject to this disadvantage and for this reason has not heretofore been used. However, the inventor has shown that by feeding the monopole against a ground plane which is symmetrical and large in terms of wavelength, the disadvantage is overcome and a monopole array can be used. This invention therefore makes it possible to use a monopole in a manner heretofore impossible. This is important because the electrical advantages of a dipole can be obtained from a monopole and the monopole has many mechanical advantages such as ease of manufacture, much less cumbersome to mount, it can be mounted at a place on the aircraft where it does not obstruct the pilots view, does not interfere with other aircraft parts, and is stronger, more durable and lighter.
It is therefore an object of this invention to provide a monopole antenna which operates where only dipoles have previously been successful.
It is also an object of this invention to provide an antenna which can be mounted at any of several convenient locations on an aircraft.
It is another object of this invention to provide an antenna which is cheap and simple to manufacture and mount and is much preferred to a dipole which is more costly and intricate to manufacture and mount.
These and further objects and features of the invention will become apparent upon reading the following description in conjunction with the accompanying drawings, wherein like numbers indicate like elements, and in which:
FIGURE 1 shows a schematic of the antenna connected with a reversing switch; and
FIGURE 2 shows the radiation pattern obtained from the antenna.
The antenna shown in FIGURE 1 is one half of a loop mounted against a ground plane 16. However, it can be analyzed as two short monopoles 10 and 11 with their upper ends connected by a bar or strap 12 which acts as a transmission line above ground. The height of monopoles 10 and 11 is not critical to the operation of the antenna. However, because bar 12 forms a transmission line above ground an excessive monopole height would require a very wide spacing between bar 12 and the ground plane. For this reason the monopole height should not exceed a height which results in a characteristic impedance exceeding 200 ohms. This is true because the characteristic impedance of transmission line 12 must be matched by the impedance matching termination 13. Because an excessively high impedance rapidly lowers antenna efficiency the impedance should not exceed 200 ohms. This is a limiting factor on the height of monopoles 10 and 11. Because a cardioid radiation pattern is required, the spacing between monopoles l0 and 11 is important. A usable cardioid is obtained when the spacing is between 0.15 and 0.35 wavelength of the frequency to be transmitted or received. A closer spacing approaches a folded monopole type antenna with its omnidirectional pattern. Further spacing than 0.35 approaches a figure eight pattern as the cardioid pattern deteriorates.
It is also necessary to provide an impedance match between the characteristic impedance of the monopoles and the transmission line 12 from the monopoles 11 and 12 to the receiver 18. This is done by the use of identical 4:1 transformers 14 and 15.
The system shown in FIGURE 1 can be considered to be two transmission lines one formed from each end of the system-that is, one from monopole 10 to termination 13 and one from monopole 11 to termination 13, each through reversing switch 17. One of the cardioid patterns shown in FIGURE 2 is obtained from each of said transmission lines. In order to obtain both cardioids shown in FIGURE 2., termination 13 and receiver 18 are switched back and forth between monopoles 10 and 11 by use of a cycle per second reversing switch 17. Switch 17 alternately connects both termination 13 and receiver 18 to the two monopoles 10 and 11. Switch 17 is a common DPDT switch available on the market and can be other than 100 cycles per second. As the switch 17 is oscillated back and forth between the two positions the two cardioid patterns are alternately formed. Because the switching occurs continuously a useful full cardioid is formed on the two sides of the antenna.
Although this invention has been described with respect to a particular embodiment, it is not to be so limited, as changes and modifications may be madetherein which are within the spirit and scope of the invention as defined by the appended claims.
1. An antenna comprising a first and a second monopole, said monopoles being spaced at a distance equal to from 0.15 to 0.35 wavelength of the frequency to be received, a transmission line connecting said monopoles, a ground plane, said monopoles being electrically connected to said ground plane, a termination and a receiver, and means for alternately connecting said monopoles to said termination and said receiver.
2. An antenna comprising a first and a second monopole, a transmission line connecting the tops of said monopoles, said monopoles being mounted on and electrically connected to a ground plane and spaced at a distance equal to 0.15 to 0.35 of the wavelength to be transmitted, said ground plane being symmetrical and large with respect to said wavelength, a termination having an impedance which matches the characteristic impedance of said transmission line, means for matching the impedance of said monopoles to the impedance of said transmission line, a receiver, and means for alternately connecting said monopoles to said termination and receiver.
References Cited UNITED STATES PATENTS Plebanski 343120 King 343-845 Sirons 343-120 Seeley 343-845 ELI LIEBERMAN, Primary Examiner.