Reduced back lobe spiral antenna

A ferrite sleeve placed about the cavity portion of an antenna reduces back lobe radiation without degrading the antenna's front hemisphere radiation pattern. The thickness of the ferrite sleeve is selected such that the sleeve highly attenuates the surface currents on the cavity at the lower end of the operating frequency range of the antenna.

The present invention relates generally to high-frequency antennas. 
Antennas capable of transmitting high-frequency signals, typically in the 
range of between 5 to 20 GHz, are employed in such varied applications as 
communications, radar systems, and electronics countermeasures systems. 
One type of antenna construction employed for such high-frequency 
applications is a cavity-backed spiral antenna in which a radiating 
element in the form of two interweaved spiral-shaped conductors is located 
on one surface of a cylindrical conductive member, the interior of which 
defines a cavity. Microwave energy is applied to the antenna by means of a 
microwave connector connected to the opposite or back surface of the 
cylinder. 
In antennas of this general construction, it has been found that 
high-frequency currents generated on the outer surface of the cylindrical 
conductive member may produce a significant backlobe pattern of radiation, 
which, when received by a nearby receiving antennas, will cause 
interference with the reception of other signals at the receiving 
antennas. 
Prior attempts at reducing this undesired back lobe radiation, particularly 
in those applications in which a broad frequency range of signals is to be 
transmitted by the antenna, have caused either a degradation in the 
antenna's front lobe radiation pattern or required an increase in the size 
or diameter of the antenna to maintain the desired front lobe radiation. 
Since these results are undesirable from either a technical and/or an 
economic point of view, no satisfactory means has yet been devised to 
effectively reduce back-lobe radiation in such antennas as cavity-backed 
spiral antennas designed to transmit a wide range of frequencies. 
It is an object of the invention to provide an antenna of the type 
described in which back lobe radiation is significantly reduced over a 
wide range of frequencies. 
It is a further object of the present invention to provide an antenna of 
the type described in which back lobe radiation is substantially reduced 
over a broad frequency range without degrading the forward lobe 
transmission or increasing the size of the antenna. 
To these ends, the antenna of the invention includes a ferrite sleeve 
placed about the cavity portion of the antenna. The ferrite sleeve is 
effective to reduce the back lobe radiation pattern without degrading the 
performance of the front hemisphere radiation pattern. The thickness of 
the ferrite sleeve is selected such that it is most absorbent of surface 
currents at frequencies in the lower end of the operating frequency band 
of the antenna.

In the embodiment illustrated in FIGS. 1 and 2, the invention is shown as 
employed in a cavity-backed spiral antenna for purposes of example, it 
being understood that the invention may also be used to comparable 
advantage in other types of high-frequency broad-band antennas. 
As therein shown, a cavity-backed spiral antenna, generally designated 10, 
includes, as is conventional, a rear conducting surface 12 to which a 
microwave connector 14 is connected to provide drive energy to the 
antenna. Rear surface 12 of the antenna is joined to a front radiation 
surface 16 by means of a cylindrical conducting surface 18, which defines 
an interior reflecting cavity portion 20. 
As shown in FIG. 2, the radiation surface 16 comprises two interweaved 
spiral conductors 22 and 24 respectively center fed at conduction lands 26 
and 28, which may, as is known, be formed by printed circuit techniques on 
an insulating surface, such as a ceramic or glass. The lands 26 and 28 are 
connected to the microwave connector 14 by a suitable feed device (not 
shown in the drawings). 
In a conventional antenna of the type thus far described, in addition to 
the desired front lobe radiation, a considerable amount of back lobe 
radiation is produced at the rear surface of the antenna, which is 
wasteful and is also capable of producing undesired interference in many 
antenna applications. The extent of such undesired back lobe radiation 
ranges from about -25 dB at the high end of the antenna's operating 
frequency range to about -10 dB at the low frequency range. 
In accordance with the present invention, such back lobe radiation is 
significantly reduced over a wide frequency range by the placement of a 
ferrite sleeve 30 about the cavity portion of the antenna. As shown in 
FIG. 1, ferrite sleeve 30 is preferably arranged around the entire 
peripheral cylindrical surface of the antenna and extends between the 
front and rear surfaces 12 and 16. Although this invention comprehends the 
use of any ferrite microwave-absorbent material as the material for the 
ferrite sleeve 30, sleeves made of a ferrite material manufactured and 
sold by the Emerson Cumming Corp. under the designations MF 124 and MF 190 
have proven to be particularly effective in reducing back lobe radiation 
in a cavity-backed spiral antenna of the type illustrated in FIGS. 1 and 
2. 
The reduced back lobe radiation achieved by the antenna of the invention is 
illustrated in FIG. 3, which represents field strength as a function of 
angle at a frequency of 10 GHz, although equally favorable results have 
been achieved in an antenna of this type at frequencies between 6 GHz and 
18 GHz. The field strength curves 32 and 34 in FIG. 3 are respectively the 
field strength patterns of the vertically and horizontally polarized waves 
produced in a conventional spiral antenna, whereas field strength curves 
36 and 38 are respectively the vertically and horizontally polarized wave 
patterns produced in a spiral antenna with a ferrite sleeve arranged 
thereon according to the present invention, as shown in FIGS. 1 and 2. 
As can be seen by a comparison of curves 32 and 34 with curves 36 and 38, 
the back lobe radiation that occurs at an angle at 180.degree. is 
considerably reduced, in the order of 7 dB, in the antenna of the 
invention, whereas the front radiation lobes that occur at an angle of 
0.degree. for both the vertically and horizontally polarized waves are 
essentially the same in both the prior art antenna and the antenna of the 
invention, differing by only about 1.6 dB. Moreover, the pattern of the 
front lobe, that is, the desired circular polarization and shape of the 
front hemisphere, is not modified in the antenna of the invention by the 
placement of the ferrite sleeve about the antenna cavity portion. 
Significantly, the level of the back lobe radiation is reduced in the 
antenna of the invention to an extremely low level, below -20 dB over the 
entire frequency band of 6 to 18 GHz; more particularly, back lobe 
radiation is reduced to -20 dB at a frequency of 6 GHz and is gradually 
reduced still further to -26 dB at a frequency of 18 GHz. In contrast, the 
conventional cavity-backed spiral antenna typically probes back lobe 
levels, which, as noted previously, range from about -25 dB at 18 GHz to 
-10 dB at 6 GHz. In addition, the beamwidth of the back lobe in the 
antenna of the invention is narrowed but circular polarization is 
maintained at the 180.degree. axis. 
The thickness of the ferrite sleeve 30 is preferably selected so that it is 
most absorbent of surface currents at frequencies at the lower end of the 
operating frequency range of the antenna, which, as in the embodiment of 
the invention herein shown, would be in the range of 6 or 7 GHz. As 
described by Naito and Suetake in their article entitled "Application of 
Ferrite to Electromagnetic Wave Absorber and Its Characteristics", IEEE 
Transactions on Microwave Theory and Techniques, Vol. MIT-19, No. 1, 
January, 1971, ferrite materials have two matching frequencies at two 
respective matching thicknesses at which the ferrite would be a perfect 
absorber of radiation. It has been found that the use of a ferrite sleeve 
having a matching frequency at the lower end of the antenna frequency 
range significantly suppresses back lobe radiation at the low frequency 
end as a result of this absorption, and the increased lossiness of the 
ferrite sleeve 30 to radiation at higher frequencies also suppresses back 
lobe radiation at the high end of the antenna frequency band. A ferrite 
sleeve having a thickness in the range of 160 to 200 mils was found to 
produce a matching or absorbent frequency at 6 GHz, and produced the 
desired suppression of back lobe radiation at the low end of the antenna 
frequency band as well as at the higher end of the band. 
It will be appreciated from the foregoing description that a cavity-backed 
spiral antenna is provided by this invention in which back lobe radiation 
is substantially reduced over a wide frequency range, while the forward 
lobe radiation of the antenna is essentially unaffected. It will also be 
appreciated that modifications to this embodiment of the invention may be 
made without necessarily departing from the spirit and scope of the 
invention.