Omnidirectional antenna with low RCS

CALCULATED SCATTERING PATTERN FOR OMNIDIRECTIONAL ANTENNA WITH LOW RCS... 

Calculated Scattering Pattern for Omnidirectional Antenna with Low RCS / 217... 

Many readers intuitively assume that it is inherently impossible to design an omnidirectional antenna with low backscatter. This assumption is based upon considering only a simple dipole as discussed in Common Misconceptions, Section 7.6.2. However, we have shown in this chapter that just as a planar array of dipoles backed by a groundplane has a very low backscatter, see Chapters 2 and 5, so can a circular array of dipoles placed around a conducting pipe. It was remarkable that the measured RCS at one frequency was 40 dB below the same array covered by foil. A 13-dB reduction was obtained over a bandwidth of almost an octave. It should also be noted that the load resistors were somewhat different than conjugate match. This is simply because the curved surface has a small residual scattering component in contrast to the planar surface which has none (in principle). No broadband technique as illustrated in Chapter 6 and Appendices A and B was applied. 

Furthermore, in Chapter 8 we will show that in order to design a parabolic antenna with a low RCS, it is crucial to use a feed with low RCS. Such a feed is readily obtained by a relatively simple modification of the omnidirectional antenna presented in this chapter. Note that no power is lost in the omnidirectional antenna or in the low RCS feed when transmitting. See also Section 7.6. 

So far we have considered planar arrays with a pencil beam. However, there is also interest in antennas with omnidirectional pattern and low RCS. Intuitively, many designers feel that such an antenna is inconsistent with the fundamentals of physics. Usually this conclusion is based on the facts associated with a simple dipole or monopole. However, we show in Chapter 7 that if we instead consider a circular array of dipoles arranged around a conducting cylinder, we can indeed obtain an omnidirectional pattern and very low RCS for signals incident anywhere in the equatorial plane. A substantial reduction was obtained over a frequency range of one octave. 

A large flat aperture is most often associated with a narrow pencil beam. However, we shall in Chapter 7 consider antennas with omnidirectional pattern and low visibility in the backward direction over a broad band. Further discussed in Chapter 8 is how to design a feed for a parabolic cylinder that will produce an RCS about 6dB lower than with no feed at all. The design of such a feed is closely related to the omnidirectional design. We emphasize, however, that parabolic systems never can attain the inherently low RCS level encountered for the flat aperture over a broad frequency band. 

In Chapter 2, 5, and 6 we considered arrays backed by a groundplane. Such configurations typically produced pencil beams. There is, however, also considerable interest in antennas with omnidirectional radiation patterns. The principles for these will be presented in this chapter. It should be emphasized, however, that we shall limit ourselves to designs with inherently low RCS. That rules out the common simple dipole and monopole—in fact, most omnidirectional antennas. 

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