7.2 Antennas with dimensions >> wavelength.
microwave and near-optical frequencies it is usually possible to build
antennas whose sizes are many wavelengths. These tend to take the general
forms illustrated in figure 7.5. The main element of the antenna is either a
mirror or a lens whose diameter is many wavelengths. The lens/mirror couples
free space fields into or out of a much smaller feed element. This
can be a simple dipole, or the ‘cut end’ of a standard waveguide
or fibre. Here we will concentrate on the use of microwave guide, but
similar arguments can be applied to other systems at other frequencies.
Standard rectangular waveguide consists of a rectangular metal pipe whose cross dimensions are approximately . This form of guide is widely used because it will only carry a single mode. This means that we can precisely define and control the field pattern running along the guide. In principle we can use an open guide end to feed signals to a lens/mirror, or place it at the focus of a lens/mirror to pick up gathered power. However, the effective area of an open or cut single mode guide is around
This means that the open end will tend to radiate power into a solid angle of around 8 sterads — i.e. about 2/3rds of a sphere! (In fact, the actual effective area isn't exactly equal to the physical opening area, but it is of a similar size so this estimate is good enough to reveal the problem.) This would require a very ‘deep’ parabolic reflector and and impossibly large lens to collect. To avoid this problem it is usual to feed the large mirror/lens antenna with a smaller feed antenna or feed horn whose dimensions are between those of the guide and the main antenna. This feed is a sort of ‘mini antenna’ which helps us couple the guide to the main part of the antenna.
The simplest way to do this is to flare the end of the rectangular guide into a conical shape. This looks a bit like a rectangular version of the ‘bell’ on a brass band instrument and it does a similar job! The open end of the resulting pyramidal feed is the base of the pyramid and we can. in principle, give this as large an area as we like. In practice, things go may wrong if we flare up to an open end much bigger than about 5l × 5l. However, a horn with this aperture size will have an effective area of
this is directional enough to feed a practical mirror or lens system. In practice, most antennas of this type use more than one lens or mirror, and may use more complex feed arrangements. This leads to improved performance, but the basic method of operation is the same as described above. The gain and directivity of the total antenna system is largely determined by the area of the main lens/mirror illuminated by the feed system.
The larger the mirror/lens, the higher the gain and the narrower the antenna angle at a given wavelength. To avoid edge diffraction problems it is usual to illuminate only the central portion of the lens/mirror. Hence the effective area of antenna is generally between 90% and 50% of the physical mirror/lens area.
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