What is a microwaves antenna and how to
design it?
The small antenna elements at
microwaves facilitate the construction of highly directive, high gain
antennas with high front-to-back ratios. At frequencies below about 2
GHz, 12- to 24-element
Yagi arrays, enclosed in plastic
shrouds for weather protection, may be used. At higher frequencies,
antennas with dish reflectors are the norm.
The aperture ratio
(diameter/wavelength) of a dish governs both its power gain and
beamwidth. The power gain of a parabolic dish is given to a close
approximation by:
Gain = 10 log10 6(D/λ)^2 × N, dBi
where D = dish diameter and N =
efficiency. Dimensions are in metres. The half-power beam width
(HPBW) in degrees is approximately equal to 70λ/D.
A microwave antenna with its dish
reflector, or parasitic elements in the case of a Yagi type, is a
large structure. Because of the very narrow beamwidths – typically
5◦ for a 1.8m dish at 2 GHz – both the antenna mounting and its
supporting structure must be rigid and able to withstand high
twisting forces to avoid deflection of the beam in high winds.
Smooth covers, radomes, fitted to
dishes and the fibreglass shrouds which are normally integral with
Yagis designed for these applications considerably reduce the wind
loading and, for some antenna types, increase the survival wind
speed.
The electrical performance of a
selection of microwave antennas is given in Table 4.1 and the wind
survival and deflection characteristics in Table 4.2 (Andrew
Antennas, 1991).
Table 4.1 2.1–2.2GHz antennas –
electrical characteristics
With shrouded Yagis and some dishes low
loss foam-filled cables are generally used up to about 2 GHz although
special connectors may be required. At higher frequencies, air-spaced
or pressurized nitrogen filled cables are frequently used with
waveguides as an alternative.
Table 4.2 Wind survival and deflection
characteristics
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