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

No comments:

Post a Comment

Related Posts Plugin for WordPress, Blogger...