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All microwave antennas are more sensitive in some directions than others. For example, your satellite TV subscribers (or your installers) had to orient the satellite dish in a particular direction for best reception. This property of varying sensitivity with direction is called directivity.
Directivity is referenced against a practically impossible antenna to manufacture, namely an isotropic radiator. Imagine that you could suspend an antenna in the middle of a giant sphere, and you could transmit a signal from this antenna equally in all directions into the atmosphere housed within the sphere. If you could measure the signal strength with a meter at any point on the surface of the sphere and read the same strength, independent of position, you would have an isotropic radiator antenna.
In practice, such a device is impossible—there will always be some directions where more signal strength is received than others. The ability of the antenna to send a signal into some directions better than others is therefore referred to as the antenna's directivity - the higher the directivity, the more directional the antenna’s sensitivity.
Gain is another term used to describe the directional characteristics of an antenna and is very similar to directivity. Imagine that you invented an antenna that was able to direct and receive radio waves to or from one direction only. This would be a highly directive antenna and would be ideal for preventing the bleeding of signals into nearby adjacent links, otherwise known as interference. Gain is derived from the antenna directivity and includes measurements of the signal losses within the antenna system. It is therefore a much more meaningful figure.
What’s the importance of antenna directivity and gain? The ability of an antenna to direct its response in one direction in transmit mode, and capture as much signal as possible on receive, is very useful to microwave system planners. They use these characteristics to compensate for the inherent losses within the link. Loss mechanisms external to the antenna system can come from atmospheric attenuation and weather effects, while internal losses arise from within the antenna itself and from the transmission lines connecting the radio equipment to the antenna.
The microwave link planners will consider the necessary strength level of received radio waves required for satisfactorily decoding the transmissions from the operator. They can then work through the system loss budget which, together with knowledge of the transmitter power levels, can be used to calculate how much antenna gain is necessary. Once this is determined, the size of the antenna can be derived—the larger the antenna diameter, the larger the antenna gain.
I hope that makes sense. If you have questions, leave a comment. If you’d like to learn more, sign up for the Microwave Radio Antenna Link Fundamentals course. There is plenty more to learn when it comes to microwave transmission. Look for my next blog post on antenna polarization soon.