Back to Basics in Microwave Systems: Antenna Directivity and Gain
Recently, I wrote about a new online training course—part of
the Microwave Andrew Antenna System Training (MAAST) series and available through the CommScope Infrastructure Academy—called Microwave Radio Antenna Link Fundamentals. This “go at your own pace and in your own time” course covers the
fundamentals of microwave transmission—the most commonly used method for backhauling to the core
network from wireless cell sites. As promised, we will be giving out previews
of the course content in this and future blog posts. Today’s lesson is about
the important concepts of antenna directivity and gain.
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
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.
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
About the Author
Jim Syme is product line manager for CommScope’s Microwave Systems division, responsible for business development in the European region with additional global management responsibility for several major OEM customers. Jim began his 23-year career with Andrew Corporation in the microwave antenna design engineering group and is a regular participant at conferences on all matters related to microwave antenna systems. Prior to joining Andrew Corporation, Jim was a design engineer for the United Kingdom Atomic Energy Authority.
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