Antenna gain figures.

This page is intended to give an insight into possible reasons why an antenna may claim a high gain figure, yet not perform as well as the figure suggests it should. People who wish to buy an antenna should be aware that manufacturer's gain figures are not always a good indication of which is best.
Before looking at how a manufacturer can get high gain figures, let's look at how your situation can affect the performance you get.

Ground plane
The radiation pattern and gain is greatly changed by the ground. A lossy ground will lose signal and gain, costing you in distance obtained. On a vehicle you may well see a difference between signal with different mounts.
If, as with most mobile types, the antenna is designed to be mounted on a ground plane the size of that ground and the position of the antenna on it can make a big difference.

Tuning
A high SWR can have an effect on the radiation pattern. It's not just the amount of reflected signal, but what effects that signal can have on the system.

Height.
The radiation pattern changes with the height above ground. Unless you fit the antenna at the same height as they did, then you will get a different result.
The picture on the right shows two radiation patterns, one in light grey and one dark grey. These are both patterns from the same antenna, the difference being the height above ground that this antenna is mounted at.

Obstructions.
Any object near to the antenna will have an effect on performance. Such objects can absorb or reflect signal. In the real World, you are usually unable to avoid such obstructions.

Does this account for figures?
Gain figures are dependant upon many variables. Unless you match the test conditions, you cannot hope to get the same results as the manufacturer claims.
However, some figures seem to beat the laws of nature. But there are tricks that can be used to get these figures...
 

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