OPTIMUM OMNIDIRECTIONAL ANTENNA
Optimum coverage omni antenna To choose an omnidirectional antenna with maximum gain is not always the best way to optimise WLAN coverage. We have made a studie on a number of omnidirectional antennas and explain why to choose an omnidirectional antenna with 6 dBi gain.What is antenna gain?
Gain, or actually directivity, comes from stacking a number of antenna elements and feeding them in phase, so the transmission is focused in a desired direction. The element, a dipole, gives 2.15 dBi gain. By feeding two elements (dipoles) with the same phase and amplitude (half the available power to each element) we get 3 dB stacking gain, ie a total of 5.15 dBi. Another 3 dB is gained for each doubling of the number of elements.
Gain and directivity
The antenna is a passive unit where no energy is inserted. The electrical power fed to the antenna can be transformed into radiated power. However, energy can be lost in the antenna due to material loss. Examples on non-optimal materials are plastics and fibreglass ie circuit boards. It is not unusual that antennas for 2.4 GHz have as little as 50% efficiency.
Gain is directivity (D) multiplied by efficiency (n).
A comparison between omnidirectional antennas
In the radiation pattern we compare a VO6/24 6 dBi omnidirectional antenna with an 8 dBi omni antenna. The VO6/24 has three elements (dipoles), whilst the bigger antenna has 6 elements. The directivity for that antenna could be 9-10 dBi. It is common to not feed all elements with the exact same amplitude and phase, to get a better nullfill or better coverage right under the antenna, at negative elevation angles.
In the diagram the radiation pattern of the two antennas are plotted using the same scale, ie with 6 and 8 dBi gain. The 8 dBi omnidirectional antenna has its maximum gain just above the horizon. At moderate elevation angles 0-2 degrees the bigger antenna is a bit better. At angles where clients usually are placed down to 15-20 degrees, the 6 dBi antenna is superior. Note the deep dips in the radiation pattern for the 8 dBi antenna. A small movement of the client can make a big difference for the received signal strength.How big omnidirectional antenna is it possible to build?
Counting backwards on what is possible to implement at a reasonable cost with a decent production technique, it should be possible to build a correct antenna with 8-9 dBi gain. There is such an antenna on the market, but sad to say it only works for 2350 MHz. The difference in coverage for an antenna with 6 and 8 dBi gain is however very small.
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