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For a yagi antenna for example, what does front to back ratio mean exactly ?

Does it mean the reverse gain at 90 deg behind the antenna compared to the forward gain at 90 deg in front of the antenna ?

Or does it mean the average or combined gain from 0 - 180 deg behind compared to 0 - 180 deg in front of the antenna ?

Or the max gain in the forward direction regardless of angle compared to the maximum gain in the reverse direction regardless of angle ?

Or is it a 3 dimensional thing ?

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Antenna gain generally refers to the maximum gain of the antenna unless the context makes it clear that some specific elevation or azimuth angle is being considered.

For a directional antenna such as a yagi, the forward gain is therefore the maximum gain of the antenna unless otherwise qualified. The gain in the azimuth direction 180 degrees opposite of the maximum forward gain (and generally at the same elevation) is the rearward (back) gain of the antenna.

If the forward and rearward gain is expressed in dB (e.g. dBi or dBd), then the front to back ratio is the forward gain minus the rearward gain. The result is then expressed in dB. For example, if the forward gain is 10 dBi and the rearward gain is -3 dBi, then the front to back ratio is 13 dB.

From a practical perspective, you can measure the front to back ratio if you have a remote signal that you can monitor while propagation is constant. Turn the antenna to find the maximum signal strength of the remote signal and note the signal strength in dB form. Turn the antenna 180 degrees from that direction and again note the signal strength in dB form. Subtract the forward dB value from the reverse dB value to compute the front to back ratio.

If you only have an s-meter available to make comparative measurements, each s-unit is generally 6 dB so you can approximate the front to back ratio by making the appropriate measurements and conversions and then subtracting the two values as described above.

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  • $\begingroup$ so in general the fb ratio is the ratio of maximum forward gain in the azimuth plane at zero elevation angle compared to the same rearward ? The reason i ask is because the radiation pattern is 3 dimensional, and an antenna has different maximum gain for each elevation angle correct ? so then really the angles should be specified so we know what we are talking about ... $\endgroup$ – Andrew Feb 27 at 4:18
  • $\begingroup$ ... it seems that the elevation angle is often not mentioned and the azimuth plane at zero deg elevation is only a small slice of the entire radiation pattern ... an antenna could have poor gain at zero degrees but really good gain at say 10 deg, and gain at 10 deg is just as important ... $\endgroup$ – Andrew Feb 27 at 4:18
  • $\begingroup$ @Andrew The elevation angle is simply wherever the maximum forward gain occurs. This angle can change a bit from 0 degrees elevation due to ground reflections but in many cases it is considered the elevation angle projected by the boom of the antenna. $\endgroup$ – Glenn W9IQ Feb 27 at 10:43

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