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Owen Duffy · September 27th 06, 11:59 PM posted to rec.radio.amateur.antenna

On 27 Sep 2006 10:36:42 -0700, wrote:

Hello, I have a question about dBi antenna gains. From what I gather,
dBi is a unit that specifies the directivity of an antenna and the
focus of the transmission power between a transmitter T and receiver
R1. A 25 dBi antenna gain then, for example, is referenced to a perfect
isotropic antenna.

For an isotropic antenna, received interference power at a receiver,
say R2, is simply the transmit power of T, P_T, multipled by the path
loss in watts between T and R2 that is computed in dB as follows:

0 dBi + 0 dBi - 32.45 - 20*log10(frequency) - 20*log10(distance)

My question is, how do I compute the received interference power at R2
for the same scenario where T, R1 and R2 all use 25 dBi antennas? I
know the positions and path losses between T, R1 and R2, and the
antenna gains of 25 dBi. Do I take a percentage of the transmit power
P_T because these antennas use directivity? How does this work?

The gain figure is not a single number (though a single number is
often quoted for the maximum gain), gain for a directional antenna
varies with direction (in three dimensions) and you must apply the
gain that is appropriate to the path (in three dimensions and
considering the effects of ground reflection) at each of receiver and
transmitter.

Your formula looks like the Friis formula for free space propagation.
Does it fully capture your scenario?

The scenario you describe is a simple one where one transmitter is
heard at two receivers. The calculation of the interference at a
single receiver from two discrete sources or one discrete source and a
host of interfering sources is obviously more complex.

Owen
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