How to find antenna gain?

Question

Given:

• Transmit power 1 W
• Receiver sensitivity -95 dBm to -85 dBm
• Frequency 2.4 GHz
• distance 2 km

I have used Friis equation to compute it. I will use 2 similar antennas.

$$ P_r = P_t + G_t +G_r - F $$

where F = free space attenuation.

$$ G = (P_r-P_t+F)/2$$

I got G<0, is it correct?

Answer 1

The Friis path-loss equation using isotropic antennas can be expressed in decibels: -

Path-loss (dB) = 32.45 + 20\$log_{10}\$(f) + 20\$log_{10}\$(d)

Where \$f\$ is in MHz, \$d\$ is in kilometres and free-space is assumed.

At 2.4 GHz and 2 km, that's a path-loss of 32.45 dB + 67.60 dB + 6.02 dB = 106 dB.

Your transmit power is 1 watt (+30 dBm) and your receiver sensitivity might be -85 dBm so, that is a difference of 115 dB hence, you have a surplus of 9 dB and, you can use isotropic antennas in free space for this. Hence your value for antenna gain is negative.

However, over terrain (obstacles) and, with weather changes and man-made noise, a general approximate rule of thumb is to add 20 or 30 dB to your path loss to cater for these extremes.

This is called fade-margin.

Now, your path-loss is 136 dB (30 dB added) and, your transmit power and receiver sensitivity fall-short by 21 dB hence, you need antenna-gain in my humble opinion.

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