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RX antenna is connected to the Spectrum Analyzer whose reading is regarded as P_RX. TX antenna is connected to a transceiver module that is set to work at the target frequency and emit certain power (P_TX). Question: Can I deduct S21 from P_TX-P_RX?

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Yes, you can deduce an S21 (forward gain) from that by dividing P_RX / P_TX but I think that that number would not mean very much.

Move the RX and TX further apart: that S21 will change

Move the TX antenna (assuming non-isotropic radiation pattern): that S21 will change

Move the RX antenna (assuming non-isotropic radiation pattern): that S21 will change

Stand in between RX and TX: that S21 will change

Place something conductive between RX and TX: that S21 will change

So I do not see how the S21 you'd get would mean anything at all. It would be the S21 under those circumstances. I fail to see how that number can be useful in any way.

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  • \$\begingroup\$ What I am trying to get is how S21 changes when I move TX away. The TX is moving inside a certain medium. Hopefully, I can get a path loss model out of such setup. \$\endgroup\$ – luw Jan 24 '18 at 16:22
  • \$\begingroup\$ OK, if the medium is the only thing you change then indeed the S21 results could mean something. I was thinking that you were interested in the absolute value but you're not, you're interested in the relative values. \$\endgroup\$ – Bimpelrekkie Jan 24 '18 at 17:18
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You can perform a measurement of scalar S21 (power gain only) using Spectrum analyzer on the medium (antenna + free space + conductor).

You won't have good accuracy on the phase information compared to VNA.

People do use SA + SG for S21 measurements especially if they want to get the channel matrix over a long distance.

In that case, to ensure a good phase accuracy, one has to use a GPS clock for synchronization, over the two remote Tx and Rx.

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