Gain of an Antenna measurement

If you want to measure the Gain of an antenna, a Standard Gain Horn (SGH) is necessary. A SGH doesn't have to be a horn antenna. It just has to be an antenna with known, repeatable gain and a polarization compatible with then antenna you intend to test. The calibration procedure for a SGH is here.

Test Setup:

The  Network Analyzer could be replaced by a Signal Generator and Power Sensor if you prefer; it will make calibration harder, in my opinion. An amplifier may be necessary to bring your signal above the noise floor of the Network/Spectrum Analyzer.

Calibration Procedure:

• Find an Anechoic chamber or antenna test range that is quiet at your frequencies of interest (FOI).
• There are no background signals (i.e. cell phone tower) large enough to be picked up by your SGH.

• Acquire two SGH's.

• Space the horns apart from one another so that we measure the Far-Field Gain of the SGH's.

• Rule of thumb: distance between antennas, R, should be >> wavelength at FOI. (R > 100λ)

• Calculate the freespace loss (Lf) from Friis Equation: Approximate the distance R between the horns with a measuring tape, aperture-to-aperture. 
• Calculator is here. 

• Set the Transmission Power (Pt): Adjust the power of the Network Analyzer to overcome the Freespace loss minus the expected Gains of the transmit and receive antennas; Signal should be well above the noise floor of the Network/Spectrum Analyzer.
• Pt = Lf - Gt - Gr + Pr, where Gt and Gr are the Gains of transmit horn and receive AUT.
• Set Pr, in this equation, to an appropriate level above the noise floor of your receiver (~ -20 dBm for most Network Analyzers)

• Align the horns as best as possible so that they both have the same polarization and are facing each other.
• Make fine adjustments (up/down, left/right, swivel) to each horn until you see a maximum amplitude for S21 on the Network Analyzer. 

Photo copied from Wikipedia. Author: T. Truckle.

• Antennas typically have a main lobe of radiation, flanked by sidelobes that radiate in all directions. The purpose of aligning the horns until maximum power is received is to ensure that Gain is measured at this main lobe

• Conduct an S21 THRU calibration on the Network Analyzer at your FOI. S21 should now read 0dB.

The Measurement:

• We know the Gain of the receive-SGH because it was measured previously.
• Enter an amplitude offset into the Network Analyzer equal to the SGH gain at your FOI.
• ex: If your SGH has G = 25 dB @ FOI, the Network Analyzer should have S21 = 25 dB.
• If you do not have this option, or if you are using a Spectrum Analyzer, make a note that whatever maximum power displayed is equal to the SGH gain at your FOI.

• Replace the receive-SGH (#2) with your AUT. 
• Align the antennas as in the calibration procedure.

• Note the S21 amplitude on the Network Analyzer (dB) has changed.
• It should now display the gain of the AUT.
• If using the Spectrum Analyzer, add the change in power to the gain of your SGH to determine the gain of the AUT.

*Please Note: It does not matter if the AUT is the transmit or receive antenna. Gain is identical in either direction. Just make sure to offset S21 by whichever SGH will be replaced by the AUT.