Gain of antenna
Gain=f(eff,directivity)
one way to do it is to go to "open place" conect your antana to knowen osilator, and then away from it, and musear the 3dB power with rx meter and chack it in a table. or to compere it to dipol (a dipol have 1.65 dB gain)
What is the best method to measure the gain of antenna?
I use EMC chamber to measure it.
General use a standard wide band antenna, we know this antenna gain.
And tested antenna to be apart from standard antenna 3m.(>1GHz)
Sweep a fixed power signal to the tested antenna.
Standard antenna receive and measure power of signal by spectrum.
Calculate the 3m path loss.
Measured power - path loss + standard antenna gain - sweeper output power = The tested antenna gain
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May be u can direct to connect spectrum and sweeper for calibration.