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# Measuring gain in dBi with VNA - s21

## Measuring gain in dBi with VNA - s21

(OP)
I've measured my prototype antenna using a known Reference Log Periodic. Test set up was the known Log pointing at my prototype Antenna Under Test (AUT) using S21 on my VNA.Cable length was calibrated out.

How do i determine the AUT's gain in dB or dBi? I thought,
i took the LogMag measured data (i.e. -30db), and subtracted the free space loss (say +40dB) and my reference antenna gain (+4dB). Would this yield:

AUT (dBi) =  40-30-4 = 6dB or +8.14dBi???

Thanks,

### RE: Measuring gain in dBi with VNA - s21

How does the tolerance (accuracy, stability, repeatability, multipath, interference) of your estimated pathloss (free space loss) compare to the tolerance required of you calculated gain? For example, if the pathloss is 40dB, but this is +/- 1.5 dB.

How about using a third antenna and *comparing* it to the known (?) gain of your reference log periodic antenna to the AUT?

And, with three antennas, then you could also perform the three-antenna self-calibrating method.

And then compare all these various results to see if they all align.

Sounds to me like a couple of fun days at work...

### RE: Measuring gain in dBi with VNA - s21

Look up Friis equation.
Measure S21 with full two port calibration.

Measure at three different lengths from your Log Periodic to your unit under test and compare the answers for all three lengths with the Friis equation.

The 3 antenna method is good too, it all depends on your environment whether it's clean or not.

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