Procedure for using NanoVNA to test Nooelec SAWBird H1 LNA

Hi Andy,

Sorry I’ve been driving all day so just seen this. I hope I’m not teaching you to suck eggs, but it seems from some of the posts that you actually need some guidance on using the VNA, then everything else should fall into place.  I am (was) a professional RfFdesign engineer specialising in antennas and LNA’s so I should be able to help! (But I don’t know much about radio astronomy – certainly not the software!)

I’m a little confused: You say it (the Sawbird) is still working – I thought your problem was that you could not get a clean measurement such as those of Alex (which I would agree look good) .

Your measurement showed a forward gain (S21) of 18dB- this should be about 40dB. The VNA will have a test output of about -10dBm. With 40 dB gain in the Sawbird this would give an expected out put of +30dBm, 1watt. This is obviously nonsense and what happens is the amplifier amplifier in the Sawbird will saturate with an output of around +8dBm, hence the 18 dB indicated gain.

To get a good measurement, calibrate your VNA with an SMA cable between input and output- when calibrated it should give a forward gain (S21) reading of 0dB. Then insert 40 dB attenuation in the path. You should get an S21 reading of around -40dB.

Then add the Sawbird (VNA output > 40dB attenuator> Sawbird input and Sawbird output to VNA input (with a dc inserter to power the SAWbird)

You should then get a forward gain measurement including the attenuation of about 0dB again. So your VNA has a gain of around 40dB as Alex states. 

Also try plotting your results over 1370 to 1470MHz range- the smaller range will make the ripple in the filter response clearer. Let me know how you get on, I’ll see if I can find any simple papers on VNA operation to send you.

Melvin