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Re: NanoVNA Under The Covers

 

I just did some very simple resistance and SWR measurements using a HP 8753ES and 85046A.

I did a very basic one port 3 point cal using a Anritsu OSL which is specified to over 3 GHz.

I took measurements at 900 MHz.

Resistance was measured using a calibrated Agilent 34401A in 4 wire mode.

Load R ohms SWR SWR notes
OSL 50.052 1.001 Flat
Cheap BNC 51.104 1.908 Sloping up with freq
Nano load 49.044 1.019 Flat
Narda 12.4 GHz 49.536 1.018 Flat
Tiny SMA 50.787 1.009 Flat


I am guessing there is some significant reactive component in the BNC terminator.? All three of the SMA loads showed a flat SWR with frequency so I'm thinking they all have a minimal reactive component.

The difference in resistances while keeping SWR low was a bit of a surprise to me.? The load that came with the Nano is over an ohm off of the load I used to calibrate yet the SWR remains at a low 1.019. Why is this?? I did the math and surprisingly this is indeed correct, per calculation the SWR should be 1.021 vs my measured 1.019.? I'd say this is darn close seeing one measurement is DC resistance and the other is at 900 MHz.

So my conclusion is that SWR is not a sensitive number to see resistance differences.

Once I read some of the references cited I can do some more advanced measurements.

Peter

On 8/5/2019 3:01 PM, Warren Allgyer wrote:
Tuck

I am not sure if you are misunderstanding (I don't think so) or simply miss-stating the concept. A load, any load, does not have an inherent return loss. It only has a return loss as measured relative to a reference. Typically the reference is understood to be 50 ohms. Minute differences in the resistance and reactance of the reference load can lead to huge differences in return loss, especially when you are talking RL on the order of 40 or more dB.

The numbers that I have shown are not the result of design or calibration routine errors but, in fact, differences in the characteristics of the loads themselves.

As a verification I have re-run the tests I made prior with the nanoVNA on my spectrum analyzer/tracking generator/RF bridge combination. It has an inherent dynamic range of 100 dB at a resolution bandwidth of 1 kHz..... 130 dB if I engage the preamp. I again used each of the three loads as a reference on the bridge with the other two measured for return loss in succession. Here is a comparison table of the nanoVNA as compared to the professional setup.

As you can see, the results are remarkably close. Especially at return losses less than 40 dB. I have no concerns whatsoever about relying on the nanoVNA results for hobby and home lab use.

Warren Allgyer
WA8TOD
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