Inconsistencies between VNA and in-line SWR meter.

[SvenMarbles]

I've been noticing that what my nanoVNA says the SWR will be across certain points and what my in-line Surecom SW-102 says while the radio is actually in use on those spots, are different. What says on the nanoVNA should be a 1.5, actually reads back 1.2 in my meter. And it sort of runs consistent in that deviation amount as I go.. What's the real number, and why are they different?

[SteveShannon]

Just now, SvenMarbles said:

I've been noticing that what my nanoVNA says the SWR will be across certain points and what my in-line Surecom SW-102 says while the radio is actually in use on those spots, are different. What says on the nanoVNA should be a 1.5, actually reads back 1.2 in my meter. And it sort of runs consistent in that deviation amount as I go.. What's the real number, and why are they different?

If the VNA is accurately calibrated it will probably be more accurate than an SWR meter. Unless you are using something like a Bird meter with the right size slug, the directional RF coupler in most consumer SWR meters are non-linear in detecting the very wide range of RF power that’s transmitted forward and being reflected at low SWR.

The good news is that it really doesn’t matter much at such low SWR values. 

[SvenMarbles]

6 minutes ago, SteveShannon said:

If the VNA is accurately calibrated it will probably be more accurate than an SWR meter. Unless you are using something like a Bird meter with the right size slug, the directional RF coupler in most consumer SWR meters are non-linear in detecting the very wide range of RF power that’s transmitted forward and being reflected at low SWR.

The good news is that it really doesn’t matter much at such low SWR values. 

 

Well they're both sort of equally budget type meters. The nanoVNA was like $65. I do run through the calibration slugs before each use. I don't really have it in the budget for a RigExpert.

Yeah it's all fine, but I do use the vna a lot for experimenting with other antennas and making wires etc..

[AdmiralCochrane]

In practice those readings are so close I would not have given it a second thought.

[dosw]

I trust the NanoVNA more. Try hooking it up to your computer so you can get nearly infinitely fine-grained scan points. Or narrow your scan range, or set the center instead of start/end points. Also there's a menu buried deep within for increasing the scan power. I haven't used it, because "auto" seemed good enough.

 

I've found the two tools disagree a bit, too. 

 

Be sure you're calibrating for the exact range you're sampling.

[SteveShannon]

54 minutes ago, SvenMarbles said:

 

Well they're both sort of equally budget type meters. The nanoVNA was like $65. I do run through the calibration slugs before each use. I don't really have it in the budget for a RigExpert.

Yeah it's all fine, but I do use the vna a lot for experimenting with other antennas and making wires etc..

There’s no reason to believe that your NanoVNA is less accurate than a RigExpert. I have both. The RigExpert is more convenient, but not necessarily more accurate. In fact because you calibrate your VNA against the standards each time you may even be seeing more accuracy from the NanoVNA. 

[AdmiralCochrane]

If both devices are 100% accurate, the difference can be explained by the different load presented on both the coax and the antenna, more likely the coax.

 

You haven't told us the type of coax or the length or the transmitting power.  What you are seeing is likely cable loss - some power not getting to the antenna (being absorbed on the way up) and some power not reaching the SWR meter on reflection (some power being absorbed on the way back down). The skew of the readings agrees with this.  (WOW, I got to use "skew" in a sentence!)

Moving the SWR meter to the other end of the coax may or may not confirm this. Also possibly changing the transmit power.

A way to help most people understand this is a dummy load on the end of the coax.  The SWR of a dummy load (in ideal conditions) is exactly 1:1 because all of the energy is consumed at the dummy load end; NONE is reflected back.

 

Some time back someone posted that they measured 1:1 SWR with a dummy load.  As a joke, I posted "I hope so."  Only a few people got it.

 

[dosw]

You need "triple modular redundancy" if it *really* matters. That means, a third opinion, basically.

 

The old sailor's saying was "Never go to sea with two chronometers; take one or three."

 

But you're below 2:1; it doesn't matter enough to worry.

[RayDiddio]

8 hours ago, SteveShannon said:

There’s no reason to believe that your NanoVNA is less accurate than a RigExpert. I have both. The RigExpert is more convenient, but not necessarily more accurate. In fact because you calibrate your VNA against the standards each time you may even be seeing more accuracy from the NanoVNA. 

Which RigExpert model do you have? I am shopping them currently because I am having fun building antennas lately and I want the convenience factor.

[AdmiralCochrane]

45 minutes ago, dosw said:

You need "triple modular redundancy" if it *really* matters. That means, a third opinion, basically.

 

The old sailor's saying was "Never go to sea with two chronometers; take one or three."

 

But you're below 2:1; it doesn't matter enough to worry.

The old saying "A man with a watch knows what time it is, a man with 2 isn't certain."

[SteveShannon]

53 minutes ago, RayDiddio said:

Which RigExpert model do you have? I am shopping them currently because I am having fun building antennas lately and I want the convenience factor.

I have two of them.  I bought a used AA-600 from one of the local repeater owners because it was a good deal even though I already have the Stick Pro. The AA-600 has a larger screen. The Stick Pro is small and convenient in a go bag.  Plus the Stick Pro has Bluetooth and I can see it with my phone. I helped a friend put up an antenna and I attached the Stick Pro right to the antenna and we would tune the antenna then put it up in the air and I could see what the SWR was in the air. If I could only have one I’d keep the Stick Pro. It’s the black thing right at the bottom of the antenna in the picture:

[WSAA254]

I believe its already been mentioned that small differences in coax, connections etc may explain part of it.  I have taken my SWR reading before and after a antenna switch and see a .1 change.  I think these small "parasitic loads" can have an effect.   When you see folks make a antenna placement change on a car and see much larger differences than you mention, it hints at the differences that these "parasitic elements" make.  Either way with such low numbers I think you are good.  All the best

[RayDiddio]

19 hours ago, SteveShannon said:

I have two of them.  I bought a used AA-600 from one of the local repeater owners because it was a good deal even though I already have the Stick Pro. The AA-600 has a larger screen. The Stick Pro is small and convenient in a go bag.  Plus the Stick Pro has Bluetooth and I can see it with my phone. I helped a friend put up an antenna and I attached the Stick Pro right to the antenna and we would tune the antenna then put it up in the air and I could see what the SWR was in the air. If I could only have one I’d keep the Stick Pro. It’s the black thing right at the bottom of the antenna in the picture:

Awesome. Thank you very much for this feedback. I was looking at the Stick Pro and it's actually in the cart at HRO right now so I guess I will stick with it.