Anyone Using Nano VNA to Tune FM Antennas?

[dinger]

I'm struggling to get believable results due to 50 ohm VNA VS 75 ohm FM impedance. Some say you need to use conversion transformers while some say a mechanical adaptor will give some loss but won't affect resonant freq which is what I'm most interested in. I've built my own 75 cal jigs so I can calibrate the VNA in that range but i'm not trusting the results at this time. Interested if anyone else has gone down this path and any tips for success.

 

[k6sti]

You can analyze 75-ohm systems using the latest NanoVNA firmware. Ask about it here:

nanovna-users groups.io Group

Users of nanovna small VNA Files: https://groups.io/g/nanovna-users/files Wiki: https://groups.io/g/nanovna-users/wiki

groups.io

Brian

 

[Bob@FM]

Describe more, and I can try it on mine. I would get some cheap 75 ohm F connector terminations, and start by measuring those. These double as loads for cal.
I use nanoSaver PC to drive it, it gives many more options, and is easier for me than the tiny touch display. The latest firmware is said to support 75 ohm via math conversion, per here https://groups.io/g/nanovna-users/message/35067
I never upgraded my firmware, it's probably not hard. Otherwise, I would use a sma or BNC 50-75 ohm impedance converter DIY or bought (resistor based has ~6 dB loss), SMA or BNC to F, and calibrate loss out with 75 ohm load, etc.
Are you tuning something like a Jpole? If so, try a simpler device like a 60" wide dipole.
This details step by step SWR https://n7tar.org/wp-content/uploads/2021/12/Using-NanoVNA-for-SWR-Measurement-Rev1.pdf

 

[dinger]

Got firmware updated. Trying a few things...

 

[Bob@FM]

Great, please let us know how it goes.
What firmware version did you use? btw, this link has relevant info on the topic as well -

A question about checking SWR with Nano vna

groups.io

The "you can brick your nanoVNA" warnings and multitude of firmware choices scattered on different sites make you feel more accomplished afterwards, I suppose.
I've used my MFJ-259B to check the entire run of 75 ohm cable from inside to the antenna itself, but not used the little guy for that yet.
I'm currently wading through the gauntlet of downloads and procedures.
<edit> - firmware upgrade worked fine, after some "fun". Used this version, which supports 75 ohm

Releases · DiSlord/NanoVNA-D

Firmware for NanoVNA, NanoVNA-H, NanoVNA-H4. Support SD Card, external Serial connection, fast measure, fast exchange vs CPU - DiSlord/NanoVNA-D

github.com

 

[Bob@FM]

Forgot to add this, if not obvious - you need to measure the antenna far away from other objects, especially metal ones - best case, outdoors in large space, up on a pole.

 

[dinger]

I have an H4 and am now at firmware 1.2.2. The update was a bit of an adventure but I managed without bricking the darn thing so I feel pretty smaht right now! I'm sweeping 88 to 108 Mhz for the cal and measurements. The antenna is in the attic and I'm measuring at the end of the 25' coax where I listen. I'm getting comfortable with what I'm seeing but the firmware update probably had little to do with it. There are alot more options to view with the firmware update but SWR and the Smith plot are all I'm really interested in. I want to compare readings right at the antenna and at the end of the coax next to see how much effect the coax has on things.

 

[Bob@FM]

Are you measuring a crossed omni, with a phasing cable tying the elements together? ?
If so, I can see where that design could make for some weird kinks on the SWR curve. Or maybe a J-pole.
I measured my antenna and lead a with the MFJ-259B and it was pretty flat SWR = 1.0, with maybe 5-10% deviance across the band.
It was surprising to me. That's not how most of my modest home made dipole variants looked.
That 1.0 SWR was with a 50:75 converter on the output. With no converter, the 50 ohm "good" SWR reading (for a 75 ohm antenna) was 1.5.
Measuring at 50 ohms and targeting 1.5 SWR for tuning was reliable and repeatable for short cable runs. APS-9 rotor'ed on the roof, through a wetzel balun then maybe 50-60 ft of coax.

 

[Bob@FM]

Forgot to comment on the effect of coax - It should measure flat, period. Put a 75 ohm load on the end (antenna end) and measure it. I learned that trick from a CATV tech who serviced our house. He found a bad buried cable, and replaced it. Solved a lot of internet connection and performance problems.
You also use TDR to measure for breaks or problems along the cable. If needed. Fun stuff. Better digital scopes may have TDR too.
I would sweep over a wider area too - try 60 -150 MHz or broader. To see the bigger picture of how it reacts outside the band.

 

[dinger]

When I look at my 75ohm load at the VNA I get a nice dot in the center of the smith chart. At the end of the coax I get a 1" diameter circle around the center. I suppose I could calibrate at the end of the coax but the receiver doesn't get the luxury of being hooked up there so I believe I need to look at the whole thing. Still playing around with it. One video I watched suggested the resonant freq is where there is an impedance match and not the SWR dip. 99% are all using SWR or Logmag for the RF. I've also introduced an SDR into the mix. and having fun with that. You can really see how aiming the antenna pulls stations out of the noise like magic. I think I might have better luck optimizing reception issues using the SDR after determining which antenna might be the best choice with the VNA.

 

[Bob@FM]

I don't use the smith chart at all. So never tried that test.
I would just put SWR on the display, and compare cable with 75 load, to 75 load alone. If you want to compare, try a 50 ohm load at the end, too, and use 50 ohm impedance option.
With a laptop, 'saver makes it cake to get a boatload of measurements at cursor values, and save them. I use Snip and Sketch to save the screenshot, on win10.
NanoVNASaver (windows) download link
Radio stations use SWR meters to check transmit antennas - they can detect cable and antenna problems with it. If nothing else, make sure you don't have a bad cable or balun.
But I get you 100% on the SDR - it shows everything, including what kind of HD mess a station is, or is not putting out.