The AM Forum

So I bought one of the nanovna units, and I've gotten the PC software working and figured out how to calibrate it and get it to do sweeps.  Really glad to get it up on the computer monitor where I can see the thing...that screen is too small for my eyes.

What I don't know is what to do with the information.  For example, I have an off center fed dipole, which is as long as can fit between the supports available to me. It is fed with ladder line down to 10' above the ground, where I have a (bad/lossy) transformer balun to go to LMR400 on into the house. I plan next week, when an SMA to clip leads adapter arrives on Wednesday, to get out the step ladder and see what the antenna is without the transformer.  Then I want to wind a new transformer (on the proper mix toroid, the one up there now is the wrong mix for the lower bands). So when I go to figure out a turns ratio, which of the many numbers would I choose for the design secondary impedance (50 for the coax side of course)? I'll be doing sweeps on 160, 80, and 40 to get numbers. So far, I see I can get a R+jX, I can get a |Z|, I can get an SWR, but I'm just not sure what I should be using when I try to make the best compromise transformer.  In all cases I'll still be using a tuner, but I'd rather make the match easier than harder for the tuner....

Anyone who's used these VNA's more/longer than the two days I have been playing around with it care to enlighten me??

Ed

Ed..   For a quick first stab at it, look at the R+jX.  If the X (reactance) is fairly low in the ham bands, try winding a transformer to bring the R down to 50 ohms.  That is, if you get something near 200 ohms, and not a lot of reactance, a four-to-one transformer is called for.  If the reactance goes to zero-ish below the bands, your antenna is too long and the dikes are called for.  If above the ham bands....you get the picture.  Go for the happy middle if you want multiband performance.
Also, if it looks like your transformer should be a four-to-one, for instance, hang a 200 ohm resistor on the high side to prove to yourself that you've built the transformer you intended.  It's a good sanity check on the bench.
One thing that's always fun to do is look at the Smith Chart and slide a frequency marker around until it crosses the axis of reals...that straight line that goes from 9 oclock to 3 oclock.  That is another way to find where zero reactance is.  There are all sorts of cool mathematical things you can do with the numbers, but this is ham radio, and sometimes we just want to get to the result without writing a doctoral dissertation about it.
If you have your laptop out in the field, run it on internal batteries so that you are not coupled to whatever "ground" the AC extension cord will bring you.  This goes for tuning mobile antennas as well.
Quarantine doesn't mean we can't get out in the yard to do some antenna work!
73 de Norm W1ITT

Hi Ed,

You might want to think about this before climbing the ladder.

Assuming you want to maintain an unbalanced feed to the 450 ohm Tline.

Build a 1:1 UNUN Get one of those put together and to make sure it works properly, use the vna to validate its operation. If all is well placing 12.5 ohm to say 200 ohm (4:1 swr circle) non inductive resistors on ONE side of the xmfr should read correctly on the vna. That is to say with little reactance shift from the imperfections of the UNUN.

The intent here is to try to keep the vna from becoming part of the antenna system.

Then your ready to probe the end of the 450 ohm Tline.

Alan

I played with my nanovna this weekend. I found a plethora of info on youtube. Scan for the thing you want to do and you will find some ham showing you how to do it. My 2 pesos...

John

Hi Ed,

I put together a mock OCF_dipole covering 160 - 40 meters. I plotted the impedance on the chart in black along with a 3:1 SWR circle. The chart is normalized, the center is 50 ohms. A 3:1 SWR usually can be matched by most transceivers with built in tuners. Dedicated external tuners can do much better, say 10:1. The black trace is sort of what you might see on the vna when you probe the antenna. Trimming and adjustment of the OCF point may rotate and even bring the Z sweep closer to the chart center, 50 ohms. I added a 4:1 xmfr mock up in the response and the blue trace is the result. Now more friendly to the tuner as the Z response from 160 meters to 40 meters is well contained within a 3:1 SWR circle.