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On Thu, 1 Jul 2021 at 13:13, Mikek <amdx@...> wrote:

Testing a couple of TV antennas for Signal strength and just curious about
what they might look at on the NanoVna.
I'll be looking at them through about 70 ft of RG-6 and a SMA to F adapter.
Measuring from 180MHz to 610MHz.
Pretty much a novice here.
Mikek

If you are only interested in measuring the impedance of the antenna, then
just use the 50 ohm cal kit on the end of the 50 ohm coax. The cal kit will
remove the effects of the cable. Just be aware that the minimum SWR, if
measured in a 75 ohm system, will not be the same as if measured in a 50
ohm system. I don't know if there are any settings on the nanovna which
will allow you to easily change the reference impedance, but I would not
mess around with transformers or anything like that.

You can look on a Smith chart view, find the 75 ohm point, then just note
the frequency where the antenna is closest to that point.

You can convert a Touchstone file from 50 to 75 ohm, so look at it later.
The software for the VNWA will do that, and no doubt will other software.

Dave

Calibrating a NanoVNA using a 75 ohm load instead of 50 ohms does NOT make the NanoVNA into a 75 ohm test instrument.

Some VNA do allow you to set the system impedance to other than 50 ohms. The popular one port RigExpert products have this capability and will do the necessary mathematics conversions in software.

When you calibrate a NanoVNA with a 75 ohm load the only traces that will give correct data are SWR and Return Loss. The others (R, X, Smith Chart, S21 etc.) will be incorrect.

You can always calibrate using 50 ohms and then measure a 75 ohm system. You can then export a touchstone file and import it into a spreadsheet and do the mathematics necessary to convert to a 75 ohm system impedance. A much easier option if you have a NanoVNA-H or NanoVNA-H4 is to use the free Windows AntScope 2 program from RigExpert. It will connect to a your NanoVNA and has an option in the settings to change the system impedance to 75 ohms in the program. Then all the displays like SWR, RL, impedance and Smith Chart will be based on 75 ohms.

** please note there is a bug in AntScope2 when using the NanoVNA. The first sweep made will be invalid due to a bad data buffer download. So just sweep twice and discard the first one.

Roger

Gert,

I have to correct a misconception there. Half turns, or quarter turns, do not exist! Two-hole cores do NOT allow making half turns. At least not in the magnetic sense.

Two-hole cores behave very much like two separate cores, which have almost no magnetic coupling between them. When one turn of wire is wound through both holes, this is one turn on each of the two functionally separate cores. When instead a wire is put through just one hole, this is one turn (not a half turn) wound on just one of those two functionally separate cores.

If you try to make a primary winding using just one hole, returning the wire on the outside, and a secondary winding using just the other hole in the same way, then these two windings will not be coupled. You didn't make a transformer, but only two separate inductors! If you don't believe me, try it, and measure the behaviour with the NanoVNA. You will find that the coupling coefficient is extremely low.

Often people will make asymmetrical windings, such as "two and a half" turns, by looping the wire two turns through both holes and then another pass through only one hole, thus ending up with the two pigtails conveniently placed at opposite ends of the core. This behaves as two inductors in series, one having 2 turns on one functionally separate core, the other having 3 turns on the other core. This does work, but since one side of the core will be providing 2.25 times as much impedance as the other, it causes a much different stress of the two sides of the core. In power applications this can cause severe asymmetrical heating of the core.

Manfred

Hi Barry,

This was really amazing, thank you so much for putting it together. I've been going through while waiting for my -F to get here from China and it's been really helpful. The "grampy pushing swing" analogy helped me probably more than anything else to understand that concept in my >20 years as a ham.

I do have one request: Could you post the "answers" to the "which of these has the lowest SWR/which is resonant/etc" for the questions on the Smith charts? I'm sure when you present this you go through it but I'm not sure if I got the correct answers.

Thanks again.

73,

David, K2DBK

Barry, Can't thank you enough. You did an excellent job putting this together.... finally, something I can understand.
73 Gerry N0DOZ in AZ

Good information in this thread.? But you might want to think about how badly you want to correct the SWR of 1.5 introduced by the 75 ohm line.? You can calculate the improvement you can achieve, but the ARRL Handbooks have graphs that show the difference.? A newer graph has been used for the past few years that plots the original loss without reflection against the loss with reflection for various values of SWR, but I think that an earlier one, used for decades, is better for this discussion because it shows the effects of lower SWRs more clearly.? If, for example, you have a line with 5 dB of loss that is mismatched at its load end by a termination that yields an SWR of 1.5, the additional loss due to the SWR in the line will be around 0.17 dB from the graph in the 1952 Handbook (my first new one).? You might introduce more loss than that by using a matching transformer or network and, in many applications, such a small additional loss won't matter.

73,

Maynard
W6PAP

Maybe of interest:

Hi Brian .
have a look in the FILES section ,
/g/nanovna-users/files/Absolute%20Beginner%20Guide%20to%20The%20NanoVNA
Jon - g0myw

On 02/07/2021 07:11, k4ard@... wrote:

Brent..
I cant find a beginners guide on how to use the vna for ham radio here ..
Can you assist me on where i can find it ..
Thanks , Brian

There's a helpful presentation here:

/g/nanovna-users/message/22977

73,
David GM8ARV
--
SatSignal Software - Quality software for you
Web:
Email: david-taylor@...
Twitter: @gm8arv

Brent..
I cant find a beginners guide on how to use the vna for ham radio here ..
Can you assist me on where i can find it ..
Thanks , Brian

See more info on the 50/75 transformer on the site of my friend ON6GD

I take it back... Nothing magic about bifilar. Best is a soldered tap to the desired wire. Next best is a twisted wire loop tap. Third best is bifilar and worst is two separate windings. This assumes both windings share the same ground. This can all be explained with keeping the leakage inductance as low as possible. (see attached plots).
Gary AF7NX

Hi Fred,
I appreciate the feedback. I will eventually post a link on some other forums.
I don't happen to have any mix 52 in my collection or I would have tested it. Looking at its complex permeability, I can see why it might be favored. The lower permeability allows there to be more turns on the primary, which will imply a lower B field for the same primary inductance -- That is probably a good thing for less loss. The competing issues would be larger leakage inductance and more winding capacitance...
Cheers,
Gary AF7NX

JMR,
Indeed I was using the two port S11, S21 provided by the nanoVNA. If you go through the Appendix in my post you can see how I generated the loss plots I presented. Once I established a calibrated system and a consistent method of doing the tests, the runs were quite repeatable and gave reasonably self-consistent data. It is what it is! You should be able to repeat the same experiment from what I've laid out, I would hope, if you have lingering doubts...

Gary AF7NX

Pete, I've modeled with 4NEC2 an end-fed wire with various source impedances and counterpoise lengths. The 2450 ohm source impedance seems close to optimum. You have to choose some step-up ratio. Once you do, then the match to 50 ohms becomes whatever it does and provides a good starting point for determining the properties of the transformer. Certainly the SWR will vary a lot depending on the antenna load. But one thing at a time :)

Cheers,
Gary AF7NX

Welcome to the forum, Brent.
There are several guides in the files section, including the Absolute Beginners Guide.?
Go through the forum's Wiki for additional information. There are also many YouTube videos available, just search for nanovna.?


On Thu., 1 Jul. 2021 at 7:38 p.m., Brent Sachnoff<bsachnoff@...> wrote: Hello.? I just received my H4 and I'm finding it difficult to understand the main website and getting this device to function properly.? I'm try to just start with something basic like SWR over 30-40 meters.? Can anyone help me out with calibration and then actually getting readings?

Thanks!
Brent

On Fri, Jul 2, 2021 at 12:35 AM, WB2UAQ wrote:

Many of us have been using a resistance of 49*50 for a test load as we both
have as a starting point but the real antenna is not a simple R.
Pete

I think that is one of the reasons the transformer should ideally be measured as a full 2 port device using a full two port VNA. This can also be done with a TR VNA like the nanovna with a little patience. Once this is done the s2p model can be loaded into a simulator and the load presented to port 2 can be changed to a complex load at will. To make things more realistic, a powerful RF simulator can define port 2 as an s1p model taken from a real antenna for example.

Interesting article, thanks! I think the best way to measure/model the transformer is to take a proper full two port model using the nanovna. This is a bit fiddly to do with a TR VNA but worth the effort I think. I then allows a much more detailed analysis of the transformer on a linear simulator.

I may have missed something but the insertion loss of the dual FT240 transformer looks to be very low below 2MHz. Is this taken with a 50R source and 2450R load? Or is it matched for lowest loss at 2MHz?

I did look at one of these transformers a while back using 2 x FT240 cores but I didn't wind it as an autotransformer. I wound it as a regular 2:14 transformer. I got the best performance up at 28MHz with the turns well squished together into just part of the core. With a matching cap and a 2450R load I think I achieved about 0.5dB loss at 28MHz. This setup isn't ideal for power handling but the insertion loss was impressively low across 2-30MHz. I think it was less than 0.7dB everywhere but the loss at 28MHz was optimised at just 0.5dB.

I just noticed a getting started guide under the files section. Reading thru it now.

Hello. I just received my H4 and I'm finding it difficult to understand the main website and getting this device to function properly. I'm try to just start with something basic like SWR over 30-40 meters. Can anyone help me out with calibration and then actually getting readings?

Thanks!
Brent