#583 NANOVNA Not all 50ohm loads are 50ohms

Episode 583
Using the VNA to measure frequency range of BNC 50ohm loads
Be a Patron:

[rjordans]

There's also a bunch of '50' ohm loads around from old 10base2 networks where things were less critical, I've seen quite a few that actually registered as 47 ohms

[mikesradiorepair]

Just a word of advice. Never "spin" on a connector. As a example when you installed the first SMA-M to BNC-F adapter to your VNA you initially tried to hold the BNC end while you tightened the nut. Had some difficulty and just spun it on. This destroys the center pins in the connector. Not something you would ever notice at HF /VHF/UHF frequencies but get into the microwave range and just doing that a couple of times will make a noticeable and measurable difference. The easiest way to destroy connectors is to allow the center pin to rotate while tightening the nut. Get caught doing that in a metrology lab and they would probably fire you on the spot for destroying their $3K cal standard. Had a friend who works at a metrology cal lab catch me doing just that one day at my shop when he was visiting and I got the lecture. I don't work on equipment at frequencies over 10GHz (honestly anything above the 23cm amateur band is usually just my microwave amateur radio setup) but he showed me some literature that demonstrates that at very high frequencies even not properly torqueing connectors make a significant difference. Mike KC3OSD

[xjdriver]

It never occurred to me that there could be such a large difference in quality between terminators. I'm going to try this on the terminators I have. Great video.

[williamdenbeste]

I especially appreciated the use of the delay format as a way of knowing what electrical delay to use. I built some 30db PI attenuators as suggested by w2aew but they didn't look as good as I expected. The smith chart seemed to indicate that I needed an electrical delay so I played around with that but never found a good value. Now I have a new trick, so I'll go at it again. Thanks.

[ahbushnell]

When you put a 50 ohm through on a scope there is the input capacitance on the scope that will impact the termination.
Nice Video!!

[JxH]

This is a simply-fantastic video. Thank you.

[JxH]

Important point that must be explicitly mentioned more often:

People with NanoVNAs spend their waking hours repeatedly calibrating them with the 'Open', 'Short', and 'Load'. Very nice. But for testing and adjusting most simple antennas, the frequency calibration depends only on the internal TCXO, and I don't see anyone calibrating the TXCO frequency 127 times a day. (Of course the frequency is more than precise enough; yeah got it. I'm trying to make the point clear here.)

Point being, if all you're doing is calibrating the length of your 80m dipole, then the only parameter worth looking at is where the impedance dip is located in the frequency spectrum. You can perform this measurement using a NanoVNA even if the amplitude and phase calibration are a bit off. You're only looking for the dip, and trimming the antenna so the dip is where you want it in the band. (This presumes that you've not set the Impedance References to something odd like 300 ohms.)

Me thinks people are bringing precision amplitude and phase calibration into the mix far too often. Likely not required in many use cases.

Something to consider.

[Chris_Grossman]

The impedance of feed through terminations will be dominated by the instrument they are connected to. I have a video that shows that.

[keithcookman]

Great advice, super instrumentation tutorial.

[barrykery]

That was a great video. Full of useful information. I liked how you zeroed out the smith chart with the delay. I learned something. I also found that all termination loads are not equal. I mostly focused on only SMA figured that would have least chance of having any stray reactance because of their size. I used a spectrum analyzer with an RF sampler to make my tests and none were as advertised. Not all were bad, just not up to spec.
Thanks for sharing. I really enjoyed your video.

Barry, KU3X

[pyrokinetikrlz]

great video! Incredible how they can manufacture a 50 ohms resistance that has no significant reactive component up to 1GHz!?!?!? We live in the future!

[VEHL]

Have you done a video showing duplexer and/or multicoupler tuning, the type used in LMR repeaters? I’d REALLY like to see how well these little nanos do for that purpose.

[azizyousif]

This is great. What is your calibration procedure? Do you calibrate directly onto the vna without the adapter or with it on? I'm assuming the delay is turned off during calibration. Also, how can you display a large frequency range at once? My understanding is nanovna has a low dynamic range since it only sweeps 101 points. Thanks for the help.

[technophil]

It supposes you have calibrated the VNA with the best load to compare with the others !

[williambudd]

any manufactured product has a tolerance.

[vettonator]

how bad was the one that came with your nanovna?

[bigpimp]

50ohm should be 50ohm regardless but goes to show tolerances, heat and even 50.0001ohm can change further up the spectrum.
not all 50ohm is actually 50ohm.