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I'm experimenting with measuring the electrical length of a piece of coax.
To understand the process, I experimented with some RG-58 I had lying
around.

I made a short adapter to connect a length of coax to CH0 of my nanoVNA H4,
and performed a OSL calibration at the end of that adapter over the
frequency range of interest (in this case, centered on 435MHz).

I cut a chunk of coax slightly longer than what I calculated as 1/2
wavelength at 435MHz, based on the published velocity factor of 0.66. I
connected this to the adapter, leaving the free end open. I snipped off a
little at a time, and watched the marker rotate counterclockwise around the
Smith chart. As I approach the calculated length for 1/4 wavelength, it
gets closer and closer to a short, and less and less inductive, until it
crosses over the central horizontal plane of the Smith chart. At the point
of crossover, it reads about 6 ohms.

All this aligns pretty well with my understanding of what should happen.
Given the decidedly non-precision adapter and homebrew cal load, I figure
it's as close as I can expect to zero ohms non-reactive. The length (as
measured with a tape measure) was almost exactly what I had calculated for
1/4 wavelength.

So, two questions:
1) is my experiment above valid, or am I missing something important?
2) the experiment above was all done with 50 ohm coax and a 50 ohm
calibration. What if I want to do the same thing with a piece of 93 ohm
RG-62? If I did a calibration with a load as close as I could make to 93
ohms, would that work? I suspect that the nano hardware is built to match a
50 ohm system, so perhaps not.

What say you?

Yes, it is a valid method.

For non 50 ohm coax, use the same process. Still calibrate with 50. The quarter wave impedance transformation will occur regardless of the transmission line impedance.

Thanks Alan!
Yeah the thought occurred to me that a quarter wavelength of any
transmission line should behave the same... I was just unsure whether I had
to somehow account for the characteristic impedance of the line for that
method to work. Thanks for clearing that up for me.

The reason for my interest is that I’m building an antenna that requires a
phasing line of RG-62 to place two crossed loops 90 degrees out of phase. I
could just use the published dimensions but I like to understand the “why”
as much as I’m able.

By the way, I love your videos and have learned a lot from them. They are
very well done.
73 de W0ZF

On Mon, Jan 18, 2021 at 7:14 AM Alan “W2AEW” Wolke <alan.wolke@...>
wrote:

Yes, it is a valid method.

For non 50 ohm coax, use the same process. Still calibrate with 50. The
quarter wave impedance transformation will occur regardless of the
transmission line impedance.

Greetings,

The process sounds pretty good to me but I have not used it.

Before my VNA I used an MFJ Antenna analyzer to create 1/4 wave lengths of coax.
I found an article that described the following.

Create a coax adapter with a 50 OHM resister in SERIES with the center conductor of the coax.
I used a PL-259 and an SO 239 fittings. Connect your OPEN length to the SO-239.
Tuning for minimum SWR you can determine the ODD 1/4 wavelengths. (ie 1/4, 3/4, 5/4 etc)
The lowest one found is the 1/4 of course.
I hope that this helps.

73 Gordon WA4FJC