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The only way to properly measure an antenna is at the antenna initially.
Once you're happy with the antenna the coax can mostly be ignored and you
should NOT change your antenna based on any measurements done with the coax
connected. Coax has it's own inductance and reactance and impedance which
you cannot change. This is also why tuners are best placed at the antenna
and not at the rig as the tuner can get misled by common mode also and coax
effects.
If the VNA allows proper calibration you can calibrate your coax by
putting the open/short/load at the end of the coax where you are hooking up
the antenna. Then when you measure at the rig side you will see just the
antenna (plus whatever common mode is coming back at you) and it should
like the measurement done at the antenna. To see if common mode is
affecting your measurement you can add a 1/4 lambda length coax jumper and
see if your VNA answer changes. If it does than common mode is affecting
your measurement and you can pretty much ignore what the VNA says at that
point.
Adapters don't cause a lot of problems so an SMA/PL-259 or SMA/UHF adapter
should be OK so at least you are using the same loads all the time (there
is (or rather should be) some minor variation between loads).
What you should look for is tuning at the antenna for j0=0 (resonance) --
the frequency where j0=0 occurs should not change with the coax even if
common mode occurs. The rest of the curve might change though. The
reason is that you don't get much common mode at the resonant frequency.
I just went through this exercise tuning a 6M beam where we found a
Cushcraft 6M4EL had the wrong measurements in the manual. Once we got the
antenna tuned properly (adjusting the feed element for frequency and D1 for
impedance) we were able to get an SWR of 1.07 and 600KHz width < 1.5. Once
we connected the coax the tuned frequency stayed the same but the SWR
increased a bit to 1.12 which is expected due to the added connectors. A
proper choke at the antenna suppressed common mode on the non-resonant
frequencies so the VNA smith chart didn't change much with the coax added.
Mike W9MDB
On Thursday, October 13, 2022 at 02:10:17 PM CDT, Connie Stillinger
via groups.io <stillinger@...> wrote:

Hi -- I would like to use the NanoVNA for measuring and adjusting
antennas in the field for relatively casual POTA and other portable ham
radio activities (e.g. tuning the loading coil on a vertical; or tuning
wire lengths on a dipole or EFHW; etc).

My antennas' feed points as well as my feed lines are all either UHF or
BNC, and my radio has a UHF connector.

The problem is that the NanoVNA has SMA connectors and only an SMA
calibration set. In order to measure my antennas and cables I need to
use adapters.

Do I need to acquire or make BNC and UHF calibration sets for this kind of
field antenna measurement? Or is calibration using the SMA cal set at
all useful for this kind of amateur radio activity? I know it's not ideal
but since I'm not looking for a high degree of precision I wonder how bad
the error will be.

How can I measure the error due to the use of adaptors?


Thanks,

Connie