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Re: Calibration issue causing ripples in SWR? #calibration

 

The 50 ohms cables as far as I remember were rated by different
manufacturers as 50 to 52 ohms years ago, so my guess is that 51 ohms
is on the ballpark. It is also a standard resistor value.

Try to find out the cable impedance, it seems to me to be the culprit.
Find the frequency where the cable is lambda/4 as a RF short circuit
with the end of the cable physically open. Set half the frequency
found before on the VNA and read the reactance, that is the cable?s
impedance.

I am writing by heart, a mistake might have slipped...

Jos¨¦, CO2JA

On 5/21/20, ed@... <ed@...> wrote:
After calibrating, I connected my nanovna to my VHF antenna via a brand new
75 foot length of 50ohm DX400MAX coax. The SWR measurement oscillates up and
down every 4MHz or so (see attached). These correspond exactly to the
half-wavelength harmonics of the cable. Looking at the smith chart, it is
clear that the impedance is tracing a circle around about 45 ohms rather
than the constant-SWR circle centered at 50 ohms, resulting in a
frequency-dependent SWR.

Is there is a good way to determine whether this is a problem with my
nanovna, with the calibration standard, or with the cable?

One note: the calibration standard load measures 51 ohms DC resistance,
which seems like a problem to me. Should my standard be exactly 50 ohms dc
resistance?
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