On 2/5/23 8:41 PM, Jim Brown wrote:
On 2/5/2023 8:18 PM, Lux, Jim wrote:
I just do a cal with the coax, putting the O,S,L at the end. Then
between that measurement, and the no coax cal, you've got the S21 for
the Coax, which most of the tools will de-embed if needed. You could
probably just do a "open" measurement and it would be good enough to
measure the coax.
I guess if you've got an antenna up on the tower, that's a bit harder.
The only antennas I can access to do that are RX antennas and 160M
verticals.
(although I suspect someone could come up with a scheme where you
look "off resonance" where the antenna impedance is wildly off
resonance, and then interpolate)
As you probably know, it's easy to get good TDR data with an antenna
connected, whether some form of impulse or inverse FFT of swept
impedance. An impulse is inherently broadband, and because a very wide
sweep is required for decent distance resolution, nearly all practical
antennas are resonant over a small fraction of the wide sweep, there
will be a strong reflection at all frequencies where the antenna is
not resonant. I typically sweep 50-550 MHz so as to avoid the part of
the spectrum where VF is most variable.
Yeah, but the typical tools don't have a convenient way to turn a sweep
from 50-290 (e.g. from a NanoVNA) extrapolated down to HF. Maybe that's
something someone could come up with a mod to one of the apps for.
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