[TowerTalk] More Radials = Higher SWR???

[Jim Brown]

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.

73, Jim K9YC