[TowerTalk] More Radials = Higher SWR???

[Lux, Jim]

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.