[TowerTalk] More Radials = Higher SWR???

[Lux, Jim]

On 2/5/23 8:11 PM, Jim Brown wrote:
> On 2/5/2023 7:57 PM, Lux, Jim wrote:
>> One of the nice things with the VNA is that you can do a calibration 
>> at the end of the coax, so you don't have to figure out what the coax 
>> is doing to the feedpoint impedance.  I mean, you also need to 
>> measure at the source end of the coax, because that's what keeps the 
>> amplifier happy, but when trying to figure out what's going on, the 
>> VNA provides a lot more information.
>
> DG8SAQ's software for the VNWA he designed 10-15 years ago includes 
> the capability to subtract out a feedline whose electrical length is 
> known. I've often used AE6TY to subtract out the length of a feedline 
> without knowing its exact length, simply be exporting the data to 
> SimSmith, adding a negative length of line, making an educated guess 
> as to its length, and tweaking the length until I see the resonance.
>
> You've got to do this anyway, even having good TDR data, given that VF 
> varies with frequency. 


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. 
(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)