[TowerTalk] Coax measurements for stacking with VNA via TDR

[Jim Brown]

On 11/12/2020 11:32 AM, Ignacy Misztal wrote:
> I am adding a tribander for stacking and need to have the same length of
> coax as the first antenna. Are TDR measurements good enough? TDR
> measurements via RigExpert seem to be +-1 ft accurate.

I'd like it to be a bit better, especially for 10M. If you have physical 
access to both ends of both cables, best accuracy is probably to measure 
them as stubs with the far end open, looking for the lowest frequency 
short.

TDR in many modern analyzers is done by computing the inverse FFT of a 
wide frequency sweep. That's how the analyzer I use, the VNWA3EC, does 
it, and it's done in DG8SAQ's control software running on the associated 
computer. The best resolution is obtained with the widest frequency 
sweep and a lot of data points.

I don't know how good RigExpert units are at TDR, and it may vary 
between models. The VNWA3EC can get closer in TDR mode, and can do TDR 
with the far end connected to an antenna. Whether the RigExpert can do 
that depends on how they do TDR.

73, Jim K9YC