[TowerTalk] Need help estimating cable length of DXE 400 MAX

[jimlux]

On 1/12/20 10:24 AM, David Gilbert wrote:
> 
> If you have an antenna analyzer that can measure complex impedance (as 
> in R +/- jX) and if you are willing to put a connector on one end of the 
> cable, I can tell you how to get an accurate length measurement using 
> TLW, the transmission line application that comes free with the ARRL 
> Antenna Book.  You don't need an instrument with TDR capability (Time 
> Delay Reflectometer) to do it.
> 
> 73,


I'll bet you could do it without a connector and using clipleads.  it's 
probably around 200 ft long (60 or so meters, maybe 80 meters electrical 
length, depending on the velocity factor)

So a few inches or feet won't make much of a difference.

You tune the antenna analyzer until you get a peak in SWR - that will be 
a multiple of a quarter wavelength. If it's an odd multiple of 1/4 
wavelength, the open at the far end is transformed to a short at the SWR 
meter end. If it's a even multiple of a 1/4 wavelength (multiple of 1/2 
wavelength), the open at the far end shows up as an open at the meter 
end.  Both are "infinite SWR".

if you sweep around 1.8 MHz and you get that peak, that's probably the 
1/2 wavelength scenario. and so forth.