[TowerTalk] Determining coax length?

[Jim Lux]

On 11/12/13 6:44 AM, Ray, W4BYG wrote:
> The circumstance you describe could be quite dramatic when the antennas are
> combined, as the phase difference caused by different branch lengths between
> the two antennas can significantly affect the vertical angle performance.
> Skewing the phase of multiple antennas is often used in broadcast TV, etc.,
> to change the vertical "tilt" of the antennas.
>
> The Stackmatch specs seem to indicate it is a 50 ohm in/out power
> divider/combiner when in the combined position.
>
> Thus, you could consider using "time domain reflectometry" to determine the
> differences in line length, especially if you can climb the tower and
> disconnect one and then the other feedline, going to a particular antenna.
>>From the time delta between the two you can calculate the difference in
> total line length to each antenna.  You should probably terminate the unused
> combiner port for the tests.
>
> You could then make up an extension cable for the shortest line to match the
> other and insert it at the power divider or split.  You should be able to
> resolve the cable lengths within a fraction of a foot or so.  That should
> certainly be better than some random length difference between the two
> feeds.
>
> A simple square wave or pulse generator and a homemade TDR adapter along
> with a scope with a good accurate time base, could be used in place of a
> full TDR instrument.  I typically use a Tektronix 465 scope, Tek PG-502
> pulse generator and a TDR adapter for cable integrity and length testing.
> The 465 can also provide the needed excitation.  Or, you might be able to
> rent a TDR for the tests.
>



Sweeping the impedance over a sufficiently large band will essentially 
tell you the same as the TDR.  A lot of TDR boxes these days actually do 
it by sweeping, and then doing an FFT to get time domain.  I think the 
AIM might actually have that as a feature.

But you actually don't need to go to that much trouble.  You can sweep 
using something like a MJF-259 and look for where the SWR nulls and 
peaks are. It will form a pattern...

Especially with a monoband antenna hooked up, the antenna presents a 
pretty big mismatch at all frequencies other than its design frequency 
(and maybe odd harmonics).  So, say you pick a "test frequency" around 
the 2nd harmonic (where a dipole has a huge input Z, essentially an open 
circuit on the transmission line).  You sweep looking for where the Z is 
highest (e.g. where there's a multiple of a half wavelength between 
meter and antenna) or Z is lowest (where there's an odd multiple of a 
1/4 wavelength, transforming the high Z at antenna to low Z at meter)


A bit of arithmetic with a spreadsheet will let you figure out what the 
actual electrical length is.