[TowerTalk] Measuring electrical length of coax...

[W0UN -- John Brosnahan]

Coaxial cable delay lines for phasing HF antennas are really not all 
that critical,
at least for peaking the signal, but if you are trying to precisely 
position a null
then better accuracy is required.

Where it really IS important to get the electrical length correct is when one
is making coaxial stub filters.  The nulls are pretty sharp, much less than
the width of the HF bands, so you really have to decide whether you want
a particular stub to be a phone or a CW stub.  Or use multiple stubs.

This is when having an accurate and easy way of tuning the electrical length
of a coaxial cable is important.  And this "short" that is reflected 
back to the
main feedline cable on the harmonic is critical enough so that I normally use
a setup with the identical Tee connector arrangement as will be used in the
final setup, since even the length of an extra barrel connector can change
the notch frequency significantly.

There are also minor issues with mechanical flexure causing some phase
shift.  We did a lot of testing in the 1970s with measuring phase shifts on
RG-213 before and after rolling up the cables.   Although the application
(HF and low-VHF radio astronomy) was lower in frequency, we did the testing
at 300 MHz in order to multiply the effect and it was significant but not
serious.  If you measure a cable that is straight, then coil it up, and then
roll it back out and measure it a second time when straight, the phase
shift will drift for a half hour as the cable relaxes back to its original
condition.  And when it settles down there may be a slight amount of
difference in the phase shift.  Again, this is only significant for very
critical applications like stub filters.

73 John  W0UN