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.
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