[amsat-bb] Antenna phasing for CP revisited

["Howard Long" <howard@xxxxxxxxxxxxxxxxx>]

All,

Many, many thanks for the dozen or so replies I got on this subject! I've
spent the past 48 hours playing about and things have improved things
although not to the extent I would have really wanted!

Readjusting each antenna separately with the other terminated has certainly
helped.

Just to remind everyone, individually all the components (power splitter,
coax switch and antennas) work very well individually, with 1.1:1 SWR or
less. It's just when they're put together things go strangely awry.

I'm rapidly coming to the conclusion I'm probably wasting my time in the
search for an elusive <1.5:1 across the band! When switching in and out one
or more 1/4 wavelengths on one feed line things get quite hairy with large
amounts of reactance and resistive changes. When measuring only one antenna
(with the other terminated), things are nearly perfect. However, I'm
measuring the impedance of the antenna down 15' of coax.

Interestingly placing varying short lengths of coax in the feeder give me
fairly significant resistance changes - 44 to 57 ohms, though reactance
remains at 1 or 2 ohms, indicating that the coax (proper RG-213) isn't
properly matching at the feed point. A dummy load in place of the antenna at
the feed point is fine. However, measuring impedance directly at the feed
point is self defeating as I don't believe it's really possible to do this
without upsetting the whole delicate balance.

I'm also thinking along the lines that a conductive boom with driven
elements connected to it is also a potential threat to getting everything
just right when switching in multiple 1/4 wavelengths.

Most successful has been to use a Polarphaser where I _can_ get 1.5:1 across
the band (1.1:1 at centre), but you can't put this at the mast between the
antennas and preamp as it has a manual control.

I think I might give it one last shot with a trusty SWR meter rather than
the MFJ 269.

Then it's back to the drawing board!

Again, many thanks for all your comments. Any further suggestions graciously
 accepted and appreciated.

73, Howard G6LVB


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