Re: [amsat-bb] Antenna phasing for CP

[Franklin Antonio <antonio@xxxxxxxxxxxx>]

At 09:51 PM 7/2/2000 +0100, Howard Long wrote:
>I can tune them _individually_ to 1:1 SWR,...
>I put them into a power splitter (which, BTW, works at better than 1.1:1
>with two 50 ohm dummy loads attached), and the SWR goes crazy - 1.7:1 at
>best.

1.7:1 is really a very small amount of reflected power.  0.067 W reflected 
for 1W in, so doesn't take much going wrong to make that happen.

I wonder if there is enough coupling between the two crossed Yagis to make 
that happen.  I have a couple of thoughts about what you might look 
at.  When you tuned Yagi #1, did you have a 50 ohm load on Yagi #2?  If you 
had it open circuit, the coupling would be different than loaded.  Of 
course loaded is still not the same as driven.  If there is a little 
coupling, and you want best match perhaps you should just adjust those 
matches a bit with both antennas driven.  You could measure coupling 
directly.  Put power into Yagi #1, while measuring power out of Yagi #2 
with a power meter and load.  That might confirm the mechanism.  Of course 
you'd then still probably have to adjust the matches with both Yagis driven.

Did you look to see whether VSWR changes when you flip the phasing 
switch?  If there is coupling, the phase of the coupling will be 
different.   That might or might not change the VSWR, depending on the 
phase of the reflection I think.  (Hard to visualize all the cases in my 
head.)  I suppose your phasing switch is one of those that switches in a 
180 degree section?

I think it is really good that you are measuring the Yagis 
individually.  Many people don't, and if the Yagis differ in impedance the 
splitter won't split the power evenly, resulting in less than ideal 
circularity.  If you've tuned both antennas individually, and then "touch 
up" the tuning by making identical small adjustments to both Yagis after 
they are combined, you'll probably be in great shape.  You could also go 
back and measure them individually after making that adjustment, to make 
sure the impedances of the two Yagis were the same, to give you confidence 
that power out of the splitter will be well balanced.

>  I've been careful to do my tests with all the coax in place both with
>and without the power splitter.

My first thought was that when you attached the splitter, you hung the 
cables and the cables interacted with the antennas, but then I reread your 
message, and saw the above sentence.  I suppose those cables were dressed 
along the boom until they got behind the reflectors?  I know there are some 
people who say it doesn't matter, but I'm not in that school of thought.

Where were the antennas pointed when you made these measurements?  I like 
to do this sort of thing with the antennas pointed straight up, for least 
interaction with the environment.




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