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Re: Antenna phasing for CP



Hi Franklin.

Wow thanks for such a complete _and_ qualified answer!

Whilst I agree that 1.7:1 isn't that big a deal, at band edges it goes up to
2.3:1 which starts to be a worry. I was also concerned that this impedance
mismatch may lead to bad patterns etc. At 2.0:1, I might as well have thrown
out the expense of a power splitter and used only a T-Piece!

Well... I think you may just have hit the nail on the head with your
coupling theory!!! No, the 'other' antenna wan't terminated when I did the
individual tuning. This also explains why the 70cm is nowhere near as bad as
the 2m unit: the 70cm antennas are a good 1/4 wavelength in front of each
other but the 2m elements are barely 0.02 wavelength. When I get home
tonight I'll give it a try (shame, I only just packed all the stuff away -
should have left it out!). What really makes this theory add up is that the
reactive component changes as the phasing switch adds and removes length.
What's _Really_ frustrating is that had I been using a normal SWR meter
rather than a fancy antenna analyser, I might have worked this out for
myself, by putting the SWR meter in series with each of the arms of the
power splitter to check for equal power division.

Regarding the phasing switch, it will switch in zero, 1/4, 1/2 and 3/4
wavelength sections to allow Vertical, RHCP, Horizontal and LHCP. I've used
this before (http://www.btinternet.com/~hlong/remotepolarization.htm) with
some success.

On the subject of feeding - well, yes I am going towards the middle of the
boom and not to the back via the reflector. I had thought about this, and
that's why I tried to get the antennas individually set. The only reason for
this is I was concerned at the cable getting caught in the elements as the
antennas dance around the sky, and a certain aesthetic look if there is such
a thing. Do you have any tips on successfully running the coax 'from the
back'?

73 Howard G6LVB


----- Original Message -----
From: Franklin Antonio <antonio@qualcomm.com>
To: Howard Long <howard@hanlincrest.co.uk>
Cc: <amsat-bb@amsat.org>
Sent: Monday, July 03, 2000 12:15 AM
Subject: Re: [amsat-bb] Antenna phasing for CP


> 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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