AW: [Moon] W2IMU feedhorn

[Dominique Fässler / HB9BBD]

Hi Peter
As SM0PYP put it many years back, the nulling post is fooling the tweaker of
the feed. It should be removed because adjusting the feedhorn without it is
already quite complicate.
http://www.hb9bbd.ch/article.php3?key=25
here you find Paul's original writing to me in the early 90's. 
As the isolation is concerned, you can get little more than -30dB and
(without nulling screw) still have circularity at +- 0.5dB. Importantly,
prior to all these parameters, I found that a perfect return loss of the RX
and TX probe is essential. Once I had broad and good return loss the
tweaking of the screws was not impossible any more.
I think that another expert next to Paul is G3LTF in this matter.
Good luck and 73
HB9BBD Dominique

-----Ursprüngliche Nachricht-----
Von: moon-bounces at moonbounce.info [mailto:moon-bounces at moonbounce.info] Im
Auftrag von Peter Sundberg
Gesendet: Samstag, 3. März 2007 15:53
An: moon at moonbounce.info
Betreff: [Moon] W2IMU feedhorn

Today I've been doing some maintenance on my 23cm W2IMU feedhorn. 

When adjusting the nulling screw on the back plate I see very little
difference in attenuation between the TX and RX port. Actually, it looks
like the attenutation is best without the nulling screw.

My question to you experts on this reflector, what difference/improvement
in port attenuation do you see when adjusting this nulling screw?

And what is the real world attentuation between the two ports?

>From what I can measure, attenuation between the ports on my feedhorn is
around 20 db.

73 de Peter SM2CEW
www.sm2cew.com
http://blog.sm2cew.com





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