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R: Circular patch behaves linear




----- Original Message -----
From: Anthony Monteiro <aa2tx@attbi.com>
To: Charles Suckling <charles.suckling@ntlworld.com>; William Leijenaar
<pe1rah@hotmail.com>; <amsat-bb@AMSAT.Org>
Sent: Monday, June 23, 2003 4:11 PM
Subject: Re: [amsat-bb] Circular patch behaves linear

>
> It also appears that the dish significantly affects the patch as I noticed
> the return loss goes all over the place as the patch is moved around the
focus.
>
> It would be interesting to hear if anyone did any polarization or return
loss
> measurements on a prime focus dish with a G3RUH patch installed. The G3RUH
> patch is more rigidly constructed so this would help but I would be
surprised
> if there was no detuning from the dish reflecton.
>
> Any comments?
>
> 73,
> Tony AA2TX
>
Hi Tony AA2TX,

I have built one patch of K3TZ and four of G3RUH designe all for 2400 MHz
and i have adjusted both for the best compromise of circularity and return
loss in my lab with the patch alone looking at  free space out of the
window.

Without to go in performance comparison i have seen the following:

Suppose the return loss with the patch alone looking at free space is 25 dB
and circularity - 1dB

When i place the patch in to a prime focus dish than the return loss changes
greatly as i move the patch up and down  around the focal point but as soon
the focal point fall in the exact phase center of the feed than the return
loss become again exactly 25 dB as in the lab.

For the G3RUH patch i found that the phase center fall exactly over the
outer surface of the active element.

If you measure the return loss before in the lab with the patch alone
looking at free space and if you readjust the patch position at the dish for
the same return loss around the focal point than you have reached the exact
position for the phase center of the feed.

If you get the same return loss with the active element positioned at the
geometrical focal point it is an indication of a good circularity separation
because the feed is LHCP and the dish reflects RHCP energy to the feed.

If the feed has a good isolation between RHCP to LHCP than the energy
received back by the feed from the dish is very small and return loss is
small affected.

For return loss and circularity measurements i have used a rotatable 2400
MHz dipole with balun + a HP8690B Signal Generator + HP11692D Dual
Directional Coupler and HP 8555A Spectrum Analyser

If any one is interested on pattern diagrams,circularity diagrams and pics
of the above test set up please send me a  message and i will replay to all
ASAP

73" de i8CVS Domenico










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