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Re: Circular patch behaves linear



Hi Charlie and William,

I also built a K3TZ-style patch and made some measurements.
I did not do any tweaking of any kind, I just built it as carefully
as possible using ordinary hand tools and I removed the nylon
spacers after it was completed. The measurements were made with the
patch only, no dish or other reflectors.

I measured the return loss at 2401 MHz as only 6 dB and the difference
between polarizations was around 10 dB - i.e. it's linear.

I think the major issue is that it is exceedingly difficult to get
the patch element parallel to the reflector plate and keep it there with
no supports. This in conjuction with the narrow bandwidth makes it
look hard to get it to work without some tuning..

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


At 07:45 AM 6/23/03 +0100, Charles Suckling wrote:

>William,
>
>I am not sure which patch you built - it looks like the K3TZ.
>
>I built one of these a while back  and some of my tests are reported on 
>K3TZ's web page.
>
>I also built the patch as carefully as possible to the dimensions 
>provided.  In the as-built state, the polarisation was nearly pure linear 
>and it was only after MUCH tweaking that I achieved good circular 
>polarisation.  My test equipment was a network analyser and a rotatable 
>sampling antenna.  The beauty of this set up is that it permitted 
>circularity to be tested across frequency.  During tweaking, you could see 
>that the patch was often working quite well, but not at the desired 
>frequency.  This helped greatly in getting a handle on what to adjust to 
>bring it on to the wanted frequency.  If I had just measured it at 2401, I 
>think it would have been more difficult to get it working.
>
>I quickly found out that patches are very "high Q" devices, are critical 
>to adjust and only have good circularity over a very narrow frequency 
>range.  If our experiences are anything to go by, I think it is highly 
>probably that most home-made patches, which have not been tuned up with 
>test equipment,  are unlikely to have good circularity and may also have a 
>poor VSWR.
>
>Contrast this to the short helix, which being a "low-Q" device, is 
>relatively insensitive to constructional tolerances.  It has adequate 
>circularity over a wide bandwidth and a reasonable VSWR .
>
>73
>
>Charlie G3WDG
>
>
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