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Re: Circular patch behaves linear


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 .


Charlie G3WDG

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