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Re: question regarding patch antennas



[amsat-bb members: This turned out to be a good explanation of linear 
patches for uses other than CP.. so I decided to forward to the group as 
well...]

One really good thing about the linear patches I discovered when making the 
1296 one (I didn't know this when I did the 1691 MHz unit) is that you can 
put a little "capacitive tab" (which you will often see in commercial units 
of good quality) to tune the unit.  If I remember correctly (off the top of 
my head) the tab should go in the same line as the feedpoint (otherwise, it 
will pull the frequency higher for some reason in the othorgonal direction.. 
I don't understand this yet).

Cut the patch to be, maybe, 5%-10% high in frequency, and slide a little 
(maybe 1.5 cm thick) piece of metal until resonance is acheived, then screw 
it down with #2 flathead screws (flush on the bottom of the patch).  Or 
alternatively cut the patch so a similiar tab sticks out and trim until you 
get the desired resonance.

Like other antennas, making the antenna a little off resonance and adding 
tuning capactance seems to increase the bandwidth a little.

On my 1296 antenna, the antenna VSWR is nearly 1:1 at 1296, but it's really 
like 36+j32 Ohms (note: made up for purposes of this explanation and might 
be a little misleading).  The reason this "widebands" the design is that the 
SWR match looks into the Absolute value of the impedance, so the SWR is the 
same at 36+j32 Ohms as 48 ohms.  At the 36-j32 ohm point, same deal.

This would play hell on a circular patch, though as the phases have to be 
exactly +90 and -90 for good circ.  So it'll only work on linears. (It 
*might* on CP's if you tuned both sides of the patch-- impractical) This 
technique, btw, works WONDERS on single band vertical HF antennas as well.  
I have a 20m vertical that is <=1.3:1 over all of 20m fed this way.

Anyway, I'll have more to say on this on the website one of these upcoming 
weekends.  I apologize to everyone waiting for my writeups, but work has 
been busy and I've had a severe case of "entropy and stupidity" at home with 
almost all of my home appliances and vehicles breaking at once.

It appears that 3 yrs/36,000 miles applies to dishwashers and refrigerators 
as well as cars... hrrumph...:-( (again, a registered trademark of Despair, 
Inc. All rights reserved! [see last amsat-bb posting!])

See comment below about WEFAX patch.

Fred W0FMS

>From: "William B Magnusson" <williamb@netc.net.au>
>To: "Frederick M. Spinner" <fspinner@hotmail.com>
>Subject: Re: [amsat-bb] question regarding patch antennas
>Date: Tue, 5 Mar 2002 05:56:47 +1100
>
> >I've built linear polarized 1691 MHz WEFAX (WEFAX is linear polarized) 
>and
> >terrestrial linear 1296 MHz patches.  Coupled with a prime focus 4' dish
> >they seems to work fairly well.  I'm going to post info to my website
> >(w0fms.org) when I have time to work on it.
>
>Thanks for the response Fred,
>I was a bit unsure of how touchy the patches are to scale.
>I have machine-shop capability so accurate sizing is no problem.
>I'm using a dipole/reflector for 1691MHz at present and I feel the patch
>should be a lot better.

I agree, but: I've not tried it enough to claim this yet, and with 6" of 
snow on the ground, and covenants against dishes >39" I do not have a 
permanent WEFAX setup anymore.  Meaning I'll test in spring.
I'd guess a "scalar" or "choke" ring would help here too...

>Look forward to your posting the 'network-analysed' dimensions on your web
>site.
>That's a great service for home constructors and I'm sure a lot of folks
>will appreciate it,
>Regards, Bill ...vk3jt
>Milawa
>Australia
>
>


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