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Re: What's Wrong?



Robert,
The G3RUH patch feed is certainly capable of receiving a good signal from AO-40 without the adornment of a parabolic dish, as it has almost 10 dB of gain.  Whether or not your dish is "transparent" is not something anyone other than you could ascertain, but I can tell you the combination you have will greatly over-illuminate the dish, allowing lots of extraneous RF into the feed.  This extraneous RF probably includes lots of S-band reflections, including AO-40's beacon.  I found testing S-band antennas anywhere within 20 meters of my house to be problematic due to the reflections off the solid walls.

I modelled the G3RUH patch pattern and ran the file through W1GHZ's FEEDPAT program and determined the feed is optimum for a f/D ratio of 0.4.  The offset dish you have is probably more like 0.7 f/D (very "flat" like most of the offset designs), thus greatly overilluminating the dish.  

You might try experimenting with extending the length of the scalar ring James has built into the feed.  Using a straight edge or a string, go from the center of the patch to the edge of the dish in the longest dimension, then imagine the scalar ring extended to intersect the string as a guide on how long the scalar ring should be to effect good edge-of-dish cutoff.

BTW, the G3RUH feed had a maximum efficiency of 71 percent--at least 5 percent higher than any other feed design I have modelled.
73,
Jerry, K5OE/VU

Standard Time, "Robert Jewell" <ac9r@home.com> writes:

> 1535Z 23 Jan 2002.
> 
> Ok guys, I am using a SSB-3000 (70 cm) fed by a G3RUH patch antenna attached
> to a 1.2m offset fed solid fiberglass dish.  There is a 70 cm to 10m
> converter in the shack feeding the receiver.
> 
> With the antenna pointed 180 degrees away from AO-40, an elevation "way"
> off, I still receive good CRC signals.  (I am positive of my antenna
> position).       Noise = S3.5,  SIG = S9 +.
> 
> With the antenna pointed at AO-40, the signals are S9 plus 20 dB with a
> noise floor on my Collins 75A-4 of S3.5.
> 
> My guess is that I do not have the patch in the proper position, although it
> appears to be exactly where the TV feed was.
> 
> Can the G3RUH "look through" the dish.  I am tempted to take the patch feed
> off, and just hold it in my hand without any dish, but it's raining too
> hard.
> 
> AC9R
> 
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