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Re: Helix Feed for a Dish




>fspinner@hotmail.com writes:
>> Has anyone figured out if the "cup reflector" cavity makes any difference?
 
>From: K5OE@aol.com
>A helix has several very large sidelobes, especially around 35 degrees and
50 
>degrees (reference the ongoing discussion with NX7U).  For a small dish, 
>especially one of the partial-parabolas like the BBQ style, the sidelobe 
>radiation can greatly over-illuminates the dish.  This degrades the G/T, 
>hence the S/N ratio.  The cup reduces the sidelobes and "splash" beyond the 
>dish's edge.  That's the theory, anyway...  
>
>It would be very educational if someone who currently has a "regular" helix 
>feed with reflector could take noise measurements and then install a cup 
>around the feed and observe/report the difference.  

Jerry, Fred:

Well I intend to do exactly that.  But now that AO-40 has moved ALAT, ALON
it may be harder to get a good test signal from it.  I can try solar noise
measurements, though.

Tonight I installed the 33 inch offset feed dish that I "borrowed" from the
satellite-TV system [actually I bought a replacment 1m dish for the TV
which got installed last night].  So the 33 inch dish [84cm, gain=25.7
dBic] is now attached to the cross boom of the G-5400 az-el rotator next to
a 45el loop-yagi [1268] and 42el x-yagi [435].  I did not install the
5-turn helix feed.  I will do that tomorrow as I need to look at a good
manner of attaching it.  Comparing the LNB mount on this dish with the
manner that I mounted the helix on the DSS dish, I now see that I had the
helix mounted about 3 inches above the focal point and probably an inch too
close [explaining my not hearing the beacon on it a couple weeks back].
Later that feed worked well with a 4-foot dish.

Mounting the 33 inch dish was easier than the 18 inch DSS dish I had
before, since the new dish mounts on a 1.25 inch pipe making alignment
easier.  I'm going to reposition it a little farther from the 1.2G antenna
and make finer adjustment to its pointing using the sun's shadow, tomorrow.

Anyway, when my preamp arrives, I should be able to see sun noise and try a
4 inch dia. cavity on the existing helix to see if it helps sidelobe
suppression.  Since I am using a 3 inch square aluminum reflector plate, I
will make up the "cup" from brass sheet metal with a 2 inch hole in the
bottom so that the cup may be slid over the exisiting helix [probably drill
four holes in the bottom for attachment to the plate using 6-32 machine
screws].  I can also try modeling the cup on the existing NEC4WIN95 program
file I have for the 5-turn helix.

Tomorow will also see the connection of the 1.2G loop-yagi to the new DEM
144/1268 Tx converter for testing.  So then I'll be set up for when the
transponder returns.  [roof of my house is beginning to resemble an antenna
test range...one advantage of a flat roof].  I also am going to mount my
1N4005 diode harmonic generator inside a 1-lb. coffee can for making
antenna pattern measurements [I'll leave it up permanently as a test source
as I can run a 150 MHz 5w signal up a coax from the shack].

If I can detect the beacon with the "bare" Drake, I may try looking at the
telemetry.  But with the extreme squint I expect that I will need the new
preamp [delivery about a week off].

Now that I have tracking 432 antenna, I may give a listen to UO-14 and F0-29.

Lots of fun in the sun [over 18 hours/per day]!
Ed

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