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R: Helix Feed for a Dish



Hi  Jerry, Fred, Ed

I plan to add a cup reflector to my 3 turns elix feed for the 2400 MHz
1,2 meter dish F/D=0,45 and see if i get less noise from spillower and
hence more sun noise.

Actually i get 5 dB of (S+N)/N from the sun without any cup reflector but as
soon it is ready it take ten minutes to slid the cup over the existing
reflector,stick up the cup with four screws and make a comparative sun noise
measurement in quasi real time.

Since my reflector is a 108 mm square aluminum plate i will made the cup
using a 120 mm diameter copper disc 0,5 mm thick with a 50 mm hole in
the center  to slid over the existing elix.

This reflector has two rings that acts as a chock to cancel out the side
lobes and they are similar to those used in the Chaparral feed.
The external ring is 120 mm in diameter  30 mm hight and made with a strip
of  0,5 mm copper sheet soldered  to the bottom plate with tin.

The internal ring is concentrical and 90 mm in diameter  same 30 mm hight.
This reflector has been designed by Matjaz Vidmar,S53MV for his 2 turn elix
feed for 2400 MHz  and a parabolic dish with F/D=0,4

As soon the sun test will be completed with and without the cup, i will
informe you about the results and measurements.

73 de i8CVS Domenico

----- Original Message -----
From: Edward R. Cole <al7eb@ptialaska.net>
To: <amsat-bb@AMSAT.Org>
Sent: Saturday, June 02, 2001 9:23 AM
Subject: Re: [amsat-bb] 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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