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Re: Helix Feed for a Dish



Hi guys,

Just a quick report of the days activity.  I have the 33 inch offset dish
mounted with 5-turn helix and "bare" Drake [433.832 IF].  AOS around 2200
local with squint>30 produced no signals...not surprizingly.  The
spreadsheet showed only 0.5 dB S/N.  Tomorrow I will set up my harmonic
gen-coffee can horn and also look at sun noise.  Without the new preamp
[still coming], I do not expect much for sun noise.  At least I can try
resolving my my dish pointing with my signal source.

Ed

>From: "i8cvs" <domenico.i8cvs@tin.it>
>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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>>
> 

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