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RE: WL7M's questions re offset dish (Answer)



Joe,
You are correct. I cut the old feed off after marking the support arms with
a straight edge across the mouth of the old horn so I maintained the proper
angle with respect to the angular focal point of the offset dish. I
estimated how mush to remove and over estimated. Then in order to place the
phase center of the patch at the focal point, (between the front disc and
the rear reflector) I had to make three 1/2 inch extenders. Then on the
patch, I just formed three angular alum brackets which fasten to the back of
the patch. I elongated both holes so I could fine tune the position.
All went well.
I hope you like your patch as much as I do. It really works!!
73
Gunther Meisse
W8GSM
AMSAT LM:594


-----Original Message-----
From: WL7M [mailto:joe@xyz.net]
Sent: Saturday, October 13, 2001 11:39 PM
To: gjmouse@neo.rr.com
Subject: RE: [amsat-bb] WL7M's questions re offset dish


Hi Gunther,

More questions - one any way!  I have a G3RUH patch feed on order, and was
curious how you located it at the feed point.  From the picture on your web
page, it looks like you just attached it by three metal tabs to the
original feed arms.  Was there anything more "scientific" than that?

73 and thanks again for your help!

Joe WL7M


At 03:22 PM 11-10-01, you wrote:
>Joe,
>Well it sounds like you are about ready to go.
>Re: the offset 2.4Ghz dish. I removed the dish from it's mounting
plate/post
>holder and turned the dish over 180 degrees. The reason: With an offset
dish
>with the feed on the bottom, you can not go to zero degrees elevation
>easily. You throw to much un-counterweighted weight forward. Additionally,
>in the original configuration the feed is looking up at the sky and will
>collect snow. Bad news in northern Ohio. By flipping it over the problem is
>solved and when you get to look angles of 30 -50 degrees ele. you are
tipped
>back, thus receiving a minimum of wind load and when parked the feed is
>looking down so snow can not pile up on the feed. An item I discovered in
my
>first attempt. Do not use an unsecured counter weight as I did. It acts
like
>a pendulum and gives the rotor to much whiplashing. I removed the cables
>from the weight and bound it to the counterweight arm with 4" stainless
>clamps. That works well..
>Good luck on your project and we'll look for you on AO-40.
>73
>Gunther Meisse
>W8GSM
>AMSAT LM: 594
>
>
>-----Original Message-----
>From: WL7M [mailto:joe@xyz.net]
>Sent: Thursday, October 11, 2001 10:32 PM
>To: gjmouse@neo.rr.com
>Subject: Re: [amsat-bb] WL7M's questions re offset dish
>
>
>Hi Gunther!
>
>Just checking back with the 2.4 GHz antenna guru!  I finally received my
>1.2 meter dish and scrounged a set of 3 original feed arms, so I'm close to
>having it put together.    I do have a question, however, since this is an
>offset feed dish.  In looking at your web site pictures, it appears that
>when parked at the horizon, the dish is aimed upward at a 22.5 degree angle
>to compensate.  Is this the case?  Thanks again for all your assistance!
>
>73,
>Joe
>WL7M
>
>
>At 10:32 AM 03-09-01, you wrote:
> >Joe,
> >In answer to your posting:
> >The dish you see on my site is an offset dish (1.2m) by Channel Master.
>They
> >may make the Primestar units. I would guess that the Primestar is an
offset
> >in any event. Most all of the DBS dishes were offset for the very reason
>you
> >mentioned. (Snow)
> >I did use the original struts on the dish. You will note that I marked
the
> >focal point on the struts when the old 4Ghz horn was still on. When you
get
> >ready to remove the old feed, take some good measurements from the top
and
> >bottom of the dish to a point in the middle of the old feed so you know
> >precisely where the focal point is. Remember, this is not a prime focus
>dish
> >which is very easy to deal with. Not only is the location of the feed
>offset
> >from the middle, but there is an angle at which the new feed must look at
> >the dish, in my case 22.5 degrees. I mounted my dish upside down for two
> >reasons. #1 I wanted the face of the Patch feed pointing down so it did
not
> >accumulate snow. #2 I used the original mount for the dish which could
not
> >point at the horizon. Reversing the mount on the dish did the trick. It
can
> >now be calibrated at 0 elevation and the working elevation is taken care
of
> >by the 5500 rotor.
> >The only notes off caution I have about the 1.2m dish on the 5500 is that
> >the dish is heavy and MUST be counter-balanced and you should set your
> >parking positions on Nova, or whatever software you use, to tilt the dish
>so
> >it does not face the wind. The wind loading is significant on a dish of
>this
> >size and I do not think the 5500 would survive sustained high winds. For
>me,
> >so far so good. I have had 45 MPH gusts.  Another thing: If I had it to
do
> >over again, I would have my mounting adapter made of aluminum not
stainless
> >steel. SS weighs to much and is over-kill. When I made my mount I had to
> >much mounting pipe and the dish stood up above the rotor which turned out
>to
> >be BAD, in that I created a great lever which put way to much load on the
> >elevation rotor. I cut off 13" and snuggled the dish right down on the
> >rotors so that I reduced the loading and the amount of counter weight
> >needed. By the way, the picture shows the dish counter weight suspended
> >from the CW lever. DO NOT DO THIS. I soon learned that I had made a great
> >Grandfather Clock with a wonderful 10# pendulum. I now have the weight
hose
> >clamped to the lever and now when the rotor says stop, I don't slap the
> >rotor teeth for a few seconds until the weight osculation runs down.
> >In closing I will tell you that perhaps I over illuminate the dish with
the
> >patch, but I would not change a thing. I can hear the transponder come up
> >with it's noise floor measurable. That makes a qso a piece of cake. It
> >almost sounds like 20meters.
> >Good luck on your project.
> >Gunther Meisse
> >W8GSM
> >P.S. Have you thought about heating your dish? We do it in commercial
> >installations. It would keep it usable in bad weather, and reduce what
>could
> >be a nasty load on the rotors. Remember you can not park the dish
>horizontal
> >to avoid wind load, and not find yourself with a great snow catcher. PPS:
> >Drill a small hole in the low point of the dish for water to run out. 2-3
> >gallons of water screws up your center of balance and the 5500 will not
be
> >able to lift it up. Don't do as I did and right the dish while standing
in
> >front of it. I felt like the coach of a winning football team. Man did I
>get
> >wet!
> >
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>
>
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