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RE: WL7M's questions re offset dish



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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