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



Cliff,
Take a look at my web site regarding the Channel Master offset dish.
www.wmfd.com/w8gsm The print I got from them is included. The dimensions
from the front face of the 4ghz horn that was on the dish are 48.9" from the
top (including the rim--Put tape ever edge of rim lip), the bottom is 28.18"
from the bottom. (not including the rim.) The measurements are to a
horizontal line drawn right to left through the center face of the old horn.
As you can see, there is a bit guestimating involved. The dimensions given
to the focal point of the old feed are what I used for the dimension to the
focal point of the patch. Focal point of the patch is midway between the
front element plate and the rear reflector plate. (About 1/8" from the rear
reflector.)
I hope this is of some help.
73
Gunther Meisse
W8GSM
AMSAT LM: 594


-----Original Message-----
From: Clifford Buttschardt [mailto:cbuttsch@slonet.org]
Sent: Sunday, October 14, 2001 4:21 PM
To: Gunther Meisse
Subject: RE: [amsat-bb] WL7M's questions re offset dish (Answer)


Gunther, the only thing missing from this note is the dimensions of the
patch and the spacing with dimensions of the ground plane.  These are very
critical from what I have been able to read.  With limited test equipment
I'd like to get fairly close the first time.  73  Cliff K7RR



On Sun, 14 Oct 2001, Gunther Meisse wrote:

> 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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>
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