[Moon-Net] Fwd: Prime-focus vs. Offset-dish

Paul Wade w1ghz.vt at gmail.com
Sat Nov 26 14:54:46 CET 2016

I was in hospital having my knee replacement when his discussion
started and still not able to sit and type for long, will add
my thoughts:

Prime focus dish
+ easy to find focus
+ intuitive pointing
+ good feeds available
+ readily available free in USA
- feed blocks center of dish
- feed is far from equipment
- moving equipment to feed point unbalances weight further

Offset dish
- harder to find focus (but EME has lots of hard parts)
- pointing not intuitive (readouts required for all large dishes)
- excellent feeds available (both W2IMU and Skobolev dual-mode
  feeds may be tailored to fit any f/D, even Cassegrain)
+ no feed blockage
+ equipment should be mounted at feed
~ conventional dish mount may be unbalanced (I believe there are
  better ways

One other note is that dual-mode feed for an offset dish at 23 cm
is physically large compared to a modest size dish and would cause
some blockage, as well as being physically unwieldy.  (the proper
version is a good deal larger than small IMU commonly used).

I am still looking for 2.4M offset dish, which I think would be
big enough to justify the effort of building a mount. I have a
1.8 meter, but it is not big enough on lower bands.

Also was given a 3 meter mesh prime focus which I started to put
on last summer, and hope to this summer if no more emergencies.
It should be fine for the lower bands, but I'd still prefer a
good offset for higher bands.


On Wed, Nov 23, 2016 at 6:30 PM, Barry Malowanchuk <ve4ma at shaw.ca> wrote:

> Hi Rex,
> You don't really want to go much lower anyway since the ground noise
> /atmospheric noise starts to hurt badly on the high bands.  But if you have
> no choice....station at other end is 12000 miles away...you have a
> problem.  The only regret I have is that I cannot put the dish on the
> horizon so that I can focus the feed on a point source.....sun and moon are
> too big at 24 GHz and up with a 2.4 m dish !  I also have a chain link
> fence that gets in the way at low angles.
> Best 73
> Barry VE4MA
> ----- Original Message -----
> From: "Rex Moncur" <rmoncur at bigpond.net.au>
> To: "serge" <ve1kg at eastlink.ca>, dl7yc at snafu.de,
> moon-net at mailman.pe1itr.com
> Sent: Wednesday, November 23, 2016 1:56:21 PM
> Subject: Re: [Moon-Net] Prime-focus vs. Offset-dish
> Hi Serge
> One issue with offset dishes that has not been discussed so far is how to
> mount them in a way that allows you to beam low at the horizon.  I notice
> that often people with offset dishes cannot work down to zero degrees as
> the
> bottom of the dish comes back against the vertical part of the mount.  This
> can be resolved by putting the feed at the top but then the feed is more
> difficult to access.
> Another solution is to have the vertical mounting extended into an L shape
> to mount the centre of the offset dish so it is clear of the vertical part
> of the mount when beaming at low elevations.  The downside of this it that
> all the weight is leveraged to one side of the mount and you need a much
> stronger mount to maintain accurate elevation.
> 73 Rex VK7MO
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