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

Edward R Cole kl7uw at acsalaska.net
Wed Nov 23 22:26:01 CET 2016


Rex,

Not that difficult unless you have a 30m dish.

Here is a picture of my 46cm 10-GHz terrestrial dish with 10/24G W1GHZ feed:
http://www.kl7uw.com/Rainyday.jpg

Of course this mounted on a camera tripod head, but the concept is 
usable for mounting the offset angle to the vertical azimuth 
mast.  Most center fed dishes are mounted on one side of the mast and 
require a counterweight.  Same here.

http://www.kl7uw.com/1.8m_dish.jpg
you can't see the back support but has a adjustable screw for 
elevation (here the dish is set to El = +19 deg which makes the dish 
appear vertical - its tipped forward 6 degrees).  This is a 
satellite-TV antenna so pointing is fixed.  A typical screwdriver 
actuator arm can serve as elevation drive and is strong enough to 
hold 12-foot dishes in 100kph winds.

My 4.9m dish elevation uses A HD 36-inch actuator:
http://www.kl7uw.com/Dish_Oct20-2013.jpg
that is a 2m stepladder.

My dish uses actuators for both az and el and survived >100kph wind 
storm.  Storm bent 6cm angle steel legs but actuators were 
fine.  Legs were replaced with 6cm square steel tubing.
http://www.kl7uw.com/DishRepairs.jpg

73, Ed

At 10:56 AM 11/23/2016, Rex Moncur wrote:
>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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73, Ed - KL7UW
http://www.kl7uw.com
     "Kits made by KL7UW"
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