[Moon-Net] Prime-focus vs. Offset-dish
Edward R Cole
kl7uw at acsalaska.net
Wed Nov 23 22:26:01 CET 2016
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:
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.
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:
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.
At 10:56 AM 11/23/2016, Rex Moncur wrote:
>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
>Moon-Net posting and subscription instructions are at
73, Ed - KL7UW
"Kits made by KL7UW"
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