[Moon-Net] using VK3UM EME calc for offset dishes

Paul Wade w1ghz.vt at gmail.com
Tue Nov 29 17:24:27 CET 2016


I didn't get the G/T for offset dishes done in time for EME2016 -
losing Doug also lost the* incenti*ve.  shooting for EME2018.

intuitively, *noise in a prime focus dish goes down as elevation*
*increases and the dish shields the feed from the ground.  *
however, in an offset, increasing elevation points the feed downward
and possibly increases noise.  won't know the answer until I do the
numbers.

73
paul

On Tue, Nov 29, 2016 at 8:48 AM, Peter Blair <g3ltf at btinternet.com> wrote:

> Simon, unfortunately offset dishes were on the long term “to do” list when
> Doug died. The first thing to note is that in EMECalc the spillover and
> efficiency of the feeds are ( nearly) all derived algorithmically from the
> work described in the paper published by W1GHZ at EME2014 and subsequent
> colaboration with Paul and Ingolf SM6FHZ. All of this work applies to prime
> focus dishes and EMECalc assumes the dish is pointing at an elevation of 45
> degrees. The exception is that the feed table includes a few appropriate
> feeds for offset dishes,( in grey)  The spillover, Tspill, and gain for
> these feeds is derived from the old (estimated) pre EME 2014 method, its
> probably not far out but Tspill and gain will probably be on the high
> side.  ( there is more about this here http://moonbouncers.org/
> WorkingTogether%20to%20Improve%20EMECalc_v2.pdf )
>
> So you can use the 2.4m diameter but, as you know, the spillover for an
> offset dish with an optimum feed is lower than for a PF and so you should
> “intelligently fiddle” with the numbers based on the above and looking at
> feed data available. For example on the W1GHZ site
> http://www.w1ghz.org/antbook/contents.htm there are sections on
> appropriate feeds as was discussed recently here. The “grey” feeds in
> EMECalc come from http://www.w1ghz.org/antbook/conf/optimized_dualmode_
> feedhorns.pdf  Also on there is a programme HDL_ANT.exe
> <http://www.w1ghz.org/software/hdl_3b4.zip> which calculates the f/D and
> gain of offset antennas with an optimised feed.
>
> 73 Peter G3LTF
>
>
>
> *From:* Simon Lewis <zl4plm at outlook.com>
> *Sent:* Monday, November 28, 2016 3:29 AM
> *To:* moon-net at mailman.pe1itr.com
> *Subject:* [Moon-Net] using VK3UM EME calc for offset dishes
>
>
> Hey all
>
>
>
> I’m interested how to adjust VK3UM to get some accurate figures for
> offset dishes?
>
>
>
> I’ve acquired a 2.4m Andrews offset which I am looking to dedicate to
> 10g (and maybe 6cm if it performs ok )
>
>
>
> That leaves the 5m dish for 23.13 and 9 as dedicated
>
>
>
> So far I simply adjusted the dish size to match the gain in the dish
> performance data so far
>
>
>
> I can play around with the feed and F/D and these can vary results quite a
> lot
>
>
>
> An offset at 2.4 should be about 3m for a full parabola?
>
>
>
> I’m keen to see what sun/moon noise a 2.4 offset should see
>
>
>
> How can I adjust the VK3UM settings to reflect (get it!) the true nature
> of the offset?
>
>
>
> Thanks
>
>
>
> Simon
>
>
>
>
>
>
>
>
>
>
>
>
>
> Sent from Mail <https://go.microsoft.com/fwlink/?LinkId=550986> for
> Windows 10
>
>
>
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