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

Peter Blair g3ltf at btinternet.com
Tue Nov 29 14:48:08 CET 2016

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 which calculates the f/D and gain of offset antennas with an optimised feed.

73 Peter G3LTF 

  From: Simon Lewis 
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?











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