[Moon] [Moon-Net] Under what angle is a feed in a offset dish ??

[Peter Blair]

Don, thankyou,  Its good to have HDL_ANT run on something above XP but the current version on Pauls website allows a curve fitting process by entering a series of points as its quite difficult to get the exact lowest point. Just for info, Using HDL_ANT curve fitting I get the focal point on my offset dish as 2553 ( top edge, and 1543 bottom edge. Using the wifi programme with my best estimate of the lowest point gives 2501 and1480 (All dimensions in mm.) Differences of 57 and 32mm.
As I had no frame and no feed arm for this dish ( nominal 2.4m) it has been quite hard to find the exact focal point. until you get close the changes in sun noise on moving the feed ( in x,y and z) are small and then when you get close it becomes more sensitive.
73 Peter G3LTF 

From: Don Hawbaker 
Sent: Sunday, May 07, 2017 6:19 PM
To: John Lambo 
Cc: moon at moonbounce.info ; moon-net at mailman.pe1itr.com 
Subject: Re: [Moon-Net] Under what angle is a feed in a offset dish ??

http://mscir.tripod.com/parabola/

If you select the wifi option of this program, you can determine the phase center for your dish.  i.e. distance from top and bottom where the mouth of the feed horn should be located.  I assume that when you change feeds, you compensate for the smaller diameter of the higher frequency feeds by raising them up from the mounting base so that their phase centers are in the same location as at the lower frequency.

I have no explanation for the azimuth change.  Very sharp pointing at higher frequencies, maybe measurement error at lower frequency and the azimuth is more accurately located at the higher frequency because of the sharpness of the beam.  At 1296, pointing is much less critical.

73 WA3RGQ

Sent from my iPad

On May 7, 2017, at 3:29 AM, John Lambo <pa7jb at ziggo.nl> wrote:



       
       

       
       

       
       

       
       
  Hello Lunatics, 
I use a 2,4 meter Prodeline  1270 offset dish for EME now for 7 years.
The support for the feed is changed to a other support.
This beacuse the feed for 23cm is so big that the old support was to high.
You can look on the home page off OK1KIR how this feed is in my dish.

http://www.ok1kir.cz/index.php?name=1296 MHz&page=gals&gallery=15&galpart=3&list=1&start=100

But now my problem..
If i change the feed from 23cm to a other band say 9cm or 6cm i see on 
sun and moonnoise a offset on my  OE5JFL readout.
This can be in elevation 2 degree and in azimute 0,7 degree.
I know it is not that much but what i think this must not be possible??
The feed i use for all my EME bands is a RA3AQ feed with a round septum.
I wonder under what angle must my feed look to the dish ?
So if my dish is water level under what angle looks the feed to the dish??
I know that more amateurs use a offset dish on EME and wonder if they see the same ??  Thank you for reply...

73  John  PA7JB



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