Re: [amsat-bb] offset dish

[Cliff Buttschardt <k7rr@xxxxxxxx>]

Edward R. Cole wrote:
> At 01:47 AM 4/26/2006 -0700, Dave Guimont wrote:
> 
>>>Hi,  I have an offset dish on my tower and would like to know what angle the
>>>dish should be pointing down so it's like my other antennas seeing the
>>>horizon. Scott ve6itv
>>
>>
>>Hi Scott,
>>
>>It can be measured, of course, but I find the following the easy way, 
>>and is very accurate.
>>
>>This assumes your dish is large enough to be able so see sun noise on 
>>what ever equipment you have..
>>
>>I use InstantTrack when ever the sun is high enough to give me a 
>>pretty good indication of sun noise.
>>I fasten a home made protractor with a bob weight to the antenna 
>>mast. Simply rotate the dish to give you max signal.  Note IT's sun 
>>angle at the time, and then use the protractor to lower the dish to 
>>the horizon...It will of course, at the same time, give you AZ at the same
> 
> time
> 
>>
>>
>>
>>           73, Dave wb6llo@amsat.org
>>                   Disagree: I learn....
>>
>>              Pulling for P3E... 
> 
> 
> Scott,
> 
> Most offset dishes are made for the ku-band TV satellite service and have
> very similar offset angles ranging from 66-71 degrees.  Most congregate
> near 67 degrees.  See: http://www.w1ghz.org/antbook/app-5a.pdf
> 
> I have several 18-inch dishes (dealers throw them away here in Alaska as
> anything below 30-inches is unusable), 33-inch used for ham satellites
> (AO-40), and a 39-inch used for Sat-TV.  On all of them the LNB support
> points very nearly in the direction of radiation, so if you mount the dish
> so the support arm is horizontal the dish will be very near pointing at the
> horizon.
> 
> If you are using an az-el rotator that is good enough as the elevation
> indicator is hard to read closer than a couple degrees (on the Yaesu
> rotator).  You need a signal source or sun noise (as Dave suggests) for
> tweaking the dish closer.  
> 
> Actually, if the sky is clear you can use the shaddow of the feed horn to
> line up on the sun:
> 1- Adjust your rotator until readout agrees with the tracking program
> elevation and azimuth angles. 
> 2- Center the sun's shaddow by manually swinging the dish in azimuth until
> centered.
> 3- Then adjust the dish angle to lower the shaddow to the bottom edge of
> the dish.   
> 4- You may have to touch-up the rotator pointing to keep up with the
> tracking program if you take very long in your adjustments.  
> Your dish pointing is calibrated...you're done
> 
> You can check your dish alignment anytime the suns is bright enough for a
> shaddow...even if you cannot see any sun noise on your receiver s-meter.
> My guess is you should see some s-meter indication (maybe 1/2 s-unit) on
> 2400 MHz if your dish is >2-foot.
> 73's,
> Ed - KL7UW  
> =========================================
> http://www.qsl.net/al7eb - BP40iq 
> 144-EME: FT-847, mgf-1801/1402, 4xM2-xpol-20, 170w
> 432-EME: FT-847, mgf-1402, 1x21-ele (18.6 dBi), 60w
> =========================================

Hi all.  Every dish I've measured in the 18 inch to four foot diameter
range seem to have an offset angle of 22 degrees.  That number pretty
agrees with Ed's comments as well.  At least it's a good starting point.
          Cliff K7RR
----
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