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Re: "Precision" Antenna Pointing



Howard,

Your thinking is on the right path.  Yes use the sun.  Center the shadow of
the feed in the bottom center of the dish.  If you have a shiny dish
surface do not point at the sun long or the feedpoint gets VERY HOT.  Most
of the "big" dishes are painted with flat colored or white paint for this
reason.

Ed

>From: howard@hanlincrest.co.uk
>Two solutions: place a small non-flammable screen at the
>focal point of a dish on the az/el system and calibrate it by
>pointing the system at the sun (eg Nova will do this). If the
>dish is at least marginally reflective to light, the sun's image
>should appear in the middle if you're correctly calibrated.
>
>Even easier if you don't have a dish... on a sunny day make
>up one of those homebrew pin hole eclipse viewers out of a
>couple of pieces of paper. Align the pin hole device with your
>antennas and attempt to automatically track the sun. Whilst
>tracking, make manual physical adustments the az/el until
>the sun is tracked in the middle of the pin hole 'screen'.
>
>73 Howard G6LVB
>
>--------------Replied message--------------
>Date: Tue, 09 Jan 2001 21:57:38 -0500
>From: "Louis A. Mamakos" <louie@TransSys.COM>
>To: Clifford Buttschardt <cbuttsch@slonet.org>
>Subject: Re: [amsat-bb] "Precision" Antenna Pointing
>
>> I confirm your thought about plus/minus two degrees as good
>as amateurs
>> usually can obtain.  It would be nice if ordinary antenna
>hardware would
>> do better.  This is barely good enough for S band use even with
>a two foot
>> dish.  Can we list here how much excess sun noise is gotten on
>2400 MHz?
>> Be sure and include you antenna system and noise figure if
>possible.
>
>My other hole-in-the-head, er, hobby is amateur astronomy.  In
>addition
>to owning some exquisitly polished chunks of glass, I own a
>couple of
>german equitorial telescope mounts.  I can routinely aim these
>things
>to a precision measured in arc-seconds because each of the
>axes (RA
>and declination since they're equitorial mounts) have an optical
>shaft
>encoder connected to them.  These provide quadrature outputs
>with
>4000 pulses per revolution.  These are connected to a small
>computer
>with some anonymous single-chip micro running on a 9V battery;
>after
>a calibration process which essentially computes a
>transformation
>from the X and Y coordinates to (in this case) celestial
>coordinates,
>it displays the current aiming.
>
>I'd guess that the hardest part of moving this to an antenna
>positioning
>application would likely be the mechanical aspects of coupling
>these
>optical shaft encoders to the antenna system.
>
>louie
>wa3ymh
>
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>
>
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