Satgen 174  Microwave Dish Alignment  by GM4IHJ                25 July 92

As the number of radio amateurs using Dish antennas increases, there are a
number of reports of poor performance. One possible cause of this is
misalignment of the microwave feedhorn with respect to the radio focus of
the dish. If your dish is one where the feedhorn is mounted centrally out
from the dish ( Not one where the feed is offset), you can use the
following alignment procedure.

1.   Connect up the dish mechanically and electricallly taking a signal
from the feedhorn to a suitable converter or LNB which outputs at a
frequency you can tune on a VHF or UHF receiver . At IHJ the receiver used
is an ICOM R7000 covering the 900 to 1700 MHz band of interest.
2.   Then on a Sunny day align your dish on the sun and move it about
until you get maximum Solar noise signal on your receiver. Even a 90 cm
dish will produce 3 S points rise in noise or more at 10GHz on the Sun.
3.   When you have maximum solar signal, take careful note of where the
shadow of the feedhorn is on the dish face. It may well be several
centimetres off centre.
4.   Readjust the mechanical supports of the feedhorn until the Shadow of
the feedhorn is central on the dish and, concurrently, the receiver is
showing maximum solar signal.
5.   Repeat procedures 3 and 4 , until you have perfect coincidence of
central shadow and maximum signal.
6.   Next, adjust the feedhorn , moving it towards or away from the dish.
Then lock it in the position where it produces maximum signal.
7.   If you have switchable or rotatable polarisation, check that changing
polarisation has little effect. If you get big changes check the polariser
and the feedhorn and associated waveguide. Eg a spidersweb which had
collected dust played havoc with this test at IHJ.
8.   Lastly, whilst you have the Sun, it pays to fix yourself a simple
visual sighting aid on the side of your dish. This is helpful on satellite
and radio astronomy targets. It is useful for example to check your
computer derived dish alignment on stars at night. You should also check
how closely you get to a designated target when turning your dish towards
it from different directions. Very few radio amateurs can afford 3rd or
4th order drive element controls. At IHJ it is accepted that the dish can
differ by several degrees turned in clockwise as opposed to anticlockwise.
So the dish is accurately aligned for run in clockwise only and elevation
only. This is OK for most microwave satellites which are south but it is
tedious on radio astronomy targets to the north if they have to be
followed as they move anticlockwise. A polar mount would be easier than
the present AZ EL but the polar mount is hopeless for Phase3 sat tests. So
please consider these points when you consider how you will use your
beautiful new dish. 73 de John GM4IHJ @ GB7SAN