Satgen492  Antennas and Azimuth 5  by GM4IHJ  (BID SGEN492) 1998-08-29

Microwave dish antennas produce very narrow radio beams, so they need
careful alignment. This is not easy. The dishes are awkward in shape,
are difficult to secure in any kind of wind, and therefore need strong
stiff supports. Lining up by magnetic compass is generally
unrealistic. Distant object alignment , as described in Satgen 489(2)
is possible in some cases. But by far the best alignment tool is the
Sun.

With a central focus dish, it is important to check that the pick
up driven element or cavity resonator is correctly aligned with the
centre of the dish.This is achieved by adjusting the element/resonator
supports to position its shadow directly around the dish centre , when
the dish is aimed at the Sun. You can then position the pickup exactly
by noting the level of Sun noise on a receiver connected to the focal
point pickup.

If your dish can be trained , you have an additional problem. You need
to check the accuracy of training over a wide arc of bearing. You can
in some circumstances do this on distant terrestrial transmitters and
beacons, or you can use geostationary satellites. But for the best
check you can use the Sun. You need a good software tracking programme
for the Sun ( Ephemeris not Keplerian element source data). Used
carefully as the day progresses and the Sun tracks across your sky,
you can compare theoretical rotator azimuth with true azimuth found by
peaking solar noise and noting the Solar bearing reported by your
software for that time. You will find you can use solar noise even
when the sun is clouded over. But please remember - the Sun is a
highly variable radio source. So do not expect to get the same max
signal levels every day.

Most trainable dishes for geosat TV use POLAR MOUNTING ie the dish
rotates about a single axis , which axis is aimed at the Pole Star.
But many satellite experimenters want to wander far from the
geostationary arc. So they will have a dish trainable in two motions,
azimuth and elevation. If your targets are relatively slow moving (
Mars probes, Lunar Orbiters, radio astronomy targets etc), the Az El
training can easily be done by hand. You still need accuracy, but at
IHJ this is achieved very simply, by mounting a 90cm dish on a tea
trolley which moves smoothly in azimuth on a hardboard surface on
which the azimuth bearings are clear marked as lines parallel to the
edge of the trolley. Elevation is achieved by means of a wooden wedge
under the rim of the dish. The wedge is calibrated in degrees using an
inclinometer.

A simple dish system as described in the previous paragraph would not
last a moment outdoors in Scotland. So it sits behind a very large
south facing window most of the time. Please remember , high quality
glass does not greatly attenuate micro waves. Though some double
glazing may do. If however your dish must go outdoors, you will need
very strong mountings and rotators, all of which are obtainable from
stores specialising in satellite TV installation. There are no regular
amateur radio rotators that have worked on outdoor dishes at IHJ.