Satgen489  Antennas and Azimuth 2  by GM4IHJ  (BID SGEN489) 1998-08-08

How do you know when you rotate your antenna to north , that it is
accurately aimed at north? There are several ways of working out the
direction of true geographic north :-
1. USING A MAGNETIC COMPASS. Guide books say you find the bearing of
magnetic north from the compass then you add to it the MAGNETIC
VARIATION for your station location. Most good maps record this
variation for your location . But please note that it is not fixed. Eg
here at IHJ it is decreasing by 9 minutes every year. It also varies
greatly from place to place because each has a different view of the
separation of the magpole and the geopole. In addition the north
magnetic pole itself is slowly shifting its location.
Worse still the compass does not actually point around the world
sphere on the great circle bearing from your location to the
magnetic pole . So you cannot short circuit the process by noting the
difference between the great circle azimuth to the magnetic pole , and
the great circle azimuth to the geographic rotation pole. You have to
get the VARIATION correction from an up to date map. Even more
hazardous.Your compass reading may be falsified by local magnetic
fields. Eg kitchen microwave or, as at IHJ where welded iron railings
on the fence next door, were heated above their curie point in
manufacture and came ready made with a terrific magnetic signature.

2. USING A MAP. Most high quality survey maps or charts show a line
indicating true north, though please note that maps based on 1km grids
as in the UK Ordnance Survey, do not have the grid lines exactly north
south, but they do have a separate line usually up the centre of the map
indicating true geographic north. Using a protractor you can measure
the angle with respect to the north line, of a separate line from your
location to some distant object such as a church steeple or mountain
peak. This gives you an exact azimuth reference not to north but to
the distant object whose bearing you have measured. Remembering that
Azimuth is measured clockwise from north so a church 30 degrees east
of your north line on the map gives you a firm azimiuth fix of 030.

3. USING THE STARS. Polaris the pole star of the northern heavens
appears to circle the celestial pole at a radius less than 1 deg. It
is a bright star easily located in Ursa Minor , and there is an
excellent guide line to it by taking a line from Beta Ursa Major ,
through Alpha Ursa Major up to Polaris. Ursa Major is the
constellation known severally as The Plough, the Great Bear and the
Dipper. This latter name is probably the easiest to remember because
the constellation does look just like a dipper laddle .On a good
starry night you can test by turning your antenna to the north and
looking from the back to the front you should see Polaris directly
along the boom at an altitude appropriate to your geographic latitude
For a further test rotate your antenna to south and you should see
Polaris again as you look from the front of the antenna along the boom
to the back. (but note, very few rotators do this accurately).
Polaris is the most reliable check  ( unless you live in the Southern
Hemisphere).Southern Hemisphere skies do not offer a simple guide star
for this sort of test , so there perhaps, map reference is the best
bet, with magnetic compass a very poor last hope.