Satgen491 Antennas and Azimuth 4  by GM4IHJ  (BID SGEN491) 98-08-22

Satgens 488,489 and 490 described some of the methods used to
accurately MECHANICALLY align your antennas, in azimuth. But that is
not the whole story . Unfortunately it is possible to have a radio
electrical alignment differing by several degrees from the mechanical
alignment. The problem is rarely serious at frequencies below 150 MHz.
You can get away with all manner of henious crimes in respect of
antenna construction at wavelengths below 2m. But above 2m , simple
mistakes in antenna construction can cause fatal misalignment between
mechanical heading and radio heading.

The easiest way to give your antenna a squint whereby the radio beam
does not align with the mechanical configuration, is to obstruct the
forward field of the antenna ( ie ahead of the driven element), with
conducting material. The worst offender is the coax feed cable. At 2m
you can just get away with leading the coax forwards from the driven
element then down the mast. At 70cms the coax must come out behind the
driven element through the centre of any reflector before dropping in
a wide curve well back and down from the antenna to reach the mast at
least a wavelength below the antenna. Also near fatal are metal
support masts coming up through the forward field of the antenna, or
even worse, horizontal metal pipes connecting a pair of beams forwards
of their driven elements. Avoid having anything metallic in the
forward field. Use only glass fibre or varnished hardwood ( obechi) to
mount or separate antenna units. Then you should have no squint. 

How can you check squint on site ? Try reading solar noise and see
what antenna bearing produces peak noise. It should be the bearing on
which the Sun is directly in line with the antenna for no squint. But
be careful if you also propose to use the Sun to check antenna
azimuth. Remember solar timing due south in the northern hemisphere is
not necessarily local noon. The earth orbit around the sun is
elliptical . Sometimes the Sun can pass south up to 16 minutes early
or late, with respect to mean time noon. Always use a good solar
ephemeris and tracking program if you need to check south ( or north
in the southern hemisphere ).

NORAD KEPLERIAN ELEMENTS in the 21st CENTURY 
Keps 2000 (see satgen 487). NORAD have said that , for example, Epoch
1st January 2000 at noon utc will be printed as   00001.5
This means that NORAD year 99 is to be followed by year 00
So you need a trap in most software , roughly as follows
IF EPOCHYEARNUM <57 THEN EPOCHYEARNUM = EPOCHYEARNUM + 2000
: ELSE EPOCHYEARNUM = EPOCHYEARNUM + 1900

This switch postpones the difficult change until 1st Jan 2057. Which
is the 100th anniversary year of the flight of Sputnik 1. Perhaps by
then distribution of Keplerian elements will have a very different
form from that used presently.

More on Antennas and Azimuth next week.