Satgen 423  Antenna Experiments by GM4IHJ (BID SGEN423)     2 May 1997

Most of what passes for amateur radio equipment is very expensive, and
as we move up to ever higher frequencies. the costs are escalating. So
that some sections of amateur satellite operating are becoming
restricted , solely, to individuals with large pockets.

How can we change this situation ?  Has all the ingenuity and
originality departed from amateur radio ? Perhaps we need to think
harder about the way we do things, and , look for inexpensive
solutions. A line of thought which got IHJs ancient chemical computer
questioning the need for another set of Azimuth and elevation rotators
for proposed future work on Mode S and L LEO sats, and experimenting,
went as follows :-

If you do not use a rotator , the satellite track is rarely amenable
to continous coverage with a single fixed antenna, with any sort of
useful gain. Gain usually means narrow beam width , which limits
stationary coverage. But we can be guilty of tunnel vision at times ,
and thoughts about simple polar mountings for covering the geosat arc
lead to thoughts of how a similar idea might help with coverage of
the various arcs followed by successive LEO sat orbits.

Checking Fo20 recently showed that with 9 orbits visible each day from
Scotland, there was 131 minutes total time when Fo20 was above the
station horizon. Of this total 92 minutes =70% saw the satellite below
15 degrees elevation , ie easily covered by a 435 MHz colinear fixed
antenna. But a further 39 minutes (on just 4 of the 9 orbits) saw the
satellite above 15 degs elevation , where reception varied enormously.

It was appreciated that the colinear had a wheel shaped polar diagram
. All round in azimuth , but only 0 to 15 degrees in elevation ,
provided the colinear was mounted vertically. But if on orbits going
above 15 degs the colinear was tilted backwards, its wheel shaped
polar diagram covered the low elevation track at acquisition and at
end of pass at either horizon and it also covered the elevated portion
of the track in the middle of the pass. At least that was the theory.

A few experiments with Fo20 suggest this is a reasonably practical
procedure . The colinear is mounted vertical for those passes which
never go above 15 degs. But on a typical high pass say near Azimuth 75
Elevation 44 at closest approach , the colinear is released against a
counter weight until it leans back about 44 degs , whilst the vertical
plane through the antenna from behind , points at Az 75.

This seems to work reasonably well. So the intention now is to build a
colinear or, an array of vertically stacked dipoles, or helices, for S
band and NOAA sats , to test the idea on their downlinks, at
frequencies we may soon start to use, to and from LEOsats

Has anyone else tried similar or indeed other ways of solving this
problem ?