Satellite Antennas Pt 2 by GM4IHJ
Almost any piece of antenna wire will give some results on HF Satellites
but the change to VHF and higher uplink and downlink frequencies demands
much more attention to antenna design. You can put up a quarter wave
vertical whip at 2m VHF and perhaps embellish it with a suitable counter
poise . In theory it should work, but it will give frustrating results,
because signal polariation is automatically wrong if you use a vertical at
the ground station which has a very different idea of what is vertical as
compared with the vertical as seen by the satellite. Worse still your
ground vertical and what the satellite calls vertical is changing all the
time as the satellite follows its curving orbit above you. Even when the
satellite is overhead your station its idea of vertical is such that you
see the end tip of its antenna and get a very poor signal albeit enhanced
because the range is so short. So in general it pays to avoid vertical
ground station antennas .
Much more successful is the horizontal dipole though it has blind spots
off the tips of the dipole, and works better when a second dipole is set
in the horizontal plane at 90 degrees to the first, giving you the popular
crossed dipole which is a good starter for any VHF station.
At extreme range where the DX is, you need more antenna gain so a 3 or 5
element yagi beam antenna with its elements horizontal is favoured.
Signals coming from your horizon are usually horizontally polarised when
they get to your antenna, so provided you know the beam can only cover a
60 degree angle at your horizon you are OK, and if you give it a masthead
rotator in azimuth it can cover all the horizon at elevations lower than
25 degrees.
When the satellite gets above 25 degrees elevation it is closer to you and
signal is stronger so you can switch to the crossed dipole, follow the
satellite until it is less than 25 degrees above the far horizon and
switch back to the yagi for the last few minutes of the pass. Remember
also that in say a 15 minute pass in range, the satellite will take 5 + 5
minutes at below 25 degrees near each horizon and only 5 minutes as it
flashes near overhead. Satellites climb and descend much more slowly at
the begining and end of a pass than they do in the middle. Equally
important their azimuth and elevation rates of change are very slow on the
yagi near the horizon where you do not really need to move the beam,
whereas during the centre of the pass the azimuth and elevation changes
are very fast but your "all round looking" crossed dipole handles this
situation easily. Indeed it cannot be emphasised too much that simplicity
of operation of your antenna system is probably much more important that
the antenna gain , particularly with low altitude LEO satellites which
move across the sky quite quickly.
So the VHF solution for LEO sats is relatively simple because their short
range from you means that you get reasonable signal strength even when
antenna pointing and polarisation are far from optimum. BUT be careful, if
you want to go for higher altitude sats or just simply must have
perfection on LEO sats then you must allow for Faraday Effect, whereby the
signals plane of polarisation is rotated as it passes through the
ionosphere. Ways to deal with Faraday Effect will be discussed in Part 3.