Satellite Antennas

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.


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