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Fall 2003 Issue |
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 The author’s two helical antennas rotated by U-100 rotors. |
SATELLITES Artificially Propagating Signals Through Space By Tom Webb, WA9AFM |
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Once the satellite rig is in place, the
antennas are in the air, the tracking software is installed, and the
current Keps loaded, you are now ready to work satellite—well, almost.
Remember that those satellites are moving targets, and we have to follow
their paths across the sky with our antenna array mounted on our
azimuth/elevation rotors, a.k.a. az/el rotors. A variety of ways exist to
control rotor systems, ranging from the good old “Armstrong” method to
highly sophisticated computer-controlled interfaces, which also adjust
your satellite rig to compensate for Doppler shift. Let’s examine our
options. You may know this type of rotator as the “Field Day rotor,” i.e., turning the antenna literally by hand (or tag line). For aiming satellite antennas, hand support and aiming are feasible because of the small size of the antennas. With the AO-40 satellite appearing in the sky at the same point every 48 hours, a fixed directional antenna is a valid option. However, if you plan to track a “fast mover,” you are going to need help. For several Field Days I’ve enlisted the aid of “biped, carbon-based antenna supports” (two people who were willing to hold antennas), and another person with an accurate watch and listing of the azimuth, elevation, and timing of the satellite pass.
The human “controller” gave azimuth and
elevation instructions to the human “rotors,” who in turn pointed the
antennas. This is a crude methodology, but it gets the job done. “BRC” (before rotor controllers), moving your satellite array was literally a hands-on proposition. Manual rotor control was the only way to keep the antennas on target. For High Earth Orbit (HEO) birds, such as AO-10, movement during usable portions of the orbit with such antennas only required adjustment every few minutes, or even less frequently. However, with Low Earth Orbit (LEO) birds, it was a different story. With passes lasting just a few minutes, you had to be able to operate rotor controls, compensate for Doppler shift on your rig, make notes on your scratch pad, and talk, all at the same time—which requires using both your hands and your feet (for the PTT foot switch, that is)! It could be done, which was part of the challenge—and the fun—of early satellite operation.
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