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Helical and satellite antennas
Circular polarization for space work.
Circular polarization for space work.
A helical antenna is a wire wound like a corkscrew. Wound the right way, it sends out a wave whose electric field rotates as it travels: circular polarization. Circular polarization is the standard answer to a problem satellites create: you cannot know, or hold steady, how a tumbling spacecraft's antenna is oriented relative to yours.
In axial mode, the circumference is about one wavelength, so the currents on opposite sides of each turn are in step and the antenna radiates along its axis. A reflector plate behind the feed points all the energy forward. Ten turns gives roughly 15 dBi and a beam about 30° wide, wide enough to forgive a small aiming error. The beam sharpens as you add turns.
A helix wound like an ordinary screw thread is right-handed and radiates right-hand circular polarization (RHCP). A left-hand winding radiates LHCP. In the radio convention a right-hand wave turns clockwise as seen from behind it, looking the way it travels, like a right-hand screw driven away from you. (Optics uses the opposite convention.)
A short helix also exists in everyday radios: the coil in a "rubber duck" works in normal mode, radiating broadside like a loaded whip rather than along its axis. See Handheld radio antennas and Loaded and short antennas.
A spinning satellite, or the ionosphere twisting a wave by Faraday rotation, keeps changing the angle of a linear signal. A linear antenna on the ground then fades deeply as the angles cross. A circular antenna receives a linear wave at half power, 3 dB less than a perfect match, but at a steady level at any angle. A fixed 3 dB loss is easier to live with than random fades to nothing.
| Transmit | Receive | Result |
|---|---|---|
| right-hand circular | right-hand circular | matched: full signal |
| right-hand circular | left-hand circular | opposite senses: a very deep loss |
| circular | linear, any angle | about 3 dB below a perfect match |
| linear | linear | full when aligned, a null at 90° |
A reflection reverses the handedness of a circular wave, so a ground-bounced signal arrives opposite to the direct one and is largely rejected.
Two crossed linear antennas fed 90° apart also make circular polarization. Satellite stations often use two Yagis mounted on the same boom at right angles, fed through a harness that makes the 90° difference. Reversing the phase swaps handedness, so some stations can switch between RHCP and LHCP.