Wind a wire into a coil one wavelength around and it stops behaving like a coil and starts firing straight off its own axis — circularly polarized, so a tumbling satellite or a hand-held node lying on its side still gets through. This is the antenna for reaching a satellite, or for stretching a 900 MHz mesh link past where anything else holds.
Axial-mode helix — gain plus circular polarization.
Wind a conductor into a coil roughly one wavelength around, back it with a ground reflector, and it fires straight off the end of its own axis with real gain — and circular polarization, so it does not care how the other end is oriented. That makes it the antenna for satellite work and for long point-to-point 900 MHz mesh links.
The headline use is pulling GOES weather images out of the sky: a helix at 1694 MHz is the standard receive antenna for the HRIT downlink, and comes out about seven inches around and a foot long.
Both ends must use the same sense. A right-hand helix talking to a left-hand one throws away 20 dB or more — worse than no antenna, because it looks fine and does not work. The wind direction is what sets the sense.
The gain figure comes from Kraus’s classic formula, which is what every helix calculator quotes. Measurement shows it running optimistic — about a dB past ten turns, and more beyond that — so treat it as a ceiling rather than a promise.
And it is a searchlight, not a floodlight: with a beamwidth that narrow, a satellite pass means tracking it, and a mesh link means a mount solid enough that it will not drift off aim.
