On 5/6/15 12:53 AM, John Marsden wrote:
Ok, I only ask becuse there seemed to be a big thing about LHCP quad helix antennas - even to the point of seein an article showing how to 'unwrap. a RHCP Q-H, and rewrap it 'inside-out' to change the polarisation to LHCP. I'm seriously considering making an active Q H if I can't find anything that looks promising - pending the answer to the question above, of course - I don't want to spend $100 making a '$40' one ;)
Helical antennas are really non-critical in terms of design. I'm not even sure that a quadrature helix is actually what you want: they tend to have more response at the horizon (for a short helix) than straight up, but I would think that for timing applications, you'd rather get strong signals from overhead, rather than low angle signals subject to multipath, etc.
(remember that the GPS satellite's transmit antenna pattern is bigger at the edges than in the middle, so the incident flux on the ground is about the same regardless of elevation angle: this is different than the typical case for, say, LEO amateur radio, where the satellite is essentially omni, and you want an antenna with more gain at the horizon)
A quad helix was popular for handheld GPS units because it's got a reasonably good pattern that's almost omnidirectional (even if the axial ratio isn't so hot in all directions), so it works well in any orientation.
A regular helix with a gain of 10 dBi or so (4 turns) will have a 3dB beamwidth of 52 degrees, and will be about 6 cm in diameter and 20 cm long. 3 turns is 60 degrees bw (3db)
The folks at JPL wind their helices on things like appropriately sized plastic cups and put them on one of those bowl shaped things that fit under a stove burner as a ground plane (hence the "helibowl" moniker). You get a decent match by adjusting the distance of the end of the bottom turn from the ground plane (making a sort of tapered transformer from the 100-150 ohm impedance of the helix to the 50 or 75 ohms of the feed line)
_______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
