[time-nuts] Lucent KS-24361/Thunderbolt antenna advice
Jim Lux
jimlux at earthlink.net
Wed May 6 09:06:46 EDT 2015
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)
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