[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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