[time-nuts] Difference in GPS antennas
Lux, Jim (337C)
james.p.lux at jpl.nasa.gov
Sat Aug 15 15:12:11 UTC 2009
On 8/15/09 7:58 AM, "Magnus Danielson" <magnus at rubidium.dyndns.org> wrote:
>>>
>>>
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>> Thats a quadrifilar helix antenna.
>
> A quite traditional antenna form.
>
> Not sure I have one of those around here.
>
>
If you're near a harbor with fishing boats, you'll see plenty of quad
helices about a half a meter in overall height, used for VHF Weather
satellite reception. They're also used on spacecraft (Mars Science Lander,
Mars Reconnaissance Orbiter, and Phoenix all have UHF quad helix antennas, I
think, for about 400 MHz)
The 4 helices need to be fed in the appropriate phase (0,90,180,270),
usually, they're fed in pairs (a differential signal feeds 0,180 and another
feeds 90/270)
There are several ways to phase them, depending on the bandwidth
requirements. A quadrature hybrid is one way. The other is to make one helix
slightly longer than resonant and the other slightly shorter.
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