Re: [time-nuts] Difference in GPS antennas

[Lux, Jim (337C)]

On 8/15/09 7:58 AM, "Magnus Danielson" <mag...@rubidium.dyndns.org> wrote:

>>> 
>>> 
>>>  
>> 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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