[time-nuts] Difference in GPS antennas

[Magnus Danielson]

Lux, Jim (337C) wrote:
> 
> 
> On 8/15/09 7:58 AM, "Magnus Danielson" <magnus at 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)

Not used by the fishing vesels near me...

I didn't mean helixantennas as such, I was just asking myself the 
question if any of my *GPS* antennas was infact of the quadra-helix 
design, a few of them have the sizes that they *could* be that.

> 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.

A typical GPS sat has two rings of helix antennas, an inner and outer 
ring. These create a far-distance shape of lobes that i circularly 
fairly even but pushes more energy towards the edge of the earth, as 
seen from the satellite, such that the additional space loss from 
increased distance is being somewhat equalized by the GPS antenna.

That's the trick being used to reduce the power variations from the GPS 
sats as experienced by the user.

Cheers,
Magnus