Lux, Jim (337C) wrote:
On 8/15/09 7:58 AM, "Magnus Danielson" <[email protected]> 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
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