On 4/21/13 10:37 PM, Stewart Cobb wrote:
Date: Sat, 20 Apr 2013 12:52:03 -0700
From: "Tom Van Baak" <[email protected]>
To: "Discussion of precise time and frequency measurement"
<[email protected]>
Subject: Re: [time-nuts] pin-wheel antenna
Message-ID: <EB790CDDE52944E1A03608CA6CFCCCA8@pc52>
Content-Type: text/plain; charset="iso-8859-1"
There's a very nice picture of a pinwheel from Novatel on the back
cover
of the March issue of GPS world..
It's a thing of mysterious beauty. And the GPS World photo saves me from
the temptation to break open my own pinwheel antenna just to see what's
hidden inside.
/tvb
In a previous job, I was actually encouraged to dismantle failed GPS
antennas. Attached please find scans of an earlier Novatel pinwheel
antenna, model 702L (I think), covering L1, L2, and the OmniStar band (1535
to 1559 MHz). Note the two different lengths of the topside slots, one set
each for L1 and L2. The termination resistor on the backside spiral feed
measures 133 ohms. Its presence confused me at first, but it's only
terminating the "wrong" circular polarization -- the correct polarization
reinforces itself toward the connector. Scanned on a desktop scanner (not
a camera) at 150 dpi. The substrate is a typical Teflon low-loss material
and measures 0.063 inches thick, and the diameter measures about 5.628
inches. If anyone wants to read dimensions off the scan and model this on
HFSS or something similar, I'd love to see the results.
The more complex flame-like structures at the tips of the slots in the
photos of the current model are probably attempts to broadband the basic
antenna to cover L5, Glonass, and/or the new Galileo bands. The operating
principle of my older one is obvious almost by inspection; not so the new
one, which probably could not have been designed without computer modeling.
The one you scanned is a pretty straightforward spiral (cavity backed, I
assume, when in the package), with two different length spiral arms for
L1/L2. And slot fed, as noted in the literature. I've seen this
strategy on linear arrays, but hadn't seen it for a spiral, but of
course, it's a clever way to do it.
I think the new one is a bit more clever in terms of broadbanding;
perhaps like a fan dipole or bowtie, with the arms of non-constant
width. The spiral pitch is also a lot steeper. It's sort of half way
between the one you have scanned and a conventional crossed dipole.
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