[time-nuts] time-nuts Digest, Vol 105, Issue 72

Stewart Cobb stewart.cobb at gmail.com
Mon Apr 22 01:37:06 EDT 2013


Date: Sat, 20 Apr 2013 12:52:03 -0700
> From: "Tom Van Baak" <tvb at LeapSecond.com>
> To: "Discussion of precise time and frequency measurement"
>         <time-nuts at febo.com>
> Subject: Re: [time-nuts] pin-wheel antenna
> Message-ID: <EB790CDDE52944E1A03608CA6CFCCCA8 at 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.

You can find phase pattern calibration data for this antenna here:

<http://www.ngs.noaa.gov/ANTCAL/Antennas.jsp?manu=NovAtel>

Unfortunately, this calibration does not include gain data, only phase
data.  IIRC, our guys that tested this antenna said it was almost as good
as a choke ring (although our tests were not nearly as rigorous as NGS or a
real antenna lab).

Hope this isn't too far off topic.

Cheers!
--Stu
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