[1] GPS antenna, a homebrew success
From: scottcr@wkuvx1.wku.edu (Chris Scott)
Date: Fri Mar 18 08:06:17 PST 1994
Organization: Western Kentucky University, Bowling Green, KY
Nntp-Posting-User: scottcr
Lines: 84
 
** Adventures in amplified GPS antenna construction **
   Chris Scott, C/E WKYU-FM Western KY Univ.

I use a II Morrow "Flybuddy" GPS receiver in a small aircraft for RF survey
purposes. This receiver uses the JRC 8 channel parallel  oem board.  The
installed antenna is difficult to remove and re-install, so I needed a
second unit to use in the lab while interfacing the equipment and de-bugging
the data-collection software.

I had tried several passive monopoles without any luck; no signals period.
Evidently amplification was required to get to the threshold of detectability
in the receiver. $385 was the price for an additional unit.  Seemed kind of
high.

Having some experience in VHF antenna design and construction, I contemplated
the possibility of getting a home-brew antenna to work.

My impedance measurement instruments are useful only to about 500 mhz, so
tuning after construction was not feasable; obviously any type of narrowband
design had no hope of working without tha ability to trim it to resonance.
A helical design, although fairly straightforward, again must really be
trimmed to get a good match, which at 1.5 ghz I presumed to be important in
minimizing losses.

I decided upon an equalateral triangle blade monopole design, which according
to Kraus, approaches the broadband characteristics of a conical monopole.
This way, as long as I was reasonably close in cutting, the resonance being
broad, it should match reasonably well.

The pre-amp was a different matter; many GPS units send +5 vdc along the
center of the coax to power the antenna-mounted preamp, allowing optimum
amplification to take place prior to transit to the receiver.

Commercial designs seem to vary between 9 and 30 db of gain, mine  being
unknown.  Probably a Gaasfet design similar to satelite LNA designs would
be optimal, but again, faced with no way to directly measure impedance at
1.5 GHZ, I chose a straight MMIC amp that 1) would be stable and provide
some gain at 1.5 g,  2) would operate at +5 vdc within the current limitation
of the receiver, and 3) match the antenna and line directly, or nearly so.
Included in the standard mini-circuits designers kit is the MAR-3 which I used
successfully, although a MAR-6 appears to have more gain and better noise
figure.  These devices are not optimal for a high-performance UHF front-end,
but easy to use and match for this application.

Construction was started using a 20 x 20 cm copper sheet with a small steatite
standoff in the center.  I attached the triangle shaped copper antenna element
to it with a small copper angle soldered to it, with a screw threading into
the standoff.  This constitutes the entire mechanical support for the blade.
This blade was cut about .24 wavelength high, my guess at resonance. the apex
at the bottom was  positioned about 2 mm above the copper sheet, where I
directly soldered the MAR-3, with its input lead soldered directly to the
bottom of the blade.

I used a BNC connector fitted from the bottom of the copper sheet with center
pin soldered directly to the output lead of the MMIC.  Naturally the two
adjacent leads were soldered to the copper sheet.

Upon connection to the receiver, the miracle of lock happened!

I installed a radome (tupperware) on the top, and placed it on the roof
connected via RG-213 to the rcvr.

Clearly this is a crude non-optimal, antenna, but can be constructed with
ordinary tools, with no alignment required.  Inside the lab, the GPS receiver
only occasionally unlocks; not bad considering the heavy tree cover around
the building.  A MAR-6 should perform even better.
Total cost (not including the copper sheet I had around): $5 PC board would
probably work just as well.
Chris
-- 
SCOTTCR@WKUVX1.WKU.EDU aka Chris Scott- C/E Public Radio- Western KY U
                      Telco: (502) 745-3834  Hm & Fax: (502) 781-1232
...just another insignificant VAX user.        \\\//
                                               (o o)
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