Adventures in amplified GPS antenna construction; an experiment...

COPYRIGHT 1994, Chris Scott

**NOTE** Due to the large volume of mail requesting more technical detail, a quick sketch of the construction method has been added. The MAR series MMIC amplifiers are available from MINI-CIRCUITS, Brooklyn, NY .  Please understand that this was an experiment driven by quick need - not an optimized circularly polarized design.

I use a II Morrow 820 GPS receiver in an 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 antenna to use in the lab while interfacing the receiver and debugging the data-collection software.

I had tried several passive monopoles with no luck; no signals period. I needed a working antenna now. Evidently amplification was required to get to the threshold of detectability in the receiver. $385 was the price for an additional commercial unit. I needed a working cheap antenna now.

Having had some experience in VHF antenna design and construction, I contemplated the possibility of getting a preamplified 1575 MHz antenna to work.

My impedance measurement instruments are useful only to about one Gigahertz, so tuning the antenna after construction was not feasible; obviously any type of narrowband design had no hope of working without the ability to trim it to resonance. A helical design was considered. Since the classical helical design uses two elements each above and below resonance to achieve proper axial ratio for circular polarization, it must be carefully tuned. Maybe a broadband antenna would be the solution. 

I decided upon an equilateral triangle blade monopole design, which according to John Kraus, approaches the broadband characteristics of a conical monopole. This way, as long as I cut the element pretty close, the resonance being very broad, it should match reasonably well.

The preamplifier 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 via the lossy coaxial cable.

The gain of commercial GPS preamp designs seem to vary between 9 and 30 db, the requirement for my unit being unknown. A Gaasfet approach similar to satellite LNA designs probably would be optimal, but faced with no way to directly measure impedance at 1.5 Ghz, I chose a straight Monolithic Microwave Integrated Circuit amplifier that
1) would be stable and provide some gain at 1.5 GHz,
2) would operate at +5 VDC, within the current limitation of the receiver, and
3) match the antenna and 50 ohm line directly, or nearly so.

Included in the standard Mini-Circuits designers kit is the MAR-3 MMIC which I used successfully, although a MAR-6 appears to have more gain and better noise figure. These devices are sub-optimal for a high-performance UHF front-end, but easy to use and match for this application. Subsequent to the first publication of this experiment, it became apparent that another device is probably a more optimal choice for the active device; MAcom's AM50-0002 is an LNA intended for GPS operation. At 1.1 db noise figure and 27 db of gain from 5 vdc bias, it would be] my choice for a repeat project.

Construction was started using a 12 x 12 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, using a screw threading into the standoff. This constitutes the entire mechanical support for the blade, which was cut to about .23 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 to the copper sheet with center pin soldered directly to the output lead of the MMIC. The two adjacent (ground) leads were soldered to the copper sheet. Upon connection to the receiver, I was pleased to see it begin to aquire the birds, and start locking normally. I installed a radome (tupperware) on the top, and placed the gizmo on the roof connected via RG-213 to the receiver.

Clearly this is a crude sub-optimal antenna, but it can be constructed with ordinary tools. No alignment is required and it should repeat well.
 

Inside the lab, the GPS receiver only occasionally unlocks; not bad performance considering the heavy tree cover around the building. A MAR-6 should perform even better.
PC board laminate would probably work just as well for both the ground plane and the blade.