Active Antenna by Carlos Puch
The idea is -like Allen Perrins states- to
supply power to the active
external antenna which can then be connected to the rear of the
GM-100.
The reason why ? - The original Lowrance EA-3 external antenna is not
very sensitive, and is quite bulky and expensive (I bought one
myself).
After surfing the web, I paid attention to a couple of excellent
articles written by Dave Martindale. One of them, devoted to the
construction of an external antenna for the Garmin 38 - Magellan
2000XL - Eagle Explorer receivers, has been the source to which I
refer the whole project. It can be found on Peter Bennett's web site:
http://www.vancouver-webpages.com/peter/extant.txt
I've constructed a couple of devices:
-The first one is specifically designed for car use. It comprises a
RF connection between the antenna and the GPS and a separate power
supply (see circuit layout, and photos 1-3). The RF bridge uses two
BNC female sockets assembled by means of 4 brass separators which
serve as the structural support. The isolating capacitor and the RF
choke are mounted close to the BNC's. A cigar lighter plug supplies
power to the small black box powering the external antenna and also to
the GPS, via a homemade connector.
(photo 1)
(photo 2)
(photo 3)
The second one (photo 4)
is intended for external use, and comprises
both the RF link and the power supply. It uses a small 9 volt battery
as the power source.
(photo 4)
Following Dave Martindale's directions, I've tried to minimize the
"non-50 ohms" section of the RF link between the antenna and the GPS:
it is just a gap filled by the condenser (see photos).
For linking both assemblies to the GM-100, I have made a small cable
with BNC and SMB male connectors.
I am using a Trimble OEM mini-mag antenna (which is identical to the
most popular Lowe), and also a Garmin GA-26. In both cases, the signal
bars on the satellites page never reach the maximum level, due in part
-I guess- to the number of BNC/BNC connections involved. This may be
beneficial, so that the RF front end of the GPS won't suffer from a signal
overload. Anyway, bear in mind that these kind of experiences fall out
of Lowrance's warranty !
Note that the internal antenna of the GM-100 seems to be ignored while
the external active antenna is connected. This is exactly the opposite
of what happens with the original EA-3... It's very confusing when
the GPS is mounted inside the car. Unless
you cover the internal
antenna with aluminum foil or something alike, you'll see both
antennas (internal and EA-3) working on a par.
Please don't forget that the source for this experiment is Dave
Martindale's advises on the web.
