Antennas are a frequent topic of discussion among GPS users. Usually the attention is focused on active antennas.† The major benefit from of an active antenna is that weak signals are amplified, often enough to allow reception of signals that are too weak for a receiver alone to lock onto.
In order to use an active antenna, a power source is required. Some receivers supply the necessary 5V DC through the antenna jack. Obviously it will shorten the battery life in the case of a portable application.
Lowrance receivers like the GlobalMap100 and GlobalNav212 have antenna connectors, but they donít supply the necessary 5V to drive an active antenna. Still, the active type can be used, except in that case an external power source is required.† For the technically minded, there is a description of such a set up in the ďLEI tips and tricksĒ section of our website. Alternately, a re-radiating antenna, such as Wolfe's Global-Link antenna (http://www.gpscables.com/gl66.htm ) can be used.
For GM100 / GN212 owners who just want to plug and play, an EA3 external antenna from Lowrance is the only option.
In the few times that this antenna has been discussed on the sci.geo.satellite-nav forum, there were some speculations that the EA3 may actually interfere with the built-in receiverís antenna, resulting in signal loss instead of gain.
For GPS Nuts, the only way to see whether this theory had any practical effect was to try it.
Thanks to the courtesy of Lowrance, we were loaned an EA3 for a few weeks. As usual, the gentlemenís agreement was in effect. We will report what we see and if Lowrance doesnít like our findings, itís just tough luck for them. We will still report it on our website.
The antenna came with a 7í 3Ē (2.2 meters) long cable and a set of mounting hardware. The hardware included mounting plates, magnetic base and suction cups, so the antenna could be attached just about anywhere, or simply be used as a portable. I have to admit here, that I failed to measure the actual size of the antenna. Going from the memory, it was about 3Ē X 3Ē X ĺĒ thick. The antenna with the cable tipped the kitchen scale at 4 ounces.
After plugging the antenna into the GM100, nothing happened. Well, almost nothing. The strength indicators moved a bit, but thatís all. No major, noticeable, gain or loss of signal.
Tests with the antenna placed in different positions, from right beside the receiver, to the length of the cable away, did not reveal any interference.
Further experiments, with shielding of the receiverís built-in antenna, revealed that the EA3 actually improved the signal reception from the overhead satellites and gave slightly lower S/N ratio for the satellites that were closer to the horizon. Obviously, it applies to an antenna when mounted in the horizontal position and is quite normal for patch type antennas.
The improved signal gain might not make a difference in most situations, but it proved to be valuable on a short train trip between Burlington and Toronto.
The GM100ís built-in antenna was not sufficient to maintain a good lock throughout the trip. On the return leg, with the EA3 stuck on the window, there was no problem, whatsoever, maintaining a good, consistent lock on 4+ satellites. The mounting plate that is used with the suction cups, is angled so the antenna was held at the most favorable position for receiving any satellite signal from that side of the train. One can also assume that results would be very similar in an airplane.
In summary, the EA3 proved itself to be nothing more and nothing less than a simple external GPS antenna. In the case of the GM100 or GN212, the receivers and their built-in antenna are so sensitive, that under most circumstances, an external antenna is not necessary in the first place. However, the EA3 can prove its worth in situations where the receiver itself cannot be placed in a good spot to receive signals. In the case of some cars, RVís or boats, especially if the windows are metallic coated, it may be impossible to receive any GPS signals without mounting the antenna outside. Thatís really what the EA3 was designed for and it will surely prove itís worth.