By Ben Ellison
More to Come
My GPS loses its signal whenever I run my radar. It’s mounted about three feet behind the open-array radar antenna on the flying-bridge hardtop. I raised it on a two-foot extension to no avail. D.D., via e-mail
The NMEA Installation Standard states that GPS antennas “shall not be installed in the direct beam of a radar antenna.” That beam is generally specified as 25 degrees vertically—12 1/2 degrees below the horizon, 12 1/2 degrees above—so your newly raised antenna should be working. However, the beam edge is also defined as where the radar signal is less than half of its nominal strength; apparently even a small percentage of your fairly powerful radar is able to smother your GPS antenna from a few feet away. Try moving it again, and bear in mind that since it is trying to hear signals from the sky, not from over the horizon, it will likely be quite content down on the side of your flying bridge or even on your trunk cabin top, lower but well away from your other antennas. That’s why most current GPS antennas can be alternatively mounted directly, and with low profile, onto a horizontal surface. But I’ve also heard of boaters who’ve drilled a hole in a metal pie plate and stuck it under a mast-mounted GPS antenna to protect it from the sort of low-angle interference you are experiencing—though it strikes me as a stubborn approach!
The whole business of interference between various active and passive marine electronics is complicated. For instance, GPS satellite signals are so weak that they can get messed up by nearby VHF, SSB, satcom, and even cellphone transmissions. It’s not for nothing that the NMEA Standard has both a table of suggested minimum horizontal spacing for all antennas and a rigorous protocol for testing each new installation for interference to or from each existing installation. —B.E.
Got a marine electronics question? Write to Electronics Q&A, Power & Motoryacht, 260 Madison Ave., 8th Fl., New York, NY 10016. Fax: (917) 256-2282. e-mail: PMYElectronics@primediamags.com. No phone calls please.
This article originally appeared in the November 2003 issue of Power & Motoryacht magazine.