Installing Marine Electronic Equipment ered. Follow the procedures described above for determining any compass deviation and getting power to the receiver. The big consideration is where to locate the GPS antenna. GPS receivers use receive-only antennas that need a clear view of the sky at all times. Even something like a Bimini top or a windscreen can affect the integrity of a GPS signal. Also, radio transmissions from other electronic equipment such as cell phones, VHF radios, and single-sideband (SSB) radios can affect a GPS signal. The rules here are simple and not too hard to comply with, even on small boats. Basically, the ideal is to maintain about a 1-meter separation, or a little less than a yard, between transmitting antennas and the GPS receiving antenna. On most boats this is simply a matter of mounting the VHF antenna on one side of the bridge and the GPS antenna on the other. Mission accomplished! On boats with elaborate gear-mount brackets and airfoils, just keep from mounting these antennas adjacent to each other. GPS is a line-of-sight system between the antenna and satellites orbiting overhead. The antenna scans the sky via a conical pattern that points straight up. So, unlike VHF, mounting an antenna too high can actually be detrimental to its performance in a rolling, pitching sea, particularly if you have selected the improved accuracy provided by a differential receiver. An antenna mounted too high will drive the GPS unit crazy as it swings back and forth trying to find its exact location. Low and as close to the centerline of the boat as you can get is the way to go with this installation. Any of the commercially available mounting brackets for these antennas will do just fine, but don’t think you’ll improve performance by using an extension pole, as you might with a VHF antenna. If you have a differential GPS receiver (DGPS), one that also receives radio position data from landbased stations in addition to satellite signals, you may need an additional positive and negative wire to the antenna. As with fish-finder and depth-sounder transducer cables, your GPS antenna comes with a fixed-length cable for attachment to the display unit. Don’t alter
its length, and keep any extra cabling coiled away from other cables and harnesses. Figure 12-13 on page 190 illustrates a typical wiring hookup for a GPS without DGPS. Figure 12-14 on page 190 illustrates a typical installation with a DGPS receiver.
Installing Your Own Radar In the old days, the average boatowner wouldn’t have dreamed of installing his or her own radar. Today, radar manufacturers have come a long way, and owner installation is not only practical but not that difficult. The big consideration with radar is the mounting of the antenna (scanner). Four basic rules must be followed:
Install the scanner on your cabintop or on an appropriate mast with a platform designed to accept the mounting bolts.
Position the scanner so that the antenna gets a good all-around view with as few parts of the superstructure or rigging as possible intercepting the scanning beam. Any obstructions will cause shadows and blind sectors on the radar screen.
To minimize electrical interference, don’t route the cabling from the antenna near any other onboard electrical equipment or cabling, just as with other gear mentioned above.
Remember that a radar antenna creates pronounced compass deviation. Keep a separation zone of about 51⁄2 feet (1.7 m) between the radar antenna and your compass.
When installing an antenna on a powerboat, consider the average angle at which your boat cruises while underway. If the antenna is mounted perfectly level with the boat at the dock, it will tend to aim at the stars while underway. Figure 12-15 illustrates the static angle for the antenna and the effect this slightly downward mounting has on the beam while the boat is underway. To determine the exact angle of the antenna to use, spend a morning determining the amount of lift your boat goes through from a standstill to cruising speed. Once 191
