The importance of antenna placementMar 27, 2013
The proliferating alphabet soup of electronic system acronyms that populate your boat’s wheelhouse have one thing in common: most of them have some type of antenna hooked to the other end. This can cause a shortage of real estate topside for mounting these antennas in a way that they do not cause radio frequency interference with each another.
The smaller the vessel (especially if your boat happens to be a sailboat) the harder it is to find sufficient space to mount all of these antennas. On the other hand, the good news for smaller boats is that the smaller your boat, the less cabling you will use in connecting to its antenna, which means it will be easier to install and it will have less attenuation of the signals.
It may help us in the proper placement of antennas to understand how radio frequencies interfere with each other in a sort of RF interference hierarchy. First we need to know that VHF and GPS can both interfere with each other and that radar and HF can interfere with just about anything electronic. Because GPS receivers need a clear view of the sky, their antennas should be mounted in a clear area topside at a reasonable distance from any VHF antennas. This should not be a problem because VHF antennas should be mounted as high as possible due to the line-of-sight range restrictions, and with VHF, height is more important than power output.
If you have more than one VHF antenna they should be separated at least one-half wavelength from each other, which is approximately three to four feet. It is always better to move them as far apart as possible or even to stagger them vertically. Separate VHF antennas are often used for DSC VHF radio and for the AIS system, but don’t forget that if space is really limited you can use just one antenna for both with the use of an AIS antenna splitter.
HF antennas can be mounted nicely on each side of the pilothouse. If you need special antenna mounts or mounting hardware a good place to look is on the following websites: www.pyiinc.com, www.shakespeare-marine.com, www.scanstrut.com, www.edsonmarine.com.
Of course, an antenna is part of a whole system and there is more to it than just placement and mounting. There is the antenna itself and whatever you do, don’t be cheap about buying a good antenna, because someday your life may depend on it. This leaves the coaxial cable, cable connectors, and a good grounding system on your boat. A large part of the problems experienced with marine communications are caused by loose, wet and corroded connectors.
If you are not good at soldering on PL-59 connectors, then I recommend the use of Shakespeare Style PL-259-CP-G gold-plated, solderless connectors. These are easy to use and will give you an excellent connection with minimum signal loss. As good as these connectors are you will still need to weatherproof them to ensure a long-lived and low impedance connection.
You can make up your own weatherproofing kit by going to the nearest Home Depot or hardware store, and procuring the following supplies: Scotch #2228 moisture sealing electrical tape ($9.27), Scotch Super 88 vinyl electrical tape 4/4” x 66’ ($3.95), GB Gardner Bender liquid electrical tape, black, 4 fl. oz. ($5.99).
After securing the connector on the end of your coax, screw it on to the antenna. Connectors should only be hand tight, do not over torque on them. Wrap some moisture sealing tape around the whole cable/connector a few times and go down a few inches below the connector, and mold with your hand…this tape is self-fusing so it molds nicely. Next you wrap the cable/connector with the vinyl electrical tape and go even farther down from the sealing tape. Finally, coat the whole outside of the taped area with the liquid tape and when that dries your antenna should be quite well sealed against the intrusion of moisture/water.
The next iteration of this newsletter we will discuss the actual coaxial cable and a little bit about grounding. Until next time, ”Fair skies and following seas.”