Greetings! I have been doing alot of reading lately, specifically your Surviving the Storm and the latest Offshore Cruising Encyclopedia books. Excellent pieces of work by the way! I came across a piece of information that puzzled me but I figured it was just a typo. But more recently I came across it again in your FAQ on setsail.com. Specifically, the reference is to the fact that a dipole antenna is superior to a long wire (i.e. insulated backstay). I’m confused about the reference to a dipole giving 8 times the output compared to a longwire. A normal simple dipole antenna has a gain of 3db which results in twice the output not 8 times. Also, the dipole is directional with fairly deep null areas off the ends of the antenna so the 3db gain figure is only really true relative to a point broadside to the antenna. If you really know of a dipole configuration with an 8x advantage (that would be a gain of 9db) please let me know. I currently use a whip for marine SSB and an insulated backstay for the ham rig. Some more gain over the unity gain solutions will be greatly appreciated. Also, thanks for the info on the WH Autopilot. You sold me, I will be ordering one very soon! Smooth Sailing!–Doug Dotson
Hi Doug: My data on dipoles is anecdotal, based on the less-than-ideal radiation patterns of long wires and whips when they do not have good ground planes associated with them (which means almost always on a sailboat, unless you have a metal boat). First, you are absolutely correct that dipoles are directional. They are most effective at right angles to the antenna and weakest off the ends of the dipole, as you indicate. Back in the late 70s, when we were cruising on a fiberglass boat, we made a number of comparison transmissions from Melanesia back to the West Coast of the US. We’d transmit on the dipole, then using a coax switch change over to the backstay. The hams with whom we were speaking would report a huge gain in signal strength. I’ve forgotten the “S” meter reading now, but they translated as factor of roughly eight. What was most surprising was the increased reception we had as well. I recall sitting in the harbor in Rabaul, Papua New Guinea, listening to the Rose Bowl football game on the AFRTS. We could hear it clearly with the dipole, but not even a scratch with the long wire. And we were the only boat in the harbor that could pick up the AFRTS signal. There was nothing special about these dipoles. The formula for length and frequency is of course well known (and in our Offshore Cruising Encyclopedia). We eventually went to a “trap” dipole, so we could work 15 and 40 meters as well as 20.
Give it a try on your own boat, and let us know what you find out.–Steve Dashew
Posted by Steve Dashew (November 30, 1999)
