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Old 08-10-2004
Tom Wood Tom Wood is offline
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In Praise of the RDF


Avoiding navigational errors mean adhering to the time-honored rule of not relying on only one means of navigation.  Who knows? Perhaps an RDF could have kept this cat off the beach.
Not long ago, I was indulging in my favorite pastime of poking through nautical salvage yards on a rainy Saturday. This is usually an innocuous waste of an hour or two, except when I bring home some remnant of a failed project or artifact of bygone sailing days worthy of the Antique Roadshow. But on this particular day, I was sifting the contents of some broken milk crates and greasy cardboard boxes when I discovered a floor-to-ceiling shelf of old, portable RDFs. Covered in years of dust and cobwebs, this wall of yesteryear's technology was like a beacon of light in a musty cellar.

Few sailors today would know an RDF if they saw one, let alone know how to use it. These items are the dinosaurs of modern electronic navigation, crude, not very accurate, huge by today's standards of miniaturization, and obsolete by any measure. But we still carry one on board Sojourner for a very good reason.

When the portable RDF first became available to sailors at a reasonable price in the 1960s, the companies that made them, mostly Ray Jefferson and Benmar, couldn't build them fast enough. Before that time, there literally were no electronic aids to navigation available to recreational sailors other than a speedlog and depthsounder. All navigation was performed the same way that it had been for centuries, using compass, chart, and dead reckoning (DR). Calculating vectors for set and drift was an art, and a gifted navigator, one who could "deduce" correctly at least half of the time, was the most valuable asset a sailor could have on board. Ending up a tad, say 20 miles or so, from where you originally intended was just no big deal.

The RDF was a great equalizer. A square metal box about twice the size of a breadbox, it contained a multiband transistor radio. On the top of the box were a round azimuth ring and a highly directional antenna, both of which rotated. The radio operated on a whole bunch of D cell batteries, for which I remember paying about 15 cents each in those days.


They don't make 'em like they used to.  This bulky, heavy, aesthetically-challenged relic could get you home, provided you know how to use it.
Operation of an RDF is simple. You set the box up on the deck somewhere, I always prefer the deck alongside the companionway. The first thing to do is to rotate the azimuth ring until it corresponds to the boat's compass reading and this is the first possible error in the system. The RDF must be square to the centerline of the boat or the azimuth ring adjusted so that the boat's heading is pointed truly forward. The helmsperson must keep as steady a course as possible during the operation or the resulting position will be in error. These errors can be large if care is not taken.

Once the boat's direction is cranked in, the radio is turned on and a station found that has a broadcast tower with a verifiable position. This is not as easy as it used to be. The number of marine beacons, or low-frequency coastal stations broadcasting Morse code initials, is declining. Older charts commonly had the call signals of broadcast stations printed next to the tower symbol, but this is becoming less prevalent. In addition, the modern vogue of pop stations saying, "You're listening to the swing sounds of The Weasel," instead of the 1960s FCC identifier, "This is W-E-S-L in Charleston, South Carolina," often makes it more difficult to locate a station tower on the chart. Try to find Z98.6 near a tower symbol.

Once the station is finely tuned in on the radio, the highly directional ferrite antenna ring is rotated until a "null" occurs, that is, all sound of the radio disappears. Most RDFs have a little round "null meter" that shows the strength of the radio signal visually since it is often hard to hear the sound level over the noise of water, sails, and crew. Some even use earphone jacks. When the null is found, the magnetic direction to the broadcasting tower is read off the azimuth ring and you have one line of position (LOP) that you can plot on the chart. A company called Vectra made a module-driven variant of the RDF that was pointed like a futuristic Buck Rodgers' laser gun with the direction being read off a compass in the top when the null point was found in the earphones, high-tech stuff in those days.

There is one inherent danger in an RDF reading. Since the directional antenna is a bar, it gets two nulls, one when the antenna is pointed directly at the tower and another when it is pointed 180 degrees away from the broadcast point. In most cases it is easy to tell. If you are sailing west of Los Angeles and the Hawaiian station appears to be east of you, you're reading the wrong null. But if you are well and truly lost at sea, this can add an element of uncertainty and navigational adventure.

"You don't know exactly where you are with one LOP, but you know a lot of places where you aren't, and you can start re-deducing with some fresh information."

You don't know exactly where you are with one LOP, but you know a lot of places where you aren't, and you can start re-deducing with some fresh information. An LOP is an LOP is an LOP. Whether from a celestial sight, a visual sighting of an object in a known place, or a bearing on a radio tower that is in a known location, an LOP gives you one dimension on a two-dimensional chart. Two LOPs taken at the same time, or close to the same time on a slow sailboat, give you two dimensions whose pencil lines will cross on the chart, providing a "fix." Three or more LOPs often form a little triangle or area within which your boat most probably resides. It is rare not to be able to get numerous radio stations, especially late at night on AM, but more on that later.


While GPS is a technological marvel, turning it off now and then using a combination of DR, Radar, or RDF is likely to illustrate the true nature of your piloting skills.
A feature added to RDFs late in their useful careers was a little motor and sensors that turned the antenna ring and found the null for you. These were called ADFs, or Automatic Direction Finders. Both RDFs and ADFs could be purchased as either portable units run on batteries as I have described, or as built-in units with a remote antenna on the cabin top or mast. Many of the early remote-ADF antennas are unmistakable since they look like huge eggbeaters. In fact, ADFs are still used in aircraft and are available in built-in format from Simrad and Koden/Si-Tex for boats.

Like all technology, RDFs came to an inglorious end. They all but disappeared from the sailing scene in a matter of a few years during the mid-1970s, replaced by a huge, expensive, and complicated piece of gear called LORAN. Texas Instruments launched the first competitively-priced LORAN units at $999, which was a ton of money back then, and the price wars for LORAN were on. If LORAN didn't deal the death blow, first SatNav and then GPS certainly drove a spike through RDFs' s heart. RDF improvements ceased, and hundreds of units have found their way from basements and bilges to the nautical curio shops ever since. Many of them were robustly built and still work just fine, thank you very much.

So, you ask, why does Sojourner still carry a big dinosaur of obsolete technology and questionable accuracy around? The simple answer is safety, it has got us home several times when all else failed.


All it takes is one lightning strike to transform your expensive navigational instruments into charred electronic detritus.
All of today's gadgets seem delicate. In the past 15 years we have suffered through three complete navigational systems failures, one with our SatNav and two GPS units. In one additional case, we were in the approximate center of the eastern Caribbean Sea when the US shut the GPS system down during the Gulf War, leaving us to find our own way. And we learned a trick using RDF then that has served us well since. With powerful marine beacons on the coast of Venezuela on our port quarter and Puerto Rico ahead of us, we found we could run the "radio rhumb line" to Puerto Rico, using the Venezuelan station to calculate our progress to the west fairly accurately. This is a kind of adaptation of the ancient "running down the latitude," but instead of using Polaris, we were using a radio signal as a homing device. We also arrived safely in Nassau, Bahamas, once using a reverse bearing on a station in Miami.

In addition to the issue of fragility, most of today's nice gadgets run on electricity and are wired into the ship's electrical system. I can't tell you what happens to these ship's systems, including the magnetic properties of the compass, when the boat is struck by lightning. And if you sail in the tropics as long as we have, your odds of being blasted out of the blue are pretty good. When that happens, you take all the D batteries out of the flashlights, put them in the RDF, bring it out on deck, and find a few good stations. If you're lucky, and you stay up past midnight when the AM stations really shine, you just might pick up WLS in Chicago and the Wolfman spinning vinyl, way cool.