From the July/August 2003 issue of Ocean Navigator
©2003 A.L. Smith
A Groundless Case For Loop Antennas
A single sideband radio (SSB) is one of the most welcome pieces of gear you can add to your blue water cruising boat. Once threatened with extinction by cheap satellite communications – apparently still a long way off – the SSB radio has enjoyed a resurgence in popularity. It has also been helped by the introduction of commercial and private shore stations linked to the Internet to allow the sending and receiving of email at sea. The bad news is installing a SSB radio is often no easy task and the antenna ground system is usually the culprit.
The typical marine SSB installation is shown in figure 1. It consists of a radio transceiver connected to an automatic antenna tuner through coaxial cable. One terminal of the tuner connects to the backstay which is insulated from the mast and the chainplate. The other terminal connects to a ground system that consists of a series of copper straps or screens bonded together and secured as low down in the vessel as possible. Electrically coupled to the surrounding salt water through the hull we have a very efficient antenna that can communicate worldwide. However, this is all dependent on whether or not you can construct a good ground system in your hull. Steel and aluminum boat owners have it easy. If your boat is fiberglass or wood you are not so lucky. Unless the boatbuilder installed a ground system before the interior went in, you will have to retrofit some compromise solution and hope it works good enough.
Those who don’t want to rip apart their floorboards and cabinets will be glad to hear that there are other types of antennas that do not need a ground plane. Not all yachts will be able to utilize these types but if so, you can avoid the complications of trying to install an adequate ground plane in an already built boat.
A good candidate for a no-ground marine antenna is the loop. As the name suggests, the loop antenna is a loop of wire. It can be almost any shape, the most efficient being one with the greatest internal area. This would mean a perfect circle. The circle shape is never used because the support structure required would be impractical. A realistic configuration for small boats is the triangle shape. Figure 2 shows an antenna known as a “delta” loop because of its resemblance to the Greek letter of the same name. A loop antenna can be fed anywhere on its perimeter. This could be at a corner or in the middle of a leg. Just break the wire where it’s most convenient and insert the antenna tuner. The tuner output goes to one end and the output which would normally go to ground is connected to other the end.
Due to the narrow shape of a boat, there is a trade-off between ideal shape and getting the most wire aloft that must be accepted. If you try for an equilateral triangle the shape is good but the overall size would be quite small. A loop that is too narrow might make it impossible for the tuner to match. Consequently, the wider you can make the base, the better it will perform. Many new cruising boats are being designed with wide sterns which helps in this area. Before undertaking any rigging modifications you should construct a temporary version out of 14 gauge wire to make sure your tuner can handle it. Make it the same size and shape as the permanent one will be.
Using Twin Backstays
Figure 3 shows a delta loop antenna incorporated into a twin backstay sloop rig. Antenna insulators are installed on both stays just as you would with a single backstay antenna. Make sure they are just close enough to the masthead so you can get to them from the bosun’s chair. Join the two stays together at the top with a heavy gauge wire. Strip one or two inches off each end of this wire. Bend the stripped portions along the stays and secure them with cable clamps or a stainless steel hose clamps as shown. Take care not to distort the rigging wire. Remember it also holds your mast up. Spray the connections with a protective coating of paint to inhibit corrosion. Don’t forget to inspect these connections regularly.
It’s best to keep the bottom section out of easy reach. If you’ve planned it right, the base wire will run right through the spot where your tuner is mounted. A radar mast or arch would come in handy here. Run a connection from the tuner terminals to each leg to complete the loop. Use 12 or 14 gauge marine grade stranded wire for this connection. It is big enough to handle the antenna current and still allows the use of a ring terminal connector that will fit on the tuner. Use some kind of strain relief if these wires will put too much stress on your tuner’s terminals. In all likelihood they will be too short to cause a problem. Do your best to keep the entire loop flat; that is, all in the same plane. Some bending of the loop plane where the tuner attaches is probably inevitable; plan for the best configuration possible. Since the tuner must be located at the antenna, it will be outside in the weather. You should construct a non-conducting cover to protect it from rain, spray and direct sunlight which can cause high temperatures inside the chassis.
What do you do if you can’t mount the tuner in line with the bottom corners? Place the tuner down low and run the base leg connections to the bottom corners to complete the loop. The result is now more of a diamond shape but if anything this will help due to the increase in loop area that results. However, part of the antenna will run close to the stern railings and lifelines. The proximity of any metal will reduce the efficiency of the antenna and alter the radiation pattern somewhat but very few antenna locations are ideal. In the end these two factors will probably cancel each other out. If you have to put the tuner down low you might consider mounting it in the lazarette to give it protection from the elements. You will have to use some kind of watertight insulated feed through to bring the connections out to the antenna. A problem here is these wires will develop high RF voltages when the transmitter is operating. You can slip a small diameter PVC pipe or hose over the accessible parts of each wire to help protect the crew.
Ketch And Yawl Rigs
A boat that is yawl or ketch rigged usually has twin backstays that lead to chain plates aft of the mizzen mast. These shrouds can have lower insulators installed at a point that will allow the base leg to pass just in front of the mizzen. The tuner can be mounted on the front of the mizzen mast like a radar antenna. This gives a fairly wide base with good loop area and all active parts out of reach.
If you can’t use twin backstays you may still be able to install a semi-permanent or temporary wire loop if the boat has a mounting location for the tuner that will keep the antenna away from the backstay. A radar arch or dinghy davits may work. This might be just the thing for crews that do delivery trips or a boat that makes only an occasional offshore passage. A wire loop hoisted on a spare or makeshift halyard could be put up temporarily and lowered when not in use. The proximity of the backstay is a concern. That’s a lot of metal sitting quite close to your antenna and its presence is not exactly good radio practice. You can’t avoid being close at the masthead but you should be able to get a few feet of separation at the lower end. Even so, you should still be able to get tolerable performance. Fortunately it doesn’t cost much to experiment.
To construct a temporary delta loop antenna use only stranded wire of at least 14 gauge, preferably marine grade. Position the apex about five feet from the masthead and run the two long legs to a raised structure over the stern to keep it as far from the backstay as possible. Make the base as wide as you can. If you are really ambitious, you could incorporate outriggers to make the base even wider and give the loop more area. Use a non-conductive material such as bamboo or wood with Dacron line for shrouds to prevent flexing.
The corners will need to be anchored with some sort of tensioning device to keep everything tight. It should be non-conducting. A trucker’s hitch tied with Dacron line is the simplest or if you want to go first class use a small block and tackle made from dinghy hardware. The antenna in the photo uses a small plastic block at the top to allow even tensioning of the legs by pulling on one side.
To maximize antenna performance follow good electrical practices with the rest of your radio system. Mount the radio as close to the batteries as possible. Use large diameter wire, only as long as necessary, for the DC connections to the radio. This will minimize the voltage drop along its length. Solder all connectors to ensure a reliable connection. It only takes one bad crimp connection to disable your whole system. Consider replacing your wire topping lift with Dacron to minimize the amount of metal near the antenna.
The delta loop as described here cannot out-perform a properly installed backstay/ground plane antenna mainly due to the difference in electrical size and the fact that it is narrower than the ideal loop shape. But this is in theory only. Radio engineers characterize antenna performance under conditions they call “in free space”, a situation that does not occur on Earth or any other planet for that matter. In practice many installations suffer from poor grounding, corroded connections, weak batteries and various other ailments. For this reason you may find the delta loop works as good or better than the backstay antenna on another boat. Every situation will be unique.
If you can, by all means install a good ground system in your hull and use the backstay for your antenna. If you are having a boat built, ask the builder if he can incorporate a ground system in the hull itself. If he’s not sure what to do, get an experienced radio technician to give recommendations. This would give you the best, least intrusive antenna you can have on a sailboat. But if you are planning an installation on a boat you already own or buying used, the delta loop may be an acceptable compromise.