Making a Dipole Antenna

Portions of this article first appeared in Cruising World magazine

Aerial Tricks - Making a dipole antenna for your SSB radio

(with notes on SSB receivers and RF ground planes)

by James Baldwin

Most boats equipped with marine HF-SSB radios use an insulated backstay antenna. When coupled to an automatic tuner and balanced by an adequate RF ground plane, the backstay antenna works fairly well over a broad range of frequencies. However, its fixed length and imperfectly matched ground make it less efficient on any one particular frequency than a properly matched antenna such as a dipole. To evaluate the performance of a backstay antenna, you must compare it with an antenna of known performance on the same boat. A practical way to do this is by using a 1/2-wave dipole antenna that isn't connected to your boat's RF ground and antenna tuner.

I experimented over several years with various marine antennas, and found that the simple dipole antenna consistently outperforms insulated backstays, fiberglass whips, and other types of random-length wire antennas. Although a dipole antenna is slightly directional, meaning that your signal will be somewhat stronger in certain directions, in my experience this doesn't significantly interfere with its use.

Dipole_3

A 12 MHz dipole ready to hoist.

Think of a dipole as a length of wire equal in length to one-half the wavelength of a specific radio frequency. That wire is then cut exactly in the center and its inner ends connected to a 50-ohm coaxial cable - one wire connected to the coaxial core and the other wire connected to the coaxial's braided shield. Basically, that's all there is to it. When a dipole is properly balanced in this way to the frequency you're using, it does not need any HF ground or tuner whatsoever. It's 0-decibel performance, or gain, is a standard reference against which all antennas performance can be measured. The cost of making your own dipole is little more than the price of the wire and coax cable and a few connectors.

Dipole_2

A dipole installed on left side parallel to backstay.

If the dipole antenna is effective as well as simple and inexpensive to build, then why are they not used more often? Some folks understandably don't like the added clutter of extra wires hanging from the mast. Another reason is because it's peak area of performance is restricted to the one frequency band it's designed for. In other parts of the band it may not be as effective as the auto-tuned backstay, and is particularly poor at the lowest frequencies used for short range communications. The final limitation of the dipole is that its overall length, and therefore its frequency range, is restricted by the height of your mast. By looking at the frequency table below you can see that my 40-foot (11.9 m) backstay limited me to a dipole for frequencies of 12 MHz and higher only. This is adequate for testing against another antenna's performance and is useful as a primary high performance antenna for long-range contacts with boats or shore stations on 12 MHz and higher frequencies.

Make your dipole for the primary frequency band you will use within the limitations of your backstay length, leaving about a 2-foot (60cm) clearance at the masthead and at the deck. Determine the required length of 50-ohm RG-8X coaxial cable by measuring the distance from your radio aft to the backstay chainplate and up to the middle of the backstay. Add 3-foot (90cm) of coax to this length to allow the dipole to be hoisted parallel to, but a couple feet away from the backstay.

For the dipole's wires, use either light, flexible stainless steel wire or, for slightly better performance, an insulated copper wire of 12 gauge or heavier. Avoid cutting the wires too short by adding 4 inches (100 mm) to the overall length; you can gradually trim them during testing to find the perfect length. Both wires must be exactly the same length and securely connected to both elements of the coax with crimp connectors. Affix the wires and coax to an acrylic or lexan insulator block as pictured above. Protect the exposed coax wires with polyurethane sealant, and finish by attaching a PL-259 connector to the radio end of the coax.

Testing the Dipole

Tie the mainsail halyard or boom topping lift to the end of the dipole wire that is connected to the coax core and hoist the dipole temporarily into position. Tie the lower dipole leg - the one connected to the coax shield - to the stern rail with a light nylon line. Attach another halyard a couple feet back from the coax-to-wire connection block and hoist the coax so that it is at a 60-90 degree angle from the dipole's wire legs. Then plug the coax connector directly into the antenna jack of the radio. If your SSB has a built-in standing-wave-ratio (SWR) meter, you can more precisely determine the correct length for your dipole. If not, you can buy an SWR meter, such as the MFJ-815b, that connects between you radio and antenna.

First, switch the radio to AM and Low Power. Turn off any automatic or manual tuner. Select a clear unoccupied frequency close to your dipole's designed center frequency and transmit by keying the mike. If the SWR meter indicates less than 2 while transmitting, you're alright. If it shows much over 2, there's a mismatch and too much power is being reflected back to the radio. In that case you should cut about 1-inch off each end of the dipole legs and check it again. Keep cutting and rechecking until the SWR reading drops to near 1. If at any time while using the dipole you notice the SWR rising above 2, or if the radio's power output meter indicates below normal, there may be a bad connection somewhere, or the dipole legs aren't the correct length for the frequency you are on, or they may be laying against some part of the rigging.

Now compare the dipole to your backstay antenna by listening to a faintly heard station on any frequency near the dipole's center frequency. Quickly switch between the dipole and the backstay and note the strength of the received signal. If possible, try to confirm your results by getting a transmission report on both antennas from another distant station. Allow for unknown variables by making several checks before you reach any conclusions.

If the tests indicate that your backstay antenna works better than the dipole, it confirms your backstay is efficient. It's more likely, however, that your dipole works better, allowing you to reach stations you can't even hear on the backstay antenna.

In theory, a dipole only works well on the frequency band that matches its overall length. In practice, it usually works reasonably well on other bands. If you want to use the dipole on frequency bands other than the one it was cut for, you can connect the center pin of the dipole's coax connector to the tuner, leave the shield disconnected, and run it as you would a standard backstay antenna. If you do this, keep in mind that it may work less well than a standard insulated backstay antenna on those frequencies.

For better performance on lower frequencies, a longer inverted-V dipole can be hung from its center at the masthead, with one dipole leg tied at the bow and the other at the stern. This arrangement might interfere with your sails, so take it down before going to sea.

Installing the Dipole

If you decide to permanently install the dipole, here's what to do. Run the end of the coax through the afterdeck near the backstay chainplate and through the interior of the boat to the back of the radio. Since you will probably use the dipole as a second antenna, you can run both antennas to an antenna switch mounted near the radio. Attach a small block to the masthead and hoist the upper dipole leg with a light nylon halyard. (Adding an aft facing extension bar at the masthead helps provide extra clearance.)

Using a hose clamp or U-clamp, attach a second small block to the center of the backstay. Run another light line through this block and attach it to the coax cable about 2-foot (60cm) from the center connector. Now hoist both halyards, positioning the upper end of the dipole leg 2-3 feet (60-90cm) below the masthead, and keeping the coax tight against the backstay. Tie off the bottom leg of the dipole with a line to the stern rail or deck fitting that's well clear of the backstay.

Aboard Atom, I sometimes use two dipoles, one on each side of the backstay for 14 MHz and 28 MHz on the amateur bands. When needed, I swap one of these for a 12 MHz marine band dipole. For other amateur and marine bands I run the core wire through a manual tuner, as described above, and I get good results. At other times I run a random length wire antenna in place of one of the dipoles and tune it manually as if it were a standard insulated backstay antenna.

Overall Dipole Lengths for Various Frequencies

Frequency in MHz                                                Overall Dipole Length

Marine Bands                                                      (meters)      (feet)

6.230                                                                   22.91       (75'2")

8.297                                                                   17.20       (56'5")

12.365                                                                 11.54       (37'10")

16.534                                                                  8.63        (28'4")

18.825                                                                  7.58        (24'10")

22.165                                                                  6.44        (21'2")

Amateur Bands

7.100                                                                   20.10       (65'11")

14.250                                                                 10.01       (32'10")

21.400                                                                  6.67        (21'10")

28.400                                                                  5.02        (16'6")

Note: Add about 4 inches to overall length to allow for individual installation variations. -Wave Dipole Length Formula 142.7 divided by frequency in MHz = Overall dipole length in meters

Dipole_1

Diagram of dipole installation.

The RF Ground Plane

This brings me to a brief description of what's needed for an adequate RF ground for antennas other than dipoles. There are numerous differing opinions on how to set up a good RF ground on a sailboat. First, you need to know that an RF ground is not the same as the negative ground on your 12 volt system. A typical RF ground requires a large surface area to act like a counterpoise to your insulated backstay or random length wire antenna. This is usually accomplished by running 3-inch wide copper foil from the ground post on your tuner to the engine block, keel ballast, bronze seacock, rudder post, or externally mounted bronze dynaplate, and from there back to the chassis ground on the transceiver.

I find it best not to connect this copper foil to any points that are connected to the ships 12 volt ground system, such as a negative bus bar or a grounded seacock. Of course the engine block is grounded to the 12 volt system, but this is an exception, and usually works well as an RF grounding point. Usually is the key word here. There are no absolute rules on installing your ground, just general guidelines to use to experiment until you find what works best on your boat.

As an alternative to the copper foil rolls available at the marine stores, you may find heavier gauge rolls of copper roof flashing at roofing supply stores. The very thin marine store foils may corrode quickly if exposed to saltwater in the bilge areas. I try to run the foil below the waterline but above the wet areas of the bilge, glued to the hull with 5200 sealant, then primed and painted on top.

I suggest you first try connecting your copper foil to the ballast, possibly at an exposed keelbolt in the highest and driest area of the bilge, if available. Test your signal reception strength after tuning on various frequency bands. Then disconnect the foil from the ballast and connect it to the engine block for comparison. Be on the lookout for stray RF interference with autopilots and other electronics when transmitting. If this is a problem then rerouting the foil to a seacock, rudder post, or dynaplate may help.

A dynaplate is basically a bronze plate that bolts onto the outside of the hull below the waterline to act as a grounding bridge between the seawater and the copper foil. But an RF ground does not require direct physical contact to seawater to effectively use the seawater as a ground plane. The RF energy appears to easily pass through the skin of fiberglass from internal ballast to the sea or from lengths of copper foil laid against the hull. A dynaplate may also not outperfom an existing seacock possibly because although the dynaplate has a roughened porous surface that in theory gives it a large water contact area, it quickly becomes clogged inside with marine growth that brush cleaning can't remove. In other words, if your boat already has a dynaplate then give it a try. But tie it in with another run of foil to the ballast or engine and you should get better results. For more about the elusive perfect ground plane there's helpful advice on the Communications threads of the SSCA bulletin board.

I plan to soon downsize from my large and heavy Kenwood TS-450 SSB transceiver to one of the smaller units with remote control panel such as the Icom 706MKIIG. This is not a "marine" radio, but, legalities aside, can easily be modified to transmit on all bands by a radio tech, or even by yourself using instructions available on the Net at Mods.dk. If you're not clear on what you're doing, print out the mods.dk info and give it, along with your radio, to a sympathetic radio shop tech and explain that the modification is only being done for use of the rig during emergencies in international waters. It's a 10 minute job to snip the diode - just don't cut the wrong one! Alternatively, the Icom IC-M802 is easier for the novice to operate, is easily connected to Pactor email modems, and is one of the few marine radios set up to transmit on the amateur bands as well, but it is considerably more expensive.

SSB Receivers For those on a limited budget and those who prefer simplicity, I'd suggest you forego the SSB transceiver and get a small SSB receiver such as the Sony ICF-SW7600GR for $160 from the Sony website. (Update: In August 2007 I replaced my 15-year-old Sony receiver with this new model after the old one gave trouble with sticking keypads. Not bad lifespan for a frequently used radio in salt air environment. I bought mine on amazon.com where prices ranged from $122 to $147 plus $6 shipping.) The Grundig YachtBoy 400PE for $150 is another popular choice, though their performance on SSB mode has been reported as less good than the Sony.

If you want reception quality nearly equal to the big rigs you need to connect a good external antenna, not just the short spool of wire antenna supplied with the radios as clip-on external antennas. With the correct antenna these radios work well for receiving offshore weather forecasts, downloading weatherfax images to your PC and listening in on the various cruising nets. Buy or make up an adapter plug for the dipole's PL-259 coax connector to your radio's external antenna jack.

Sony_1

Adapter plug for PL-259 coax connector to Sony external antenna jack.

Sony_2

Coax adapter plug sealed in epoxy.

Power requirements for a receiver are minimal (less than 0.5 amp). I ran my Sony ICFSW7600G receiver (similar to the newer "GR" model described above) direct off one of my four 6-volt golf cart batteries that make up my main battery bank. You could also use one of those voltage reducers that typically stick in a cigarette lighter plug with adjustable outputs from 3-9 volts. You can find them at shops like Radio Shack or Wal-Mart. Radio Shack has the correct mini-plug fitting that connects between the radio's external power supply jack and the voltage reducer.

Important features to look for in a SSB receiver are: good sensitivity of the receiver, external antenna and DC power jacks, memory channels, fine tuning adjustment, small size and portability, and all-band reception including FM. I've tested both Sony and Grundig radios mentioned above and found individual units that work equally well, although I have heard there's a higher percentage of folks not satisfied with the signal reception of their Grundig. (Check here for more Grundig reviews.) This is possibly a quality control problem giving varying performance of units of the same model. It's a good reason not to buy one anywhere that will not give you at least a one month no hassle refund, if at all possible.

Also, you may not know if you have a good radio unless you compare it to another radio of known performance on the same boat. When you buy your radio, try to find someone else who has any model and compare them together before you decide to keep it. The great thing about credit card purchases is that if the store you bought it from tries to refuse a refund, you can return the defective merchandise before making payment. Then dispute the bill with your credit card company, who usually side with the customer in this situation.

For an SSB receiver antenna you can use any of the antennas used for a transceiver, such as an insulated backstay, vertical whip or a random length wire hung near the masthead and tied off at the corner of the afterdeck or stern rail and then run under the deck to the radio. Or you could make a dipole by following the instructions above, except that you will obviously not be making any fine tuning adjustments by transmitting and checking SWR readings.

If the above mentioned antennas are too elaborate for your needs, you can connect a piece of insulated #12 AWG copper wire a few feet shorter than your backstay to the center pin of a mini-plug in your external antenna jack, connect the side of the pin to ground, and hoist the antenna wherever it's convenient, keeping the wire at least an inch away from other wires or grounded metal fittings as it runs aft under the deck to the transom.

If you want to try something very simple that may be better than nothing, try connecting a wire from your external antenna jack to the backstay or shroud or even a chainplate bolt on the boat's interior with an alligator clip. However, with a little more effort, such as the right dipole antenna, I found I can receive any station on my little Sony that I can hear on my Kenwood TS-450 transceiver with tuner. As with a transceiver, a bad antenna will absolutely cripple a good receiver.

TransAtlantic Weather Routing

Veteran mariners in the North Atlantic and Caribbean know to contact Herb Hilgenberg (Southbound II) on marine SSB frequency 12.359 MHz at 20:00 UTC (4pm ET) for excellent weather routing advice. Herb is a volunteer weather forecaster based in Toronto who has been providing free weather routing service for Atlantic sailors since 1987. Log-ins start after 19:30 UTC. At 20:00 UTC Herb signs on and acknowledges vessels he has heard log-on and will ask them to stand by as he works boats by area. If you have trouble getting through, try emailing him at: This email address is being protected from spambots. You need JavaScript enabled to view it. . For more info visit Herb's website.

If you have an SSB receiver you can listen to offshore voice forecasts from the National Weather Service. Their new schedule beginning 15 June 2005 is on their website.

Emergency Offshore Messaging

The Amateur Radio Maritime Mobile Service Net operates daily from 16:00 to 02:00 UTC (12pm to 10 pm ET) on 14.300 MHz USB. Licensed MM amateur radio operators can check in here and speak to volunteer operators in the US who will provide phone patches, one-way messages, or send brief email messages. In emergencies anyone can check in. For further information, log on to the nets website: www.mmsn.org.