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The Secret Life of Dipoles

Tom Roderick , WA4GIM

How do you make a wire dipole antenna for HF? Well, everybody knows you have to have it resonant at the frequency you want which means the right length, so you dig out the books and find that formula you memorized for the test, but then forgot. Uh, wasn’t it 468/F with F in Mhz to give the full length in feet. Then you cut that in half to form the two sides of the dipole connect your coax in the middle, and a 1 to 1 balun wouldn’t hurt and might help keep some RF out of your shack. You get it up as high as you can and either hook up your antenna analyzer or, lacking that, test it out and check the SWR and hope that the antenna is a bit long for the frequency you picked (most of the time it will be) and then bring it back down prune a bit off each end being careful to keep each side the same length. Hoist it back up and repeat until you get too tired to do more, the antenna breaks or you get it “close enough.” However, there is a better way.

A dipole antenna does NOT have to be resonant to work! The old maritime distress frequency of 500 Khz would require a dipole of over 900 feet according to this formula and back in the day most of the ships were not quite that long. A dipole DOES work better if it is at least a half wavelength on the lowest frequency you want to operate on but they can work lower. The reason resonant dipoles have become so common is that they are relatively easy to feed with coax and coax is the most common transmission line. Why? Because 50 ohm coax is a pretty close match to the impedance of a resonant center fed dipole, and maybe the SO-239 connectors on all the rigs has something to do with it. If the dipole is NOT resonant it will reflect more energy back down the coax and that causes STANDING WAVES to exist in the coax. In coax, a high Standing Wave Ratio is a bad thing since it means that most of your power is being used to heat the coax. However, there is a trick. For a wide or all band dipole don’t feed it with coax! Use open wire, window line or even old flat TV twin lead. All of these have lower loss than even the best coax and can operate with SWR well over 10 to 1. Since these lines have low loss they don’t convert much of the energy to heat and the power is NOT lost.

Most modern rigs, even with built in tuners, don’t like SWR over about 3:1 so a real external tuner is needed. A balanced tuner is best, but many of them use a balun on the output to APPEAR to be balanced but really are not. Most baluns can’t handle high SWR either. You can use a hot tuner, but that puts the entire tuner at high RF voltages – might be a tad uncomfortable at times, but it has been done. Another approach is to use a remote tuner but “float” it above ground at RF using RF chokes on anything that would ground it. The tuner is still at DC ground but can float at very high RF voltages without problems. This can give you one antenna that will operate on almost any frequency higher than its half wavelength. A final note is that there are some feed line lengths to be avoided from the tuner to the dipole. Anything around a quarter wave length on any frequency you operate on can cause an impedance inversion of the feed point making the impedance the tuner sees way too high for any tuner.

As for the non resonant dipole itself; make it as long as you can and at least a half wavelength on the lowest frequency you want to use it on, get it as high in the air as you can, use a wide range external tuner and don’t use coax for a transmission line to feed it.