Considering "equivalent lengths", bending the sections of a doublet more than 90 degrees significantly reduces the effect of the additional length of wire. Folding the wire back on itself a few inches from the original radiator has the effect of lengthening the radiator only slightly. RF flows over the wires following the broad outline of the area occupied by the wire rather than following the wire around sharp bends, so the folded-back wire tends to look simply like a "fat" wire, rather than a wire making a circuit.
Such "linear loading" works, but the total length of the wire for a given effective length of antenna is much, much greater than would be necessary if the wires didn't fold back. Another approach to loading is to make the wire a huge coil with a very large length/diameter ratio (small diameter coil compared to the length of the radiator). Doug DeMaw (W1FB,sk) and others documented experiments with these in several ARRL publications some 30 or 40 years ago. I have done some tinkering with them too, and found that DeMaw's estimates were quite close: a continuously-loaded radiator required just about twice the total wire of a linear radiator. In my case, 260 feet of wire wound in a 3-inch diameter helix (coil) with a spacing between turns that yielded a 40 foot long radiator was self-resonant at 3.7 MHz. Of course, stretched out it would take only about 130 feet of wire to resonate at that frequency. Still, what's to argue with a 40-foot self-resonant 80 meter antenna? Many operators have used or at least heard of "slinky" antennas made from the child's helical spring toys stretched along a plastic insulating rope that made use of this property to form a shortened antenna. At least one company sold them commercially. Even so, there's one major consideration. The radiation resistance of such an antenna is very, very low. That is, the portion of the load resistance that actually converts RF to electromagnetic waves is miniscule compared to a full-sized antenna. A dipole, if fed at the center, will show about 50 ohms radiation resistance at typical heights above ground most Hams encounter (it's closer to 75 ohms in free space). A loaded dipole, no matter how it's done, may show a radiation resistance of only a couple of ohms at resonance. Actually, many short loaded antennas show only a small fraction of an ohm of radiation resistance at resonance. That's why such antennas are often a disappointment when used against ground or a simple counterpoise. The resistance of the ground is often hundreds of times greater than the radiation resistance of the radiator, so the efficiency of such an antenna may be much less than 1%: 5 watts in, < 50 mW radiated. Loaded antennas fare much better when center fed since there is no "ground" connection. However, the resistance of the wire becomes significant. Remember, the resistance of a conductor is much greater at RF than it is at DC or low-frequency AC due to the skin effect. If the radiation resistance is only a fraction of an ohm, the effective resistance of the wire may still consume more than half the power. Still, there's a huge advantage to be gained by using a balanced, center-fed loaded radiator arrangement over an end-fed loaded radiator arrangement. Linear loading (wires folded back and forth in a zig-zag) and continuously loading (wires in a helix for the length of the radiator) became more popular than simple loading coils because loading coils tended to have higher ohmic losses, being made of smaller diameter wire with correspondingly lower surface area. Remember, because of skin effect it's all about surface area: a paper-thin tube of copper an inch in diameter has the same low resistance to RF as a solid bar of copper an inch in diameter. So, in general, a larger diameter conductor bent around showed lower losses than many loading coils made out of small-diameter wires. And, under it all, years of experience by thousands of Hams tends to confirm the belief that a small-diameter radiator outside 'in the clear' beats just about anything that can be erected inside a building. That's why I was quick to endorse Ray's idea of a stealth wire outside. The quest for the perfect antenna has continued ever since Marconi figured out that if he hooked his spark gap to a metal plate suspended over his laboratory table the signal could be detected farther than ever before. That quest still goes on. Every antenna, no matter how big or small, high or low, cheap or expensive, is a huge bundle of compromises. The challenge to the Ham is figuring out which compromises yield the best results in any given situation, and we're doomed to doing it largely blind. SWR is no indication of how well an antenna will get out. It only indicates whether the feed system is working as designed. On-air checks are dubious, at best, in spite of great care and efforts. Antennas are reciprocal - they receive like they transmit in spite of what some claim - but it's as hard to evaluate received signals by ear as it is when doing on-air transmitting checks. When listening, changes in signal-to-noise ratio can mask actual changes in actual antenna losses (or gain), and gain is all we care about when transmitting. Changes in signal strength of 1 or 2 dB (a small fraction of one S-unit) are often very difficult to measure under typical band conditions. Sometimes it's impossible to evaluate the gain within a relatively huge range of 6 to 10 dB because of QSB. And in such cases we're evaluating a single path to a single distant station at a time. But, hey, if it wasn't so crazy and confusing, would it be half as much fun? Ron AC7AC _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
