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The theoretical efficiency of an electrically short antenna is proportional to the effective volume of the antenna. A number of short antenna designs have appeared during the past two decades that claim to exceed the theoretical limits of short antennas. The oldest of these is the Crossed Field Antenna (CFA), which was introduced to the public in an article in “Wireless World,” a British electronics magazine, in 1989. The authors claimed that they had eliminated the near field, which is the interface between the antenna and free space. With the CFA, the far field is generated right at the antenna, and so the antenna size is unimportant. It is possible to make the CFA as small as desired without any loss in efficiency. Most antenna experts have thought that this is nonsense. The question was settled recently by Prof. Valentin Trainotti, the world’s leading authority on short AM broadcast band antennas. In the September, 2006, issue of IEEE Transactions on Broadcasting, Trainotti gave a thorough theoretical analysis of the CFA. The CFA is no more efficient than other small antennas the same size. It, of course, has a near field, like other antennas. This finding has no effect on the promoters of the CFA, because they believe that they have discovered a new physical principle which is not explained by conventional antenna theory.
The EH antenna was developed by an author of several articles about the CFA. He simplified the structure of the CFA considerably to make the EH, but claims that the EH works on the same principles as the CFA. The EH antenna also has no near field, and it can also be made arbitrarily small without any loss in efficiency.
The promoters of the Isotron antenna claim that their antenna works according to conventional antenna theory, thus avoiding some of the criticism the EH and the CFA have received. They say that the good performance of their antenna is because of its large cross-sectional area. It’s true that a fatter short antenna works better than a thin one, but the cross-sectional area does not explain the performance of the Isotron.
Another short antenna that is less well-known than the others is the CWTH (Contrawound Toroidal Helical) antenna. The “inventor” is a registered patent attorney with a PhD in EE. The theoretical explanation of this antenna in the patent starts out pretty well, but falls apart when the inventor tries to explain why his antenna is more efficient than other antennas the same size. The patent is one of the most intricate I’ve ever seen. The inventor is lucky that he’s a patent attorney himself. Maybe he wouldn’t be able to afford to pay somebody else to draft such an elaborate patent.
All of these antenna designs actually perform better than ordinary short antennas the same size. This is not because of any unusual properties of the antennas themselves, but because they all involve the use of a long radiating conductor that is not counted toward the antenna length. Part 15 AM enthusiasts are very well aware of this trick. Well-known authors of articles about Part 15 AM antennas have called it “whip and mast” (“Crash” Knorr) and “elevated” antenna (Richard Fry).
CFAs have been found to work much better than other short antennas the same length. The promoters claim that a 6.5 meter CFA works better than a quartwe-wave broadcast tower. Actual measurements show that the CFA performance is typically 6 dB to 10 db poorer than a quarter wave tower. Even so, this is pretty good, and such a short antenna should not work that well. The CFA proponents do a couple of tricks to improve the performance of their antenna. One trick is to mount the antenna on a flat roof with a copper ground plane. The copper is connected to earth ground through several metal straps along the walls of the building on which the antenna is mounted. Most of the radiation comes from the straps. This trick is well-known in Part 15 AM. Some Part 15 AM transmitter manufacturers recommend that a long conductive ground path be used between the transmitter and earth ground (supposedly for “lightning protection”). The “whip and mast” antenna is a grounded vertical dipole, and the longer the conductive path to ground, the greater the antenna efficiency.
The CFA promoters also use another trick that I’ve never heard has been used in Part 15 AM. In at least one case, the CFA was mounted close to the original quarter wave antenna (of an AM broadcast station) that was disconnected from the transmitter for the purpose of performing the CFA test. The CFA coupled energy to the broadcast tower, which radiated the RF energy with higher efficiency than if the CFA were used alone. In another case, a CFA mounted at the surface of the earth was located near a microwave antenna tower with a height approximately a quarter wavelength of the carrier frequency of the signal to be transmitted. While, structurally, the use of a large resonator near the short antenna seems to be different from the “whip and mast,” it is really equivalent. The principle of the “whip and mast” is to use a long radiating conductor that is not counted as part of the antenna length.
The other antennas I mentioned work the same way as the CFA. They all have something that you are not supposed to pay any attention to (like the man behind the curtain). They all have a long radiating conductor that is much longer than the “antenna” itself. The CFA has attracted more attention than the others because it apparently has strong financial backing, allowing the promoters to perform CFA experiments all over the world.
