That information in the site Paul referenced, as it relates to radiators, is also available in just about any antenna design handbook printed since the 1930's, so I'd trust it.
The author does mention that the angle of radiation is strongly affected by the length of the antenna. That's also well documented. In a vertical, the main lobe rises quickly above the horizon as the antenna passes 5/8 wavelengths tall. A little rise in angle is unnoticeable in the "real world" since everything radiated by a vertical below about 15 or 20 degrees elevation is lost to ground absorption (unless one is sitting in the middle of the ocean). So 43 feet tall is not optimum for 10 meters but is ideal on 20 meters. There's actually an advantage in lower angle radiation to extending a vertical beyond 1/4 wavelength up to 5/8 wavelength. As the length approaches 5/8 wavelength, the familiar "donut pattern" is squashed with more power radiated at the lower angles while less is radiated at higher angles. That is, the "donut" pattern starts to resemble a fat disk. However, when one exceeds 5/8 wavelength the pattern quickly breaks up with much of the energy in higher-angle lobes. As the author notes, ground losses are the major issue. One resistor is the radiation resistance of the antenna and the other resistor is the ground resistance. Applying Ohm's law to series resistances shows that the most power is dissipated in the larger valued resistor. So you want the largest "resistance" in the system to be the radiation resistance. Ideally, you'd use a 1/2 wavelength radiator since that presents the largest radiation resistance but, obviously, such a radiator arranged vertically would have very poor low-angle radiation. When considering ground losses, think of two resistors in series with current (RF) flowing through them. A 1/4 wave vertical has a radiation resistance of about 35 ohms. That resistance drops very quickly as the antenna becomes shorter. A 1/8 wavelength radiator, for example, has a radiation resistance is about 7 ohms and 1/16 wave is under 2 ohms. It's very easy for a compromise ground such as used by many Hams to show a resistance of 200 or 300 ohms. That's why many *short* vertical installations have an efficiency of only a few percent, throwing almost all the RF energy away heating the earth. On the other hand, a 1/2 wave radiator with its very high radiation resistance, perhaps in the 2000 to 3000 ohm range, is a very efficient radiator, even with a poor or virtually no ground. But, as noted, a 1/2 wave end-fed vertical has an very undesirable radiation pattern. (Of course if it's center fed and oriented vertically, the familiar "donut" pattern is preserved, but not if it's end fed against a ground.) The author seems to be focused on how to feed a vertical efficiently across a range of frequencies. That is a big challenge if one doesn't want to put an active matching network at the feed point of the antenna. (Such active matching networks are commonly used in multi-band vertical installations commercially, such as on ships.) As the author notes, there are added losses since all matching networks have some losses. The more elements - inductors especially - the more losses. He states his figures do not include those added losses. The more common approach is to use a trap vertical. The traps divide the radiator into 1/4 wavelength segments on the various bands so it always "looks" like a common 1/4 wave vertical to the feed line. That's quite easy to match for a consistently low SWR on those bands for which the traps are designed. The disadvantages of a trap vertical (or any trap antenna) is that the traps add ohmic losses to the system and, most important, the antenna always shows a low impedance at the feed point. That low impedance requires a decent ground system to avoid most of the RF "heating the earth'. Good "engineering" has always been, IMO, not a matter of making things perfect, but a matter of managing the compromises for the best results given the limitations imposed. Nowhere is that more obvious than in an antenna system. Ron AC7AC ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
