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"Mark Keith" wrote I beat Reg's vertload program to death finding the best overall coil height for my mobile antenna. Basically I ended up putting it as high as I could. Which ended up a center load at 5 ft up, with a 10 ft whip. =============================== There are 3 losses - coil loss resistance, ground loss resistance and radiation resistance. To calculate and maximise efficiency all 3 values must be transformed to a common point - the base feedpoint. The length of radiator between coil and base behaves is a transmission line transformer which transforms the coil loss resistance (XL/Q) to another value at the base. Would-be modellers should take this into account. The Cosine current distribution along the radiator is a direct consequence of its behaviour as a lossy line. (Actually, it is not an exact cosine shape because of end-effect) Efficiency = Rrad / ( Rrad+Rcoil+Rground ). Rground is constant. For short antennas Rrad is the smallest of the 3 resistances. As the coil is moved further up the antenna both Rrad and Rcoil increase. But even if the coil is located at the extreme top of the antenna, radiation resistance cannot increase to more than 4 times the radiation resistance when the coil is located at the base feedpoint. Usually it is considerably less than 4. So the rapidly increasing coil loss resistance very soon overtakes the increase in radiation resistance. Even if coil Q remains constant, coil loss resistance increases just by virtue of its necessary increase in inductive reactance. To maintain resonance coil inductance increases inversely proportional to the length of the whip above it. So when the coil is located 95% of the way to the top of the antenna its loss resistance is TWENTY times greater than that of a base loading coil even when Q is unchanged. In practice, a coil having 20 times the inductance but with the same overall dimensions will very likely have a lower Q and an even higher resistance. Its easy to see the fixed value of Rground in the above efficiency formula has the following effects - When coil loss is less than ground loss, higher radiating efficiency is achieved by placing the coil nearer to the top of the antenna. And vice-versa. When ground loss is very small (zero if antenna is a pair of two back-to-back radiators to form a dipole) efficiency is relatively high anyway, maximumum efficiency perhaps occurring with the coil located in the lower half of the antenna. The slight improvement relative to base loading (as part of a tuner) may not then be worth the mechanical inconvience of fitting a coil in the antenna anyway. An important factor, not considered quantitatively by anybody, is that a mobile antenna is not just a loaded vertical - the vehicle body, just by looking at it, obviously forms the major portion of the antenna and is floating above ground. The vehicle body plus loaded whip should be considered to be an off-centre-fed, short, 1/2-wave resonant vertical dipole and modelled as such. --------------------------------------------------------- Regarding antenna modelling - program LOADCOIL considers all 3 parts parts of the antenna, the mast, loading coil and whip, as consecutive lengths of transmission line each with its own Zo and loss resistance. It is obliged to do this because it covers actual antenna heights approaching 1/4-wavelength as may be erected in your backyard. And it continues to do this for very short antennas with very short loading coils even where there would be negligible error by assuming the current going into one end of the coil is the as what comes out of the other. There's a companion program TOPHAT2. There's no coil in it. ---- ======================= Regards from Reg, G4FGQ For Free Radio Design Software go to http://www.g4fgq.com ======================= |
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