Ok, let's not make this black and white, then. :-) It goes without saying that both halves of the antenna (radials and the vertical) must be present in order for the bottom-fed vertical monopole to radiate. But I maintain that any far-field radiation from the radials is way down from the radiation from the vertical. It only exists at all because in the real world, nothing is perfect and there will always be losses, imbalance, and imperfect cancellation of fields.
Maybe this is not a good comparison, but there is also a strong field in the immediate vicinity of a transmission line made from two parallel wires. But as long as it is straight and balanced, the field rapidly decays as we move away from it. Below is the question I posed to the other forum (with very minor edits): --------------------------------------------------------- On another forum (somewhere), someone that I very highly respect for his technical expertise once seemed to imply that an elevated counterpoise under a vertical monopole radiates energy into space, the same as the vertical element does. But is that really the case? In other words, BOTH a bottom-fed quarter-wave vertical antenna AND an elevated counterpoise (consisting of ... symmetrical 1/4 wavelength radials all connected together at the feedpoint under the monopole, ~6' high) contribute to the far-field radiation. I have always believed otherwise; that while there is certainly a field around a symmetrical elevated counterpoise, that field actually very rapidly decreases as we move farther away from it, as is the case with the field surrounding a transmission line that consists of two straight close-spaced parallel conductors. If such an elevated counterpoise DOES actually radiate, then what about the radials buried just below the surface of the earth under a vertical monopole with its feedpoint just the above the earth --such as is installed at many AM broadcast stations throughout the USA? Do those radials buried in (or lying on the surface of) highly lossy soil radiate? Based on everything I've ever studied, I simply cannot believe that is the case. Now suppose that we were to raise that same buried radial system up and out of the earth. At what point above the earth would they then begin radiating a significant portion of energy that would contribute to the far field? I say that such a counterpoise capacitively couples to the earth (the same as non-resonant radials lying on the ground certainly do). If we walk under an elevated counterpoise, then we become part of the dielectric of a capacitor: one plate of which is the counterpoise, and the other plate of which is the planet itself. (We could take this one step further and make a counterpoise of an infinite number of radials (a disk). Would such a disk radiate?) Having said all this, I doubt that the radials on a VHF ground plane antenna, many wavelengths above the earth, have much coupling to the earth itself. Maybe a counterpoise radiates, after all. :-) I don't have a problem with understanding that both legs of a 90 degree center-fed dipole (such as with one element vertical and one element horizontal) radiate. But everything I've ever studied seemed to indicate that in the case of a symmetrical counterpoise (balanced, equally spaced wires and with the monopole directly under their apex) parallel with the earth and with a sufficient number of radial, >99% of the far field comes from the vertical monopole. What is actually the case here? --------------------------------------------------------- I don't pretend to understand this. I'm not sure that anyone does. And I don't know what practical use knowing this would be. But it's something that fascinates me. 73, Mike www.w0btu.com On Fri, Aug 24, 2012 at 11:15 AM, Tom W8JI <[email protected]> wrote: > > The question is really all about proportions. ... Two radials can pretty > much cancel in the farfield, but not perfectly. ... Near earth, there is > an "image" that helps cancel radiation, but it isn't perfect. ... We have > to watch making things black and white. > _______________________________________________ UR RST IS ... ... ..9 QSB QSB - hw? BK
