This is an answer to an off reflector conversation, relating to a "too long" electrical length over radials reducing performance. I am writing to the list since the subject and it's objection occur in so many posted conversations. Reduction of gain by too high current max has been touted by some and called myth by others.
The reduction applies in fairly NARROW circumstances, which I think explains the APPEARANCE of myth. Used in certain other circumstances, raising of the current center on the vertical IMPROVES the results. The confusion comes from trying to lump divergent circumstances together, to run the equation with too few variables. If the vertical run plus the T (or an L) exceeds an electrical quarter wavelength, the current maximum will begin to rise away from the base of the antenna. Some designs deliberately put the current max well up the vertical. What is it that goes on? Let us begin with a WAY overkill, "gold standard" 1/4 wave vertical with 120 uniformly spaced 125' elevated radials at eight feet. This will work very, very well, little disagreement on that. There is deserved argument about where LOSS of performance kicks in as the number of radials is reduced and/or made non-uniform in length or spacing. And there is deserved argument about the DEGREE of loss in such circumstances. The NECx series of modeling programs underestimate that loss, how much depending on ground factors (what kind of dirt) that are difficult to measure. In many cases practically speaking these factors are unmeasurable unless one wants to dig up the back yard to measure them. The above gold-standard vertical will present a very predictable measured gain. It does present the very best avoidance of induced current-in-dirt losses, assuming the "dirt" is not a salt water marsh. So therefore one can expect that only REDUCTION in efficiency is possible from here. This IS assuming we are talking about overall efficiency, NOT measuring gain at a SINGLE takeoff angle (pattern changes). Part of the excellent performance of that antenna has to do with the radials' cancellation of the RF fields headed toward the ground immediately underneath. This is so because along the radials, the shape of the current is a very even exact opposite of the current in the vertical wire. The current maximum on BOTH the radials and the vertical wire are equal at the feedpoint and reduce in equal proportion moving away from the feedpoint. This means that the sum of the RF fields from vertical and radials are hugely MINIMIZED below the radials, EVEN THOUGH in this configuration, the current maximum in the vertical is as low, as close to the dirt, as it can be. The cancellation effectively makes the lossy dirt under the radials close to invisible, and forces the energy, that otherwise WOULD have been dissipated as heat in the dirt underneath, to be spent as useful radiation at other, more useful angles of radiation. It also has the maximum circle of cancellation. An exercise for illustration: elements are a dark room with a dark floor, an inch thick book, a circular white dinner plate, and a small non-focusing flashlight bulb. Put the book flat on the floor. Put the plate on top of the book. Turn on the bulb and hold it just touching the center of the plate and note the circle of darkness. Then raise the bulb and note the circle of darkness shrinking. Vertical radiation aimed at a non-sea-water ground surface is effectively lost. The vertical, although this is a very imprecise analogy, has a similar issue with raising the current center as the cancellation under the radials is gradually lost as the current center is raised. There is no complete vertical-radial field cancellation shadow as with the plate and flashlight, but gradual deepening of shadow toward the center. The more shadow, the less loss. Because of the inverse cube behavior of magnetic fields, the degree of cancellation is reduced much more quickly, counter-intuitively, as the current center is raised. AND THEN the RADIALS are now inducing fields down at the dirt which are no longer balanced by the REDUCED opposite phase field from the vertical. The performance *IS* reduced by the increasingly UNCANCELLED current in the RADIALS inducing lossy current in the ground. Now let us move away from our ideal to the much more common back yard that has zero chance of supporting a DENSE and efficient radial system. If a dense radial field cannot be done, then TO START WITH there was NOT the degree of possible cancellation under the radials to UNDO, and at some degree of sparse, or with no radials, RAISING the current center NOW IMPROVES performance. This is because near magnetic fields return their energy as the field collapses if they are not dissipated in close resistive conductors within the field (e.g. dirt), allowing the energy to be dissipated elsewhere. Therefore, in sparse radial or no-radial situations like the 5/16 wavelength single wire folded counterpoise, raising the current center and reducing current at the base improves the performance significantly. This appears to be one reason that an up 66', out 66' inverted L antenna does so very well on 80 meters if one goes through the PITA to handle the very high Z feed point. First, even the worst of grounds in series with the 2-4k ohm feed is only a small PERCENTAGE loss, making a low Z counterpoise unnecessary. Second, as inferred from above, there is proportionally very little E-field at the ground to heat up the dirt because the current is mainly up at the bend, and inverse cube factors have knocked it way down by the time it reaches the ground. For those with EZNEC, you can watch the fields at the ground by running near field plots (NF Tab) with z variable (height, not impedance) = 0. For vertical antennas, higher fields at the ground have higher loss. A ten times increase of the integrated magnetic field times unit area at the ground, results in a 20 dB increase in loss. Be prepared to be surprised by the ugly if you run these calculations to see what happens. I have not seen studies with dense radials where the radial length was shortened to reoptimize the field cancellation as the current center was moved up. Mostly I see pages and emails dedicated only to making the match to 50 ohm coax as easy as possible. This is a degree of ignoring efficiency that would never be tolerated with an amplifier, where every last watt is bemoaned and begroaned. :>) 73, Guy. > > _______________________________________________ UR RST IS ... ... ..9 QSB QSB - hw? BK
