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I read the paper by Brown, Lewis, and Epstein with considerable interest, back when I was kid in college! I believe it was the same Dr. George H. Brown who wrote fascinating (and witty) stories for the journal of Eta Kappa Nu, to which I belong. A short piece on Mr. Brown appears here: http://en.wikipedia.org/wiki/George_H._Brown_(Engineer)
Anyway, the main issue with the figures you referenced is that all of the interesting stuff is in the very lower left corner, insofar as a Part 15 AM station is concerned. A typical FCC-compliant antenna is about 5.8 degrees at the upper end of the AM band, and most Part 15 hobbyists (myself included) could only dream of putting in a radial system of more than a few, SHORT radials at this wavelength (definitely MUCH less than 240 feet)! It also seems likely that the data in that region of the graph may have been extrapolated, since those conditions were outside the main focus of the work at hand.
In order to gauge the expected differences in field strength due to ground conductivity, it would be instructive to see a family of curves, with smaller numbers of shorter radials, across a range of ground conductivities. This, of course, would be a formidable task if it were not for the availability of antenna modeling software (which unfortunately, I don’t have). In any event, one must be very careful to understand the modeling requirements and limitations, which implies a significant learning curve (GI=GO).
Questions of interest include, for example, whether or not it really matters to have extremely long radials if you have such a short radiating element, as well as the perennial question of how many are worth installing. Ray tracing would suggest that long radials may not necessarily be that important. And, you can somewhat see from those plots that the difference between say, 15 and 113 radials may not be all that significant with a 5 degree stick, but it is hard to tell with good accuracy. Another question that the data doesn’t address is what effect a fairly dense (but electrically small) ground screen right at the base of the antenna might have. For example, what if you installed a 3 meter radius dense screen (e.g. a metal sheet) right under the antenna and augmented it with a few longer radials? We don’t have any good answers on how much that would help. The paper didn’t address that situation.
Much has been written about how “bad” it is to bury radials in lossy earth, which suggests that using elevated ground systems would be a great way to go. But this invokes the problem of FCC interpretation, an area where fewer and fewer experimenters are interested in going these days (and with good reason). Now, it seems to me that all ground is not necessarily equal from a loss tangent standpoint. For instance, in Michigan my ground is 100% sand, which has very low conductivity, but which I suspect behaves very differently than the kind of lossy ground one encounters in excavated urban areas. I once tested a horizontal full wave 40M loop on 1′ high supports mounted over this sandy ground, and it tuned up very nicely and worked effectively for high angle polarization. I suspect that a buried radial system in sandy soil may not be as lossy as people might think, because the sand has properties that are more like an insulator. The same might be true of certain types of rocky soil, but I have no measurements to support this.
Back in the 80’s, there was a very informative article on ground systems for vertical antennas for the ham bands in QST, in which the author expanded on the work of the RCA engineers to focus on the frequencies and conditions applicable to a typical ham radio station. I wish I had this article handy, but I don’t. But even if I did, I’m not sure the conclusions would be exactly the same at 1.6 MHz as they are at 14 or 28 MHz. Based on this article, I built a system of 16 radials 0.1 wavelength long for my 40M quarter-wave vertical at my cottage in Michigan. As I recall, this particular configuration was supposed to be only about 4 dB less efficient than a “perfect” ground system. I don’t know if that’s true, but it did work well.
There must be somebody out there who has EZNEC or MiniNEC and who could do us a favor and model some of the most common situations faced by Part 15 station operators.
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