Running way, way, way behind in responses to postings. Getting hopefully well-thought-out responses the same day or even the same week or month is apparently not always going to happen.
Short Version: Thank you Rudy. And there is more work to be done. Long Version: Please do not consider this to be dismissive of Rudy's work in any sense. That's not at all what I intend. His (and our) proposition that an inert BOG just laying there can be grown into the ground over time, and thereby harshly deteriorate performance, is absolutely confirmed in our collection of anecdota. For long term performance, it is necessary to fix (make permanent) a BOG's *electrical* relationship to ground by some mechanical design or process. Lacking that, regular effective maintenance/adjustment must be kept up in all but very arid environments. The deterioration in BOG performance is without sudden drops like someone cut a wire. So it's very sneaky, and in many cases sneaks toward extremes to the point of losing 10-15 dB and even reversing pattern. The QST article is rather severely edited for space. The full version will apparently be in July QEX. For those who do not subscribe to QEX, or don't want to wait, the full version can be found directly at http://www.arrl.org/files/file/QST%20Binaries/June2016/QST-in-Depth-0616-Serverns.pdf NOTE: My comments refer to the FULL Severns article and set of graphs found at the arrl.org URL above, and presume the reader is looking at the full article, not QST. I will remain differing with Rudy on whether NEC is accurate. I personally think that has to do with how we separately define "accurate". I would say that NEC with regard to ground and MF antennas invoking ground is *approximate*, and only to a point carries a correct graphical "shape" of expected results in presentations. That does not mean I am at odds with Rudy's measurements. I will trust his measurements long after I give up on NEC. Just looking at his graphs, I see those as evidence that NEC is *not* accurate in the sense that *I* use the word "accurate". Brown, Lewis and Epstein, and Rudy, make current measurements along the line that show some degree of wavelength related wiggle. NEC4.x shows no wiggle. If NEC were calculating with all the variables at play, NEC would show the wiggle even if located differently or with different amplitude or distance between nodes. That makes the NEC curve only approximate, however useful that approximation may or may not be. Without knowing WHY the wiggle is absent in NEC curves, it's hard to make a case to the authors to fix that. Some of the divergences between NEC 4.2 and his measurements portrayed in Rudy's graphs would render an antenna diminished if NEC was followed blindly. I have learned the hard way not to follow NEC-based programs blindly and have the scars to prove it. I believe Rudy's measurements. To my eye NEC cannot reproduce Rudy's measurements. Further, Rudy is measuring at a single location, which is NOT a criticism. I give all due respect for the time and effort testing there. A project like this can be full of mechanical-blowing-up-the-electrical-results issues needing to be avoided. Realizing an issue at some point well into the procedure can force one to redo everything all the way back from the start. Having the experiment in one's own back yard, with zero travel time to the experiment, under one's own self-permission is a huge advantage to reforming/restarting/finally completing the experiment. Or sadly, as seen in other cases, running into "issues" having exhausted resources and time, one has to give up on the project. To further carry this exercise to the level needed to publish is even more work. But it IS a single location, and we have to remember that. We must excuse location centric for cause because picking sites and repeating the testing in a dozen places all around the USA would create a huge, time-consuming and expensive undertaking. Even then the case can be made that not everything happens in the USA. Then how expensive does it get? Our testing (I call them the Rowdy Raleigh Radio Researchers out of earshot) only in the 12 county area around Raleigh/Durham North Carolina showed huge variations in the primary electrical length of a 151' (46m) dipole laid on the ground. The often referenced FCC ground conductance maps calls us all 2 milliSiemens. However... The velocity factor of that Dipole On Ground (DOG) varied from 0.45 to 0.8 across all sites and placements. That's +/- 22% (twenty-two percent not two point two). Consider what would happen to a Yagi if the manufactured element dimensions could only be guaranteed to lengths +/- 22 %. An intended 15 meter yagi +/- 22% could actually be on 17 or 12 meters. Or if all yagi elements did not have their errors vary in unison, could render the antenna completely dysfunctional. Enter into the world of "wonderful" to "d*mned waste of time" customer performance reviews. Sound familiar? This measured variation in eastern North Carolina VF was not a gradually changing figure with area changes in geography. Reorienting the compass bearing of the DOG around its center in the same back yard, or placing the DOG at another part of the same back yard, or just linearly sliding the DOG up it's line for 50 feet could generate large variation in VF. This even without buried pipes, wires, or septic fields in the yard. What effect that may have had on Rudy's graphs if measured by his procedures in a dozen locales scattered around the US is anyone's guess. It certainly would have been varied. Varied quite enough to take a SINGLE instance "good" layout for a 160 BOG in a specified location (like Rudy's back yard) thrown down anywhere else and produce results varying from "works wonderful" to "doesn't work worth a d*mn". What do you do to take the BOG construction and have it respond to a normal "wild variation" in VF of specific chosen spots of ground to lay out a BOG? Is it to measure the ground characteristics? Rudy hints that NEC 4 is accurate if the ground characteristics are accurate. There is a long and difficult discussion that could be had to show that even the FCC does not believe this in their administrata for commercial LF/MF AM broadcast stations, and they have a we-will-not-get-on-your-case cobble to get around it. But Rudy does get approximate correlation in his back yard. And I trust his measurements. But even if we let that stand without challenge to whether it works everywhere, there is another problem. Rudy's methods are full-on lab and academic quality. And he has the equipment, software and expertise to do it. Certainly not a criticism in *any* sense, Rudy attacks the problem with 1) a sophisticated knowledge base from an enviable employment experience, and 2) a practiced experimenter's hand using 3) expensive equipment and 4) expensive software, and 5) with a gift or two for excellence in technical writing and publication skills. Then there is Joe Average Ham, hereafter called Joe A H. While Rudy's methods are full on lab and academic with adequate equipment for those methods, Joe A H lacks the means to use *those* methods to do a BOG on his own property. We need to arrive at something workable for Joe A H with stuff *commonly available* to Joe A H. Rudy's stuff and outstanding background makes him rare among the army of Joe A H. Rudy is using export controlled NEC 4.2. NEC 4.x is the only software from the NEC family that can deal with buried conductors. One must pay a fee for a license from a government agency to use Unix NEC on a Unix platform. If you are not a naturally Unix person, then the high end professional EZNEC Pro4 has a NEC 4.2 build that runs inside the EZNEC shell for Windows, That's yet more $$ for the EZNEC Pro4 license, which is enabled by a key that goes in a USB jack on your PC. You will not find any midnight copies of Roy Lewallen's (W7EL) high end pro stuff on a Russian web site. (Read around sometime about how hamdom screwed K6STI and shut him and his excellent programs down.) In addition to NEC 4.2, Rudy is using a VNA, and equipment sufficient for accurate ground conductivity measurements. All his stuff and programs together cost more than a high end state of the art HF transceiver. Many Joe A H cannot find that kind of money in their budget for anything other than necessities, if even that. And if they did have transceiver level money, they would spend it on the transceiver, not the test equipment and software. So who will be providing the instructions that allows Joe A H, with typical Joe A H equipment, to hit the nail on the head with a BOG, and maintain it? I *personally* have found these expensive investments to be very worthwhile, even if just for hobby and entertainment value since I find this stuff extremely fun and interesting. But that's just me, and some folks look at me just a bit askance... I have had it unkindly hammered home to me that merely modest means precludes the availability of Rudy-worthy equippage for practical Joe A H construction of a BOG. Or for that matter, that even having time to do it up to Rudy-grade standards just isn't going to happen. Again this is *not* a criticism or dismissal of Rudy's article. This is as well a hard-as-nails lesson regarding FCP kinds of things for Joe A H. We need to write for Joe A H, design for Joe A H, and learn how to do it with tools that can realistically belong to Joe A H. To maintain a BOG that is working, Rudy's conclusions from his experiments in the article and a large pile of anecdotal trial and error known to me, some posted here, show that one cannot allow the wire to change its effective height with respect to ground by allowing natural processes like accumulating rotting leaves, etc, to gradually bury the wire, or bury it deeper. Getting the BOG working well in the first place is a separate story. For MF ground-low-velocity-factor antennas, NEC requires a single monolithic uniform ground medium. Real underfoot ground is most often anything but uniform. NEC using "high accuracy ground" frequently underestimates ground loss and can miss VF by a mile. The reasons for this are not yet clear. And we got no Daddy Warbucks interested in the problem to pay for the likes of the RCA funded Brown Lewis & Epstein study by people PAID to keep at it and do it right with equipment and support provided by their employer. Thank you, Rudy. And there is more work to be done. 73, Guy K2AV. _________________ Topband Reflector Archives - http://www.contesting.com/_topband
