Hi Joel, The ground beneath the antenna can have two effects:
1. It can become a part of the antenna's equivalent electrical circuit. 2. It has an effect on the antenna's far field due to "reflection" of the antenna's field By burying the radials and mounting the antenna on the ground, you increase the losses of the antenna itself, but improve the reflection coefficient of the ground, because most of the field is reflected very close to the antenna (where conductivity is high because of the radials). By increasing the antenna's feedpoint height, you start to "decouple" the ground from the antenna's equivalent circuit (the capacity between the radiator and the ground decreases) but at the same time, the reflection point is further away from the antenna, so the field "sees" a ground with lower conductivity. This reduces the far field at low elevation angles due to a nearly 180 deg phase shift (related to "pseudo-Brewster angle"), if the antenna is low in terms of the wavelength. So, the "best" verticals are ground-mounted (if you have something like 15 radials at least and there are no large structures around the antenna) but antennas up about 1 wavelength are also very good if ground conductivity is rather low, because at low angles, the fields experience about the same 180deg phase shift that horizontal antennas see. If the antenna is high enough, the incident and reflected fields add up by almost 6dB at low angles. In ON4UN's book "low band dxing", he refers to some measurements performed by W8IJ and what he found was that low antennas with few elevated radials are inferior to antennas with (many) buried radials and that the difference was higher than could be expected from NEC modelling. I think he gave the recommendation to put up the feedpoint at least 1/8 lambda above the ground. In these "no radial" verticals, the feedline becomes the counterpart of the antenna, however, the current can be low, so that this is not necessarily a problem (depending on the feedline length), but basically you need a counterpoise because your trx does nothing more than periodically moving charges from one point in space to another. If there is no counterpoise, it has to take and move the charges on the feed line's shield (maybe by capacitive coupling) , the trx housing or the ac-mains (it only moves charges, but doesn't create or destroy them, so charge conservation applies). So, there is a counterpoise, even if you don't see it, there has to be one if the feed point current is not equal zero (which would mean tx off or infinite impedance). I don't remember for the R5, but I think the R7 had very short "radials". In these antennas, the short radials are part of the antenna, radials + radiator together form a resonant structure (like in the off-center-fed dipole antenna), so they are not truly end-fed, and require some broadband impedance transform plus a current choke because of the imbalance of the load (that is what is in the box at the feed point). If you provide a current choke, you may have shorter radials plus longer radiator, both together have to be lambda/half (electrically, taking all traps into account), however the feedpoint resistance is higher than 50 Ohms and some matching is required. Vy 73 Ralf, DL6OAP Am 18.05.2013 um 14:37 schrieb Joel Black <w4...@charter.net>: > A lot of talk has been going on about radials on the KX3 Yahoogroups > Reflector, but there is so much FOD on that reflector, I usually delete most > of the messages. Although it may have been mentioned there, I have probably > missed it. > > Other than for portability, why are elevated radials so important? I have a > ground-mounted 6BTV (not my main antenna) in my backyard. I have four radials > for each band and they were all put in with yard staples. I did this in the > Fall after the last grass cutting. By Spring, the grass had grown over them. > Now, several years later, there is at least one inch of dirt over them. In > all honesty, it's only a backup antenna and probably needs some radials > replaced. > > Now, in my situation, there was no way I was going to use elevated radials. > Someone recently posted a link to the SteppIR vertical - the CrankIR. > Looking at the one page from the link, it only mentions elevated radials. > Now, I've never seen a loaded-tower broadcast antenna with elevated radials > either. > > Is the only benefit portability? > > Please, I do not want to also be accused of perpetuating FOD on another > reflector. I'd prefer you reply directly to me. If needed, I'll summarize > and repost. > > Thanks, > Joel - W4JBB > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:Elecraft@mailman.qth.net > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
