1) For the low bands, the overwhelming consideration is reduction in near field losses, both dielectric and I squared R (I2R) losses.
2) We need to remember that for non salt water situations vertically polarized RF is NOT reflected from dirt and rock. For all intents and purposes it is absorbed. Vertically polarized reflection enhancement happens over salt water, but certainly not over rocky or rock-based surfaces. 3) Feedline loss is an issue for distant antennas, **particularly** what the loss will become long term, after extended exposure to the environment. 4) On the low bands are 1/4 wave radials are really the great deal on low bands that ham mythology says they are? We need a counterpoise, and 4 by 1/4 wave radials are only *one* choice with little to recommend it except simplicity at the feedpoint. The worst thing about many counterpoise schemes is that they induce current in the ground, and that is loss. The mechanics that govern the induction are a bit murky and not at all well known. We need a counterpoise to store electrons for a half cycle. Whether that device is naturally resonant on frequency only saves you complexity at the feedpoint. What you will be stuck with permanently is however much LOSS is invoked by the design. Again, the word is LOSS. Mitigate loss in your complete antenna *system* design. Regarding your particular situation: Stand at your two possible installation points. Look at your most distant horizon in any important direction. ANYTHING below tree top, or mountain ridge-top line, even 15, 30, 50 miles away, is severely attenuated, if not lost to -30, -40 dB effects. If you have a clutterless view to distant horizon, for vertical polarization this will advantage you the most because it removes dielectric and I2R lossy clutter in the path to very useful low angles. Remember that reflection enhancement in your situation only applies to horizontally polarized antennas. Designing your antenna, 1) use a counterpoise with least loss from ground induction, 2) move the RF current maximum up on the vertical conductor, 3) **expect and prepare yourself** for matching a miscellaneous impedance at the feedpoint. This misc feed Z is most likely for the best combination of 1) and 2). If you really are concentrating on efficiency, get the current maximum up in the air, and use a low ground induction loss counterpoise. If you go distant installation due to clutter considerations, PAY UP for a permanent hardline coax feed, unless you are willing to put up ***BARE*** wire open wire feedline you construct yourself. Buryflex flooded coax would be a medium solution but has all the critter and outdoor accident issues. Commercial balanced line deteriorates in time. Been there, done that, never again. Even with the open wire, you can have common mode issues on the balanced line nearly impossible to rectify, unless you take steps in the last quarter wave or so before the antenna. The balanced feedline run will need to be an odd multiple of a quarter wave accounting for velocity factor. Common mode current on the line will go almost entirely to loss. 73, Guy K2AV On Tuesday, July 12, 2016, Dauer, Edward <[email protected]> wrote: > So long as antenna discussions on the reflector haven’t been met with the > “OT” cloture lately, I have an antenna question of a different sort. I am > contemplating a ¼ wave vertical with four elevated radials for 80 meters. > My choices for siting it are two – one is near the top of the property > (about 8,600 feet ASL), somewhat in the clear, and within 100 feet from the > operating position. The other is in a meadow near the property boundary, > which is much more open and a just a bit higher – but it has two other > significant characteristics. One is that the land slopes away from that > site, over about half the compass from NNW to SSE, at a slope of 10 to 15% > for about a half mile. > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [email protected]
