Topband: Low band antenna project questions
For 160/80 some features under discussion and used, both at home and for V7, were a 55' vertical + drooping T (2x or 3x wires at ~ 45 degrees but reachable from the ground for initial tuning) + two/few experimentally tuned elevated radials for each band. The raw antenna is long on 80 and matched by a series cap (air with enough gap for power). It is short on 160 and can be matched by a hairpin coil across the input. Of course, you must somehow switch between the cap and the coil to change bands -- not completely trivial. In my case, this done by hand at the base amid the dark/rain/snakes. Especially on 80 this antenna is significantly off-center-fed and required substantial extra coax coiled up as a choke. The story is written up on my website. This method probably has limits on 80 if using just a field of 160 m ground radials, although adding a couple of elevated tuned 80m radials might do the trick - or so says standard EZNEC. EZNEC also hints that you might also be able to have a mix of 160 m and 80 m radials to get a similar effect but the limits of EZNEC when radial ground effects may be important are well known. This antenna had acceptable (second-tier in pile ups) performance on 160m using 700 watts and was quite good on 80. The V7 version with 2 drooping T wires and 2 elevated radials for each band was just okay on 160 but good on 80, probably limited by trees proximity and restricted geometry, although some might claim enhanced by being on a beach with adjacent ocean. Bill N6MW _ Topband Reflector Archives - http://www.contesting.com/_topband
Topband: Current Distribution on Buried Radials Used With Vertical Monopoles & Brown paper
The famous Brown et al. paper from 1937 on buried radials ground losses -- a brief summary Thanks to TB posts and R. Fry, I have been tempted down a path to ancient paper analysis. The essential results from this have been given before in terms of trades of height, number and lengths of radials. Generally, not surprising, more and longer radials are better, with various qualifications and considerations. A brief version of that most relevant to hams is provided here in simple tabular form. Only two vertical heights are considered, ~1/4 and ~1/8 wavelength (88 and 44 degrees) in comparison with the stated theoretical ideal of perfect ground. Only results for radial lengths of 90 and 45 feet are given. They are in dB loss (or gain if you don't like the minus sign) and might be interpreted as the ground loss as measured by the signal at a mile (1609 meters) away but near the ground (not sky wave). Since the data translation from 0.3 miles to 1 mile assumed the perfect ground scaling, the loss results are really those out to 1/3 mile all at 3 MHz. dB loss using n 45' radials n 88deg 44deg 2 -4.17 -6.30 15 -2.29 -3.57 30 -2.18 -3.43 60 -1.95 -3.16 113 -1.84 -3.03 Ideal 0.00 0.00 dB loss using n 90' radials n 88deg 44deg 2-4.17 -6.02 15 -1.25 -2.29 30 -1.05 -1.71 60 -0.85 -1.12 113 -0.65 -0.81 Ideal 0.00 0.00 So more is always better but how much you are willing to pay for the next dB always becomes the question. Impedances are in the paper. Full painful story at http://n6mw.ehpes.com/BrownNotesFinal.pdf _ Topband Reflector Archives - http://www.contesting.com/_topband
Topband: Modeling "Ground" and losses
ver, it seems possible that if you have very good soil (high sigma) it might not be smart to bury radials a lot relative to the skin depth. I don't think any of this is in contradiction of the quoted material from Brown, Terman and Laport although some of their phrasing might be quibbled with along with just what soil property parameter regime they are working in. Also, saying that the surface radials shields the ground loss seems consistent. Finally most of our standard antenna models (up to and including NEC4) do not claim to provide excellent solutions to Maxwell's equations in all of space. Some near field approximations have been made to get, primarily, far field performance evaluation. And in this context, using loss estimates, based on changes of the peak in the pattern from the models, may be a red herring leading away from understanding. Bill N6MW _ Topband Reflector Archives - http://www.contesting.com/_topband
