Where is the helicopter you insisted we need? On Dec 18, 2012 10:03 AM, "Carl" <[email protected]> wrote:
> I guess I wasnt clear enough so lets start again. > > We both agree that the .38 db increase is at all elevation angles since > the increase in efficiency at the feed doesnt change the pattern shape, > just levels. OK ? > > Where the differences are is in the initial far field signal strengths > from zero to lets say 20 degrees. With a perfect theoretical ground the > levels are the same. If that held in reality then no matter what the ground > losses are the BC stations would not be spending the big bucks in radial > fields, even for 1/2 waves. > > The city lot ham would be readily competitive with the antenna farm > operator or the little guy in a coastal salt water swamp. > > My point all along is that ground losses change the shape of the main lobe > curve at low elevations and reduce signal levels there. Total power doesnt > change but it is no longer all radiated, some is now dissipated in the > lossy ground. Basic physics tell us you cant have both at the same time. > BC stations arent allowed to do that since it is the ground wave they are > required to radiate to their local audience, the sole reason of their > existence except for the few clear channel flamethrowers. A good ground > wave signal means a good amount of power in all of that main lobe which > results in the nightime skywave BCB DXers crave. Hams want some of that low > angle just above the ground wave to work DX and those that radiate a high > percentage of the output fed into the antenna to cover all those angles win > the gold. Other than saltwater there is no magic fix as some want you to > believe. > > Many years ago there was a BC station in Lowell, MA that had a tower on a > 4th story industrial building metal roof, that was the total ground. Im > about 6 miles LOS from there and the selective fading was intense. Their > ground wave was minimal but somewhere along the way the FCC allowed them to > operate. I dont remember the details but there were several "stories" > floating around about why they kept operating. It all went away during the > urban renewal of Lowell, establishment of an Urban National Park, and a > huge city investment in its future. > > I suppose hams can use a high end local BCB station to evaluate changes as > they make them. Find a moderately strong steady station and monitor/chart > its strength for several days of the same weather. Then by adding radials, > rods, screens, perimeter wires, etc progress (or lack of) can be tracked. > By doubling radials each time from 4 to 32 or even 64 and having them all > precut and ready to unroll this can be done in a few hours especially with > a helper. Next comes the screen. > > Carl > KM1H > > ----- Original Message ----- From: DAVID CUTHBERT > To: Carl > Cc: Tom W8JI ; Donald Chester ; [email protected] > Sent: Tuesday, December 18, 2012 8:00 AM > Subject: Re: Topband: Ground mounted 1/2 wave > > > > > > On Mon, Dec 17, 2012 at 8:41 PM, Carl <[email protected]> wrote: > > > Subject: Re: Topband: Ground mounted 1/2 wave > > > ** All that means is that the elevation peak of the wave as seen in the > typical 2D plot increases by .38dB and as expected. It does not say what > happens from that peak down to zero elevation which is what 160M DXers care > about. > > What is the FS at 5, 10 degrees when going from a ground rod to a full > bore radial field over a wide range of ground conductivity? > > > Carl > KM1H > > Yes, it does say what happens from that peak down to zero elevation. It > says that the signal increases by 0.38 dB. > > To test this I ran two EZNEC simulations. One is a 90 degree vertical over > thirty 90 degree radials over medium ground. The antenna is driven with 1 > kW and the E-field at one mile is recorded from a height of 10' to 1000'. A > second 90 degree vertical over four 23 degree radials driven with 1 kW and > the E-field at one mile is recorded from a heights of 10' to 1000'. The > difference in E-field AT ALL ELEVATIONS is 0.86 dB. > > Dave WX7G > > > > > ______________________________**_________________ > It is undesirable to believe a proposition when there is no ground > whatsoever for supposing it is true. — Bertrand Russell > > > > > > No virus found in this message. > Checked by AVG - www.avg.com > Version: 10.0.1430 / Virus Database: 2637/5468 - Release Date: 12/18/12 > _______________________________________________ It is undesirable to believe a proposition when there is no ground whatsoever for supposing it is true. — Bertrand Russell
