> > On Fri, 6 May 2016 15:37:59 -0400, James Wolf wrote: > > This is interesting, it seems up until now, I was under the impression > that once a beverage was at or below ground, it lost directivity. > Thanks, Jim - KR9U
The pattern is essentially under the influence of several items: 1) the electrical length [physical length times velocity factor (VF)], 2) interference between the speed of incoming RF in the air and the speed of already received RF on the beverage wire and 3) the proximity loss of the earth moving the antenna toward a self-terminating behavior. In a regular beverage, the VF of a beverage over deer antler height is .95 or higher. This "normal" beverage plays based on its *electrical* length, just as does one buried or laying on the ground, but it's fairly constant and that small reduction in the VF is already in the beverage design length. So it's not so much that the design changes as ground is closer, the constant *physical* length no longer produces the beverage's design *electrical* length. Once you get within a few inches of the ground, the VF starts to decrease massively. This reduction continues until the wire is buried. Since this VF has been measured from 0.45 to 0.8 in various circumstances, a design length for 6 feet in the air will be cut in half by the time the wire is buried. Then the difference in speed of incoming RF and RF on the wire to the feedpoint will produce effects not in play on a regular beverage. At this point the antenna needs to be accurately modeled in order to evaluate the changes in effect. Effective termination resistance is now the self-termination resistance plus the physical end termination resistance. Plus, modeling demonstrates that some improvement can be made with simple grounding of the far end, or even no termination at all. These last two indicate that the standard beverage procedure of terminating for least SWR looking into the wire no longer produces best pattern. It really ain't a beverage at all in these circumstances. Assuming it still is a beverage and caluculating accordingly often produces the "doesn't work worth a d*mn" outcome. The antenna must be designed for ground low velocity factor conditions. Once very close to, or on, or buried in the ground, ground moisture changes will vary performance. Those who have put BOGs down in the woods discover that an accumulating layer of wet, rotting leaves over the wire, often to four or six inches of leaf layer on top can completely detune what used to be a decently performing BOG, rendering it nearly deaf. Woods BOGs need to be laid on top of the leaves, and pulled up out of the leaves from time to time during the contest season. Lawn BOGs should be lightly buried to start with, dropping the wire into a blade cut just barely into the dirt, and the lawn over top kept mowed. Even then, the wire needs to be checked annually for *electrical* length when the soil is damp and the wire length adjusted as needed. For this it's useful to retain a center connection, which is shorted and weather-proofed for normal operation. BOGs are cranky RX antennas in the best of times, and one needs to disconnect from Beverage thinking when dealing with them. 73, Guy K2AV _________________ Topband Reflector Archives - http://www.contesting.com/_topband
