Bruce, "...in my area, the most stable earth moisture is at grass/vegetation root level..."
Are you are in fact saying that to be "stable", that is to say, remain the same day in, day out, a "BOG" needs to be evenly sunk to about not more than one inch below the perceived surface? 73, Charles, W2SH > From: [email protected] > To: [email protected]; [email protected] > Date: Mon, 9 May 2016 23:27:24 -0400 > Subject: Topband: re 2 wire BOG antenna update > > > A measurement of My 2 wire directional BOG antenna, with calculations for > measuring over shoulder high dense thorn bushes in two places, came in at 176 > feet > I could see the wire atop soil level a few places to stay on track along > the way. > > Optimum length will vary from location to location with earth conductivity, > moisture content. > > I have a more simplistic view of the BOG process. The velocity factor drops > as the wire comes in contact with more damp/wet earth. Earth being the other > half of > capacity to the wire. I am finding, in my area, the most stable earth > moisture is at grass/vegetation root level. > > Stability is important as the great front to back pattern is only at its > resonant frequency. > If anyone has missed IV3PRK's research at HC1PF please go to my > bogantennanotes, and.see the IV3PRK links...... .my page address is below > > > We need to stop thinking "Beverage" when we talk about 'this' antenna. Any > ideas for a name change? > > > 73 > Bruce-K1FZ > http://www.qsl.net/k1fz/bogantennanotes/index.html > > > > > 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 _________________ Topband Reflector Archives - http://www.contesting.com/_topband
