>
> 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
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