Topband: Beverage on Ice

[Guy Olinger K2AV]

The 1000 feet of wire trenched in ice was doomed from the start.

160m BOG's longer than 220 feet start to not model well, and start to not
perform well. One can easily model a BOG that has a pattern *reversal*. The
various serious quirks of BOG's make them unpopular. And so they are not
well-researched.

Based on BOG-related measurements and experience (not modeling) I can
pretty well guarantee that an Ice-BOG of 1000 feet would not work well. It
has been interesting to figure out how to model a BOG so that the model
agrees with reality. The most accurate results require NEC4, which can
handle wires which are slightly underground.

The beverage technique of terminating the far end of a wire in what amounts
to a characteristic impedance, to zero the standing wave on the wire, does
not produce an optimum BOG.

As Tom has stated, the antenna is dominated by the fact that signals on the
wire and incoming from the air move at vastly different speeds.
Understanding the velocity factor on a BOG wire in its *individual* setting
is key to success. The best modeled patterns USE that difference to
advantage, to optimize pattern. Not at all like a beverage.

To that end, "beverage on ground" is a misnomer. What is called a BOG is
really a ground mounted low velocity factor RX antenna, which has its own
set of rules.

For just one, notching the BOG into the ground at installation prevents a
large change in velocity factor as over seasons the wire gradually works
itself through the grass and into the dirt. To get the BOG adjusted and
with a somewhat constant behavior really requires that the BOG be UNDER
ground, notched into the actual dirt, not up in the grass.

In truth, both "beverage" and "on" the ground, the "B" and "O" of BOG, are
misnomers for optimal installations. If you want the acronym to actually
mean something correct, what we call a BOG is a GLVF ( Ground installed Low
Velocity Factor) antenna, following its own set of rules. Doing a GLVF
antenna in ice would imply possible issues, also correct, reference Tom's
commentary.

The BOG's pattern will also vary with the ground's water content, which is
in turn varying the velocity factor and the best termination strategy. This
IMHO, along with the wire gradually growing down into the grass, is mainly
responsible for the difficulty in obtaining repeatable results and
unsatisfactory results. This implies setting up the BOG in wet conditions
anywhere other than desert. At contest time, a dried out BOG can be brought
back to the setup performance with a garden hose. Try doing that in reverse.

I stand by my original statement of 220 feet for an Ice-BOG. And for
mechanical reasons, notched into the ice, using insulated flexweave.

73, Guy.




On Sun, Jan 18, 2015 at 2:57 PM, Tom W8JI <w8ji at w8ji.com> wrote:

> I laid out 1000' of #14 stranded, insulated wire.  I terminated it with a
>> 200 ohm resistor to a 1/4 wave wire and several short radials running
>> nearly parallel to the antenna/grounding wires.  At the feed end, I used a
>> 4/1 homebrew transformer using one of "Tom's" binocular cores.  The
>> transformer was grounded to a conventional 8' ground rod.
>>
>> The antenna never worked at all, as far as I could tell.  There was no
>> discernable, certainly not usable, directivity.  Why, I don't have a clue.
>> The techniques chosen were the result of all the best advice I could get at
>> the time on the topband reflector.
>>
>>
> I'm afraid antennas like that are severely length constrained, because
> they are slow wave structures.
>
> The velocity factor of an antenna laying on or surrounded by ice is pretty
> slow. This will limit how long you can make an antenna before the pattern
> falls apart.
>
> Insulation will not mitigate this problem, because the issue is the
> proportion of electric field in the media (ice) around the antenna compared
> to other dielectrics.
>
> If we consider the dielectric constant 3, maximum length would be 250 feet
> and it would be quite length critical. Too short or too long and pattern
> would fall apart.
>
> The impedance is also a lot higher than you might expect. It is nothing
> like a BOG laid on normal dirt. It doesn't even act like a Beverage because
> of the extremely slow wave velocity.
>
> 73 Tom
>
>
>
>
>
>
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