Topband: Modeling a BOG

[Guy Olinger K2AV]

Below:
On Sun, Feb 22, 2015 at 1:00 PM, Richard Karlquist <richard at karlquist.com>
wrote:

> Jerry Burke, one of the developers of NEC told me that
> they tried to characterize ground by measuring open
> wire lines laying on the ground.  It is a straightforward
> exercise to back out the complex characteristic impedance
> and propagation constants of the line.  The problem they
> found was that there was no value of complex ground
> conductivity that you could plug into NEC and get the
> modeled NEC line to correspond to the measured data.
>

Amen.

So-called "natural laws" depend on finding a mathematical way to generate
what is actually measured. Then the Math is called a "law". Since all math
is a model, model-that-matches-real-so-far is a better name. This leaves
room for discovering boundaries of paradigm in never-before-tested
situations, without a lot of the current day arrogance that presumes we
have everything solved.

Our local experimentation came to the same conclusion as Mr Burke. It HAD
been assumed that the NEC math would generate results in those situations
corresponding to reality. Now we know better.

Ground, as it affects numerous wire on/in/around ground situations, is
poorly understood, and there is no Daddy Warbucks out there willing to pay
the bill for the research it would take to fill in those blanks.

In any event, if you measure a BOG, you will see that after a
> few hundred feet the signal has been attenuated away and
> you don't need to waste wire making it any longer.
>

Amen, again.

Take a 250 foot insulated wire, and break it into a dipole with a coax
connector in the middle. Go to where you intend to put the BOG down. **Very
important**, install it on/over/notched into the ground the SAME IDENTICAL
way you were going to *permanently* put down the BOG. Bend the last couple
inches up at the ends of the wire so they don't short into the ground.

Measure the LOWEST resonant frequency of this "Dipole On Ground" and its R
at resonance. Most likely resonance is in the mid to upper middle BC band.

Trim the DOG equally at its ends until it's self-resonant at 1.140 MHz,
then solder the center together, re-insulate and weatherproof wires at
solder points. Then terminate/install the ends as a BOG. Multiply the R at
resonance by 2.5 for a decent termination resistor.

Some MFJ and some others won't go that low. Beg, borrow, or steal one that
does. But this method makes it the right length over YOUR particular
placing over/in YOUR ground. You should start long so that you change
length by cutting instead of soldering.

You can make it shorter than this length, but there is no point going any
longer.

Why this works is another discussion.

73, Guy K2AV