Topband: How Good is Good Enough?

[Guy Olinger K2AV]

>
> I dont know if Id trust trying to use a resonant dipole on the ground to
> determine the VF of a non resonant wire. In any case keep the VF at about
> .6
> or higher when determining the maximum length of the BOG, Slinky, or any
> other Beverge form.
>
> Carl
> KM1H
>

It was clear there was a lot of guessing and assumptions in the world of
BOG.  So we decided to investigate.

We MEASURED a lot of wire on ground VF a couple of years ago in and around
Raleigh.  Not speculated, or guessed.  MEASURED.  Fellers went out here and
there and spread out 151' DOGS, and reported resonance, plus X and R
readings for resonance and +/- 50 kHz.  The 151' was chosen to have a
simple (46m) metric equivalent and a resonance up where the MFJ's could see
it and roughly in the 160 meter band or above.

They were measuring anywhere between .45 and .8 VF. Sometimes, nearly that
variation in the same back yard just by reorienting the wire. In an
individual case you could get a lot of variation by NOT getting the BOG
down to the dirt.

In plugging that kind of variation into BOG models, that was such a drastic
effect on pattern and performance, that if you didn't begin with a VF
measurement in your exacted intended spot for the bog, you were just
throwing away front to back, or in some cases, reversing the pattern.

Finally settled on what ever length for the BOG was gotten by 1) getting
the wire right down on the dirt as you were going to leave and use it, and
2) pruning the DOG to 1140 kHz, and then reconnecting the wire in place to
use as a BOG.  There was also a formula for computing the length from the
151' DOG resonance, so people could use the MFJ meters (they don't read
down that far in the BC band).  The problem with that was that the 151 feet
did not cover the entire BOG length, and you were ASSUMING that the
remaining seventy or whatever feet was over the same kind of stuff as the
151.

If the VF happens to be one of the 0.45 jobs and you use 500 feet, you can
actually get a pattern reversal.  That's a pretty huge variable to leave in
the unknown.

I spent some time digging up BOG tales.  They were all over the place as to
whether they worked or not.  There was also a notable occurrence of
assuming that regular beverage dimensions worked for BOGs, and that long
BOGs would always work better than short BOGs.  There was also a clear
majority of reporting failure and abandoning the concept.

The length you get with the 1140 kHz DOG trick was the *ONLY* length I was
ever able to get to work in a BOG model that had a satisfactory pattern.
 Never able to get 500 feet to model anything with the possible max front
to back.  I am left having to assume that folks using those are simply
using the BOG's ability to reject nearby noise and hear the skywave, which
is there regardless of the length unless the pattern has reversed.  That
could be very useful in a given situation.  But that property also applies
to shortie BOGs as well.

If a northeast BOG is done right, from our NC part of the country, an S9
station from Arkansas or Texas SHOULD be knocked way down, indicating
significant front to back, and also killing a lot of QRN that comes from
that direction.

I have also found the BOG's local rejection quality to be severely
compromised by close elevated conductors and particularly by resonant
overhead conductors, fence wires, above ground power wires, ladderline
feeds to other antennas.  BTDT.  Talking about 20 dB difference in
rejection of back side power buzzes, by removing the overhead.  I was able
to get one of those situations to model out and the fix tracked the model.

73, Guy.