Topband: Measure dipole on ground to get VF of BOG.

[Guy Olinger K2AV]

Hi, Mikek,

BOGs have a very miscellaneous performance, some love 'em, some hate 'em,
nobody telling a lie. One part of a back yard can have a heated BOG
argument with a different part of the same back yard, both sides verified
with MEASUREMENTS. The secret has to do with the BOG in DOG disguise, or
BOG's DOG.

When BOGs are working right, they are very good at getting rid of local low
angle noise off the sides.

If you have modeled the pattern of a BOG, you will find that the pattern
varies with the ELECTRICAL length of the BOG. When the electrical length of
a BOG gets too long, the pattern can deteriorate to zero front to back
ratio. And if it gets longer yet, it can actually reverse to a back to
front ratio.

To derive a physical implementation of a BOG model that has generated a
good pattern, change the bog wire to a dipole (do not change the height or
ground characteristics). Remove all other wires, then measure the resonance
(the R component of R+jX, when X=0, not SWR)  of the wire fed as a dipole.
That is the BOG's DOG.

For one reasonable and fairly successful BOG design, that resonance for the
BOG's DOG is 1140 kHz for 1.83 MHz BOG design frequency. For a BOG, the end
termination is a good deal less significant than having the BOG's DOG at
1140 kHz.

You put down the DOG in the place you intend to install it, at the height
and relation to grass, dirt, etc, you intend to install the BOG, and with
the final site preparation already done. You prune the DOG to get 1140 kHz
resonance, and then solder and insulate the center to a single wire again,
and ONLY THEN add the feed and termination elements of the BOG.

In '09 a dozen or so of us MEASURED quite a number of 151 foot (46m) DOG's
in the counties around Raleigh/Durham/Chapel Hill. We found that the ground
was WILDLY variable in its effects on VF. VF was WILDLY variable from 45%
to 85%, sometimes WILDLY variable in the same back yard by rotating the dog
90 degrees about its center.

This nailed down the principle that a BOG must be set its length for the
particular VF at its EXACT placement, or suffer what for a long time has
been a history of mixed "great antenna", "awful antenna" reports without
rhyme or reason for the variance. No one size fits all unless large
variation in results is OK.

The VF also changes with moisture in the soil, meaning that unless in
perpetual arid conditions, it needs to be set in the area's damper
conditions. If a moist location has a dry spell threatening a contest
weekend, you can always go "water the BOG" with a bucket to return it to
installation day behavior. A little hard to do the other way around with a
hair dryer on a 100 yard long extension cord.  Also, as grass or leaves on
top "grow the BOG down" you can experience well-documented deterioration of
pattern, unless you maintain the BOG and its installation location in the
same condition as it was when installed.

You cannot reliably tune a BOG using SWR, quite contrary to
well-established successful technique with beverages. A BOG does not behave
like a beverage. The beverage paradigm has been monstrously overwhelmed by
the BOG's intimate relationship with the dirt.

Also remember that for most people a BOG is a LOW SIGNAL, negative gain rx
antenna, and often needs a preamp. Procedures dealing with common mode
noise on the feedline need to be followed, particularly using an isolation
transformer (primary and secondary no connection other than magnetic flux
and minor stray capacitance) and the feedline not grounded in the area of
the feed, or you can bury the BOG's lower signal in preventable noise.

73, Guy K2AV

On Sat, May 2, 2020 at 1:42 PM Mikek <amdx at knology.net> wrote:

> I'm about to do some measurements on a BOG.
>   I've been a little slow getting things working, I've now got my inside
> and outside boxes assembled,
> I understand I can break the BOG in the middle and find resonant
> frequency as a dipole.
>  From that I can calculate the VF. I may leave this for a while and hope
> for rain to get some
> different measurements.
>   I also wonder about measurements, on the ground, at 1" above ground,
> and at 2" above ground. Is there any advantage to
> the pattern or signal strength. At 2" inches I could weed whip to keep
> it weed free.
>
>
>   On a second note, I'm building it long, for the AM band and
> considering using KD4Zs stretch modules (in my case shrink modules)
>   for 160M and 80M bands. Although I think I would just terminate
> instead of reflect.
>
>   Your thoughts? any good pages about the Dipole on ground measurements
> appreciated.
>
>                              Mikek   KF4ITA
>
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