We really gotta quit analyzing BOGs as if they were actually a Beverage.
(This is a late response to a thread some months ago.)
A BOG, laying on the ground, regardless of the "B" word represented in the
acronym behaves quite differently than the typical real Beverage antenna up
6 to 8 feet. It would be better to call a BOG something different, so
people would expect something different, but that misnomer name is so
ground in now it's beyond all hope of recall. A BOG is among a class of
antennas, Ground Low Velocity Factor, which have their own nasty little set
of issues separate from similarly shaped antennas up in the air.
Because the BOG lays on the ground, there is very little difference between
the VF of the wire laying right on the ground and a buried wire. Even with
the frequently diminished sensitivity of NEC based programs to ground
effects, a model of a BOG returns quite different patterns than a
Beverage well in the air for the same length of wire.
*Measurements* (not models) of VF on a 151 foot (46m) Dipole On Ground
(DOG) placed in various spots by multiple observers around North Carolina
showed velocity factors ranging from 45% to 80%, with lower values most
common.
This is quite different than the 95% to 99% VF seen on bare wire Beverages
up 6-8-10 feet.
Take a physical halfwave wire laid on the ground. Assume a signal coming
from the east, with the BOG wire laid east-west. Assume a 50% VF. The
from-the-east incoming RF wave and actual wire current at the east end will
be in normal phase relationship, just like the Beverage. BUT, by the time
the incident RF wave has traveled to the west end of the wire at the speed
of light, it will have caught up with the slower-moving reversed current
phase from the previous half cycle. This produces a completely non-Beverage
behavior, where the interaction of varying incident/induced current phase
must be calculated along the length of the wire.
By monkeying with ground constants in NEC 4.2 to get same VF in modeled
wires, we see rather different patterns and termination needs than a
beverage. This includes the realization that the best pattern was *NOT*
gotten by monkeying with the BOG for best SWR. A BOG is
partially terminated all along the length due to induction of lossy ground
to a far greater extent than in a Beverage. Best pattern has to do with a
messy calculus of all the various factors.
It will be seen in a properly constructed BOG model, that going beyond an
*electrical* half wave in length gradually morphs into a situation where
the pattern loses all front to back and frequently *reverses*. Reversal
does not occur with up-in-the-air Beverage antennas. Yet another reason
that the BOG does not deserve the "B" in the name.
This also makes the BOG a one band antenna for optimum off-pattern noise
rejection. This is more painfully obvious on the east coast where NE
orientation for Europe really needs the SW f/b rejection for all the
southern hemisphere band noise, and for all the midwest/Gulf/Florida
originating QRN. A well done true Beverage retains all these useful
characteristics for multiple bands.
A BOG *does* reject close in noise off the sides regardless of electrical
length at listening frequency, it's just that off the primary band a
regular Beverage or a BOG cut specifically for that frequency will do far
better.
A BOG is also very sensitive to ground moisture and such things as a
season's accumulation of wet rotting leaves over top. The current advice is
to "tune up" a BOG in moist ground conditions so that returning it to tune
in a dry season can be done for a contest with a garden hose.
Given the BOG's high sensitivity to height above *apparent* ground, the
"act" of using lawn staples to fasten down BOGs and allowing the normal
grass growth to gradually embed the wire, will also create large changes to
the behavior, and may need periodic shortening as the wire gradually goes
lower.
In the BOG model, changing the ground characteristics changes all the sweet
spots around, specifically messing with the best physical wire length for
pattern, and messing with the best termination value.
Some yard measurements with the 151 foot DOG varied wildly with placement
and orientation, insuring that "model and immediately cut" methods of BOG
construction, or duplicating what your neighbor did, are doomed to frequent
failure. This glum prognosis is validated by the range of anecdotal reports
on BOGs that vary between "worked great, finally have some ears" all the
way to "didn't work worth a d*mn, total waste of time".
Mental simplifications of the messy BOG design problem, e.g. "model a
beverage" and then just "lay it on the ground" is a doomed approach, as is
any attempt to understand how a BOG works by applying verified and working
theory for a true beverage.
The characteristics of a BOG would be altered immensely by stringing it on
mucky salt marsh. The highly conductive media underneath results in a very
high wire VF. The resultant high VF will relocate sweet points. And the
degree of BOG "self-termination" will go way down, due to the highly
conductive (less lossy) media underneath. Although my gut says it probably
starts behaving like a Beverage in this specific situation, the model is
very, very strange.
Beware any simple explanation of a BOG.
73, Guy K2AV
On Wed, Jan 6, 2016 at 12:08 AM, Milt <miltn5ia@gmail.com> wrote:
> Val,
>
> Beverages of any type will NOT work over salt water or earth where the
> chemical makeup of the earth produces a highly conductive material.
>
> Here is the way I understand the theory.
>
> The Beverage works on the principle of a two wire transmission line where
> one side of the transmission line is your Beverage wire and the other side
> is the earth.
>
> The wire half has RF voltage induced in it by 'tilting' of the wave due to
> the difference in the velocity factor of the 2 halves of the 'transmission
> line'. The signal travels faster in the wire than it does in the earth,
> producing the miniscule amount of RF voltage in the wire, which is
> impedance matched to your coaxial cable.
>
> If the medium which comprises the other half of your Beverage system has a
> VF at or nearly so the same as your wire half, no RF voltage is produces
> and you will NOT have any signal.
>
> Congratulations on your success. It means the earth comprising the other
> half of your BOG is not salt saturated. You are getting signal tilt
> producing an RF voltage, and therefore signals.
>
> If you were to move the BOG over the salt water and have your grounds at
> each end connected to the salt you will find that there will be for all
> intents and purposes ZERO signal produced. It is then truly just a
> balanced open wire feed line and nothing else.
>
> Keep up the good work. Always try and prove things for your self. That
> is what it is all about.
>
> 73, and good luck, de Milt, N5IA
>
>
> -----Original Message----- From: Hugh Valentine
> Sent: Tuesday, January 05, 2016 9:15 PM
> To: TopBand
> Subject: Topband: BOG near salt water
>
>
> Some say a BOG is not effective at/over salt water.
>
> For grins I installed a 200’ BOG to Europe. 4:1 Binocular XFMR, #18
> single wire, (92 Ohm Termination= 1.1:1 SWR on RG6 cable) 4’ Ground Rods,
> No radials.
> Works about same as K9AY. Maybe 8DB down in sig strength.
> Installed approximately from 1’ to a maximum of 30’ from salt water.
>
> I would say it works and can be installed rapidly.
>
> Val
> N4RJ
>
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