Topband: BOG Beverage on Ground Help
Guy Olinger K2AV
k2av.guy at gmail.com
Sun Dec 1 14:02:41 EST 2019
BOG is not a Beverage. So don't think about or treat a BOG like a Beverage.
The relationship between a Beverage and a BOG is like the relationship
between an Otter and a Platypus. Both are aquatic mammals, but the Platypus
lays eggs and has other very un-Otter characteristics.
Early in the decade, a bunch of hams in the counties around Raleigh/Durham,
North Carolina, ran a bunch of Dipole on Ground (DOG) MEASUREMENTS, to try
and explain the vagaries of BOG behavior. MEASURED (not modeled) velocity
factors of a 151 foot (47m) DOG varied from 45% to 80%. In some cases
simply rotating the DOG around its center 90 degrees in the same spot
created very large changes in VF. This absolutely underscored the need to
install a BOG using a method accounting for the MODELED VF at the ACTUAL
site using the ACTUAL placement of the BOG. We also MEASURED variations in
VF with changes in moisture.
Most issues with BOG's relate to its severe closeness with ground. It
really does not behave at all like an up in the air beverage antenna.
As to modeling, that's the number one misunderstanding and/or omission in
thinking, which I hear over and over again:
The BOG's pattern depends on its ****ELECTRICAL**** length, not so much the
termination. SWR does not properly tune a BOG. Emphasis intended. The
incoming signal in the air can be going twice as fast as the signal on the
wire, in phase at one end of the wire and out-of-phase, cancelling at the
RX end of the wire. Beverages do not have such problems unless they are
very, very long and uncharacteristically low.
THE BOG'S PATTERN DEPENDS ON ITS ****ELECTRICAL**** LENGTH.
That, as a percentage of its physical length or Velocity Factor, can vary
WILDLY all over the map, explaining the WILD variation in result
satisfaction. And if that were not bad enough, VF can vary WILDLY with
changes in ground moisture content. It can vary with a sneaky slow change
as it grows down into the grass if it wasn't notched down to the dirt to
start with. It can change if leaves pile up on top of it. Again, all of
these are a problem because...
THE BOG'S PATTERN DEPENDS ON ITS ****ELECTRICAL**** LENGTH.
You cannot model a BOG and directly use the modeled physical dimensions if
you are trying to optimize a pattern. Get the pattern you want in the
model, then model a DOG at the same physical height, wire size and physical
length as the BOG model, and use SWR or repeated SRC DAT's to find it's
resonance. That will tell you the ELECTRICAL LENGTH in the model and
therefore the self-resonance frequency needed to create the needed
electrical length in the actual physical BOG.
Then when you go to put down an actual BOG based on that model, do a DOG,
IN PLACE, EXACTLY WHERE, AND IN THE EXACT SAME MANNER YOU INTEND THE BOG TO
BE LEFT, trimming or extending the DOG to match the MODEL'S RESONANCE. Then
rewire the DOG's component wires into the BOG. Solder and insulate the
center, hook up the ends to termination, etc, but don't move or adjust the
length of the DOG wires for any reason. You should redo this exercise after
it's been there for a while to account for sink and cover, etc.
And remember, BOG's are a LOW SIGNAL antenna. Most BOGS really do need an
amplifier somewhere. They are superb at rejecting vertically polarized
local noise off the side.
That suggested for non-desert locations, that you install it with things
good and moist. If it's been dry for a while and it's VF is drifting, you
can always "water the BOG" to return it to installation moisture levels. If
you put it down with no leaves on top of it, you have to keep the leaves
blown off for operation. Don't lay it on the grass, and let it sink (get
closer to the dirt) over time. That will lower the VF and change the
electrical length, and detune it from the model.
You may have guessed that all the above makes it a single band antenna for
the desired prime pattern. Quite unlike the beverage that covers multiple
Been there, done that, beaten by that, fussed at that, cussed that, cried
over that. Need to understand a BOG well and play by its rules.
73, Guy K2AV
On Sat, Nov 30, 2019 at 10:28 AM Grant Saviers <grants2 at pacbell.net> wrote:
> The goal is admirable. I did some modeling of inductor shortened BOGS
> with EZNEC Pro4 and found it is extremely sensitive to height above
> ground. A half an inch change in height can reverse the pattern. Thus,
> I consider it an impractical antenna given the nature of most real
> ground surfaces, i.e. exactly where is the modeled "perfectly flat
> ground of uniform properties". This is in addition to the practical
> realization of the needed inductors.
> Grant KZ1W
> On 11/21/2019 19:09, K4SAV wrote:
> > Mikek said:
> > "I'd like to apply the loading to a BOG to slow the VF and make it seems
> > longer.
> > ie. make a 80 meter BOG length work on 160 Meters. But then make the
> > reactance go away for 80 Meters
> > My actual goal is to have a BOG that covers 500kHz to 4MHz. "
> > Mikek, I see you haven't given up on this project. Experimentation with
> > antennas is one of the things I like to do too. Good luck with this. I
> > won't offer possible alternatives to do this (there are several) but
> > will consider only your desired approach.
> > I don't see any switches in your circuit diagram to switch series
> > inductors, so I assume you are trying to make them variable and have
> > them operate from 500 kHz to 4 MHz.
> > There are some difficult tasks ahead to make this work over a frequency
> > range of 8 to 1.. One is the RF choke. EZNEC simulations show you need
> > about 1mH or more to get enough impedance at 500 kHz. The problem is
> > that the self resonance of the choke has to be well above 4 MHz. A
> > distributed capacitance of the choke of 1.6 pf would cause the choke to
> > resonant on 4 MHz, so the distributed capacitance must be significantly
> > less than that. It may be possible to build an inductor like that but
> > it will have to be an air wound coil using very small wire and well
> > separated from its surroundings. A ferrite or powder irom core won't
> > work. Accepting some degradation of the pattern at 500 kHz would
> > decrease the choke requirements. Other than that, I don't have a
> > solution for this, other than something complicated, like switching
> > chokes as a function of frequency.
> > Those values of inductance were derived from simulations and there is
> > some possibility the numbers may not agree with experimental results.
> > Position of the wire carrying the current may affect the pattern if it
> > is close. Too close (like in the same cable) and the capacitance
> > between the wires will effectively short out the inductors at RF.
> > The resistive component of the impedance of the series inductors is
> > important. Inductor material is important. If the resistance is too
> > large, it will kill the response of the antenna.
> > DC blocking caps should present no problems.
> > I assume you have breadboarded the series variable inductor and
> > determined the inductance range needed to cover this frequency range.
> > Mikek said:
> > "I see certain measurements for the length of a BOG, such as 200ft for
> > for 160 meters."
> > You will find various recommendations for BOG lengths from different
> > people. I didn't trust NEC to give a correct answer for this so I spent
> > a month comparing a 250 ft BOG to a 366 ft one. I posted my results
> > http://lists.contesting.com/archives/html/Topband/2018-08/msg00073.html
> > Jerry, K4SAV
> > _________________
> > Searchable Archives: http://www.contesting.com/_topband - Topband
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