There is a major difference between a resonant antenna and a one wire
transmission line whose sole purpose in life is to be unbalanced in respect
to ground.
Trying to compare them doesnt work well and neither does modelling these
types of grounds unless you work for the government and have access to
software that has been especially developed for such things. It is clear
that there are still a lot of assumptions being made with questionable
conclusions. This is often followed by statements that it is to be taken as
gospel by everyone. Internet sites are full of that stuff since these
"assumptions" ignore the huge variability of Mother Earth.
A resonant antenna on or very close to the ground is strongly affected by
different conductivity conditions beneath it even if just in small areas. A
BOG is not affected since it doesnt require or even want resonance to work
and a portion of ground that is only a fraction of a wavelength is
meaningless at 160M.
A BOG 's impedance can be anywhere from 200 to well under 50 Ohms depending
upon the RF ground conductivity. A 269B, a mutitap matching transformer, and
variable carbon pot for a termination can be used and having a helper is a
good addition.
Dont be surprised if the transformer turns out to be a 1:1 or even 2:1 with
the feedline being the higher number if you have well above average ground.
A BOG will have a naturally high F/B since the losses smooth out the VSWR
ripple making a perfect transformer and termination match a bit less
critical. A BOG is also more sensitive to common mode feedline currents and
requires those extra steps to eliminate it.
Since an elevated 2 wave length Beverage still has a high VF even over
average ground it holds that a 1 wave BOG hasnt reached its reversal point
yet. At a prior QTH here in town I found a 1500' Beverage 10' high to be
excellent and at 2100' it was useless. Ive done a lot of walking over the
30+ years Ive been using Beverages.
Finding the maximum useable length should be done after a rain storm as that
will be the point of lowest VF. Or simply start at 500' or so and chop off
50' at a time if it has already flipped. Conversely keep adding if you want
to get the benefit of length. Use a 4' ground rod and a couple of 30'
radials at the far end to establish a half a**ed ground since that part isnt
critical yet and simply move it as needed.
Carl
KM1H
----- Original Message -----
From: Guy Olinger K2AV
To: ZR
Cc: Pete Smith N4ZR ; topband@contesting.com
Sent: Monday, March 12, 2012 1:02 PM
Subject: Re: Topband: How Good is Good Enough?
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.
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