Topband: BOG data

[Guy Olinger K2AV]

It is tricky nasty to model a BOG, but can be done. One of the exceptional
problems is to get a ground characteristic for EZNEC that causes the model
to mimic reality. Sometimes people just can't come up with a set that
works.

If you are measuring current, then you are transmitting into it. Your model
will have to include the feedline shield with EZNEC loads to model the
isolation of the feedline from the feedpoint as actually occuring at the
feedpoint. An easier way to see the current behavior of an "ideal" BOG and
the currents in effect on RX, then the BOG should be modeled without a
source on it. Place a one foot long vertical antenna at a particular
distance and height a mile away to set the azimuth and elevation of the
incoming signal, and set your power to a megawatt. This is a point source
on 160. Then note the modeled currents in the BOG wire for different
incoming angles/directions and the power across a 50 ohm resistor at the
feed point. That a megawatt on a one foot wire in reality would melt the
wire is beside the point.

You can also see if any value of termination gives you a smoothly
decreasing current on a signal from the back side. What you are looking for
is the minimum voltage at the 50 ohm feedpoint termination.  On a signal
from the front side, you want the maximum voltage to appear at the 50 ohm
feedpoint termination.

EZNEC Pro will do this "plane wave" analysis directly.

As to what to use for ground, lay a 151 foot dipole on ground at your
placement point and obtain the resonance and R reading at resonance. Monkey
with the ground constants to get a modeled 151 foot dipole on ground to get
the same resonance and R reading. It sounds like you already may have done
something like this.

You may not be able to do that, because for a BOG as with any 160 antenna,
nearby conductors, including buried conductors like water and gas pipes,
can "pull" actual results way off the monolithic, uniform ground in NEC
based programs.

Incidentally, a 364 foot BOG on 160 is in the magnitude of twice as long as
needed for ideal results. Some times wire your length has been reported
with a reversed pattern. 364 divided by .67 gives an electrical length of
543 feet or two halfwaves. The problem is the slow propagation on the wire
vs. full speed of light incoming.

73, Guy K2AV



On Mon, May 9, 2016 at 6:25 PM, K4SAV <RadioXX at charter.net> wrote:

> Before you put a lot of faith in modeling a BOG, you should look at the
> data below.  I measured the current along a 364 ft BOG and compared it to
> what NEC predicts.  On 160, NEC is in error by 400% in some cases.  NEC is
> in error on 80 by 1340% in some cases.  Since NEC uses the current in the
> segments to predict the pattern, the predicted pattern doesn't have a
> chance of being correct.
>
> My meter doesn't make accurate measurements below 5 ma.  I would have
> repeated some of the tests to increase the drive current but it was
> apparent that an accurate measurement was not necessary for comparison to
> NEC since the error is huge.
>
> My current meter adds a series resistance of about 1.5 ohms when it is
> clamped on, so that should not contribute any error in this system.
>
> EZNEC model of this 364 ft BOG showed an RDF of 10.6 on 160 and I even
> developed a method (with simulation) of increasing the front to back by 20
> to 25 dB.  Unfortunately, in light of the measurements, I think this falls
> into the kidding yourself category.  Incidentally, that 20 to 25 dB
> increase in front to back only improves the RDF by 0.11.  The BOG actually
> worked pretty good on 160 when comparing it to another receiving antenna
> that has an RDF of 11.15.  The reference antenna has much better front to
> back and side rejection.  The BOG had just a little less performance on
> average but sometime it was equal.
>
> I am now struggling with obtaining a method of predicting BOG
> performance.  I have been unable to play with any of the parameters in
> EZNEC to get it to predict my measurements.  Since NEC can't do it, the
> only thing left is experimental and that is not going to give the pattern.
> All you can get easily is front to back at very low angles, which is likely
> to be very misleading, and certainly not an indication of better RDF or
> lower noise (reference the particular example above of a 0.11 increase in
> RDF for an improvement in front to back of 20-25 dB).  Comparison to a
> reference antenna seems to be the best, but sinking and pulling ground rods
> is not fun, and obtaining experimental data is a slow process.
>
> K2AV recently posted a couple of interesting topics on BOGs, one on Apr 15
> and another one today.
>
> Jerry, K4SAV
>
>
> DATA:
>
> Current measurements on a 364 ft BOG. Height above ground estimated to be
> 1.5 to 2 inches, lying on the top of short grass mowed just before the
> test, dry high ground, red Alabama clay.
>
> Load data taken at the end of the BOG:
>
> Freq MHz     Source ma     Load ma     EZNEC  predicts ma
> 1.84                 120                 25                  99
> 3.52                 150                 5                    67
> 7.01                 150                 <1 22
> 10.11               130                 <1 9.9
>
>
> Measured at the 212.25 ft (58.3%) point from the source:
>
> Freq MHz     Source ma     58.3%, ma     EZNEC predicts ma
> 1.84                 120                 70 102
> 3.52                 150                 73 71
> 7.01                 150                 35 44
> 10.11               130                 15 2.7
>
> I also measured the velocity factor of a wire in the same place where the
> BOG was.  On 2.25 MHz it was 0.67.  At a second place the measurement was
> close to the same.
> _________________
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>