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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