Topband: BOG data

[K4SAV]

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.