I may as well give you the results of my measurements with BOGs and
dipoles on the ground. It seems to be a lot different from the data I
am hearing from other people.
This experimentation started when I tried to correlate the current in a
BOG as predicted by EZNEC to an actual measurement. The results were
worlds apart. It seems the current on a BOG disappears much faster down
the wire than NEC predicts. It really dissipates in a hurry as
frequency as increased. If NEC can't correctly predict the current in
the wire, it can't correctly predict the pattern.
So then I tried a dipole on the ground to try and estimate the velocity
factor of that wire. The dipole was 119 ft total length (because that
is what the length of the piece of wire I picked up was). I left that
wire on the ground for a month and periodically measured it. The
results were highly unpredictable, at least by NEC. I can only
attribute this variablilty to ground moisture variation, although I
never measured it right after a rain. To give you a feeling of the
variability, here are the results of a few of the measurements.
1. F = 2.75 MHz, X = 0, R = 186 ohms, also f = 6.25 MHz, X = 0, R = 522
2. F = 3.37 MHz, X = 0, R = 182, also close to resonance at frequencies
above 8 MHz with R of about 440 ohms
3. F = 3.00 MHz, X = 0, R = 178 ohms, also F = 4.6 MHz, X = 0, R = 710
ohms, also F = 7.0 MHz, X= 0, R = 562 ohms'
4. F = 2.70 MHz, X = 0, R = 167 ohms, also F = 4.5 MHz, X = 0, R = 675 ohms
You won't be able to predict those numbers using NEC. NEC says the
resonant frequency of this dipole should be about 2.8 MHz with R = 97
ohms. You can get some variation in that depend on the ground constants
you select but you will never be able to come close to the measured data
no matter what ground constants you choose. (Actual ground is typical
Alabama red clay with dipole 1 to 1.5 inches above it.) You may be able
to get NEC to give agreement to the feedpoint impedance at the lowest
resonant frequency by selecting a ridiculous ground constant, but
everything above that frequency will be way off. NEC says the impedance
should rise rapidly above the resonant point and and there are no other
resonant points until you get to about 9.1 MHz. So once again, like the
BOG, NEC can't predict the pattern of this dipole on the ground. Degree
of error is unknown.
Incidentally, N6LF's pattern for a BOG computed with NEC4 I was able to
duplicate.using NEC2. So I have no confidence that NEC4 can predict the
pattern of a BOG either.
So what to do? If I can't compute the pattern of a wire on the ground,
the only thing left is measurements. I was also concerned with all the
rules generated by other people for building BOGs because I know that
data was generated from NEC analysis. I duplicated those rules by doing
the same analysis. I also generated some new rules for improving BOGs
generated from NEC but when I implemented that and made actual
measurements, there was no improvement. Thankfully I measured the
results before publishing those rules.
So I set up an experiment using a 366 ft BOG and a 250 ft BOG both
pointed in the same direction (to EU) I spent a month taking data on
signals from all directions, both close stations and DX stations, and
compiling the differences in performance between the two.
A brief summary of these two antennas was that the forward gain of both
was the same and the front to back was the same, at least within a
degree of accuracy that made any practical difference. (It was not
measurable.) That was the same on 80 and 160. The forward pattern of
the shorter BOG was wider so the response at 90 degrees off forward was
stronger. The response for high angle signals was greater for the short
BOG. The shorted BOG has a wider front lobe both in the azimuth and
elevation directions. The response at 120 degrees off forward was
greater for the long BOG. That means the resulting signal to noise ratio
should depend on where the noise sources are located. In general with
low noise sources, which is usually the case at my location, there was
no detectable difference between the two. Both of these antennas played
well on 80 and 160. The shorter antenna was better on 40.
If you do a NEC analysis of 250 and 366 ft BOGs, the beamwidth
difference between the two will be as I measured, although the degree of
difference may or may not be the same. I didn't try to make an actual
dB difference measurement, which is very difficult with over the air
signals. I also can't speculate the error in pattern produced by NEC
when it has big errors in the current in the wire.
I also don't believe the RDF number for a BOG produced by NEC. I can
get an RDF for about 11 for a 366 ft BOG. A BOG is a good antenna but
it's not that good. Many years ago I chose 366 ft based on a NEC
analysis and it has worked well. I will arrive at a better RDF for a
366 ft BOG later but based on crude measurements (comparison with my EWE
array), I'm estimating it's probably between 9.5 and 10. NEC shows that
a 366 ft BOG has 2 dB better RDF than a 250 ft BOG. I don't believe
that either (based on measurements).
An interesting observation was that the 366 ft BOG self terminated at
about 4.5 MHz and the 250 ft BOG self terminated at about 5 MHz. The
antennas were still usable at higher frequencies but probably not as
good as they could be if they were shorter (not tested).
I have done some testing with reversible BOGs also, but all the data
above is for a single direction BOG.
I also compared the performance of these BOGs to my phased EWE array.
The EWEs have a better S/N but not by a huge amount. Pattern for the
EWEs is much cleaner with much deeper nulls. Incidentally, I just
modified my 4 direction EWE array to 8 directions and it will be
interesting to see how that performs this winter.
Jerry, K4SAV
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