Models of BOGs in NEC 4.2 show less sensitivity to termination resistance
than to electrical length. Practically the far end ground will exhibit a
stubborn variability that makes any accuracy in the termination approximate
at very best.
Some have used a plain ground rod at the far end (no resistor) and adjusted
the length using the DOG at 1140 kHz method which adjusts for the wildly
varying velocity factor over wildly varying ground composition. The QRN
front-to-back null method should deliver the same results, with some
caveats about picking a day with correct conditions for the test. See
below.
The typical 450 ohm termination resistors (available from DX Engineering
and the like) in parallel configuration to produce 225 ohms do nicely. BOGs
are hugely susceptible to induced current from close lightning strikes
(close equals very roughly 1/4 mile or less) and blow termination resistors
and teensy transformers with regularity. Lightning produces strong currents
in the ground just under surface which produces very strong induction
spikes in the BOG wire. Quite more so than in standard beverages.
I have yet to hear of blowing up eight of the 450's in series parallel to
get overall 225 ohms, but I probably should not have said that out loud.
I also wind BOG transformers with teflon sleeved #14 double polyimide
insulated copper (Wireman #635 and others). The usual teensy wire used to
make the enclosure and core small and cheep just evaporates. A BOG
practically speaking is not a good application to go cheep for lightning
concerns. The #14 will not evaporate from I squared R heat, and induced
voltage will not arc across and carbonize through the 10 to 15 kV Teflon
sleeved pdouble polyimide, particularly not wire to wire which is 20-30 kV
insulation. This is the same wire I specify for winding FCP isolation
transformers, now well-proved to withstand QRO RF, wind static, and
lightning-induced pulse voltages.
The procedure of adjusting BOG length for heard front-to-back speaks to the
variability in electrical length noted by myself and other sources.
An important caution is that one must know where all the lightning is
within a three or four thousand mile radius. One cannot do this with radar
based displays. Impulse based detection is required. In many cases there
are storms quite well out over ocean that will not display on radar based
maps. The following are excellent displays that show strong QRN sources
well out to sea that could completely bolex a front to back adjustment.
http://www.blitzortung.org/Webpages/index.php?lang=en
http://www.blitzortung.org/Webpages/index.php?lang=en&page_0=30
A severe caution -- attempting to make such an adjustment for a 160 meter
BOG by listening on a band OTHER THAN 160 is doomed to failure. A BOG is a
single band antenna, and any usefulness on other bands is purely a gift
from the tooth fairy based on aspects of local ground composition.
It would be exceedingly interesting to get a BOG adjusted for front to back
by minimizing QRN, then temporarily converted to a DOG and measured for the
primary resonant frequency.
I suspect that they will exhibit a narrow bell curve about a frequency
around 1100 to 1200 kHz in the broadcast band. This would confirm ground
composition based highly variable velocity factor in wires in/on/just above
earth.
Presuming this applies, one needs to perform the QRN based adjustment with
generally moist (not sopping) earth below (excepting arid climates). One
can always soak the BOG length with a hose to moisten things, but the
reverse always requires an obedient dry spell to accomplish.
Another item is that a BOG should really not be done with delivered signal
strength in mind. The output voltage decreases with nearness to ground, but
that nearness is where the best rejection of local near ground vertically
polarized noise. Expect to need a preamp (external or in the rig), even if
you are pleasantly surprised. Don't raise the wire to make the signals
louder.
73, Guy K2AV
On Saturday, June 13, 2015, K1FZ-Bruce <k1fz@myfairpoint.net> wrote:
>
>
> 2015 BOG antenna notes update:
> http://www.qsl.net/k1fz/bogantennanotes/index.html
>
> 73
> Bruce-K1FZ
> _________________
> Topband Reflector Archives - http://www.contesting.com/_topband
--
Sent via Gmail Mobile on my iPhone
_________________
Topband Reflector Archives - http://www.contesting.com/_topband
|