A BOG is an antenna that suffers from incoming waves propagating at
the speed of light while propagation along the wire can show a
velocity factor from .45 to .80, sometimes varying wildly with
placement in a single back yard. The velocity factor varies wildly
with placement height above the actual radio ground (e.g. laying on
the grass vs. tucked into the sod) and the composition of the actual
radio ground. Given the low VF, at some length the incoming and
propagating wave are actually cancelling each other and the pattern
will reverse. The BOG is a SINGLE BAND antenna.
Starting with 250 feet of insulated wire, place a BOG in it's
permanent placement and depth. Bend up the ends so they are not
touching anything. Break the center and attach an antenna analyst.
Shorten the BOG equally at both ends until it is resonant as a
dipole-on-ground (DOG) at 1140 kHz. This will be seen as either a
zero-crossing or minimum reactance point. The resistive reading at
zero reactance may be any where between 40 and 200 ohms and varying
significantly as zero reactance is passed, so ***SWR will NOT be a
good indicator for tuning***. You must have an analyst that can go
down to 1140 and measure reactance separately. It usually helps to
note the frequency above and below resonance at the first meter scale
mark and average them, which will be more accurate than the bottom,
which will be a "visually wide" indication.
It is common for a graph of the DOG impedance to NOT be symetrical
above and below resonance.
Reattach and insulate the wires at center.
Terminate the far end with two 450 ohm beverage rated resistors in
parallel to a ground consisting of a pair of right angle 25 foot
buried bare copper radials (length not critical, but keep both sides
the same). At the feed end use the same method except a 4:1 isolated
winding transformer to flooded, ungrounded (in the vicinity) 75 ohm
satellite cable.
You will need a preamp, if one is not provided in the rig.
The mutual coupling between two BOGs is very low, so you can phase
them with good effect.
BOGS of the type above can be entirely buried in a front yard lawn
with some care to waterproofing, making them completely stealth.
73, Guy.
On Thu, Jul 29, 2010 at 1:37 PM, Bruce <k1fz@myfairpoint.net> wrote:
> Have been experimenting with BOG antennas. They are cranky to get going.
> They have a velocity factor of +- .5
> When mine was too long, I was hearing an LU station and everything else was
> 30 db down. I shortened it twice ending up at .5 it started acting
> somewhat like a Beverage. If "way too long" the antenna can reverse
> direction.
> It had a strange reactance bump at 4.5 MHZ and found it was caused by an
> old, in ground, iron water pipe from some bygone era. Sweeping frequencies
> to get the correct termination, I found that the dew on the grass caused a
> some shift.
> Ground conductivity may be a known value, but in some locations it can vary
> along the length of an antenna.
>
> A preamp is a must as the gain is low.
>
> A standard Beverage antenna is a poor transmitting antenna. The on ground
> Beverage would be very poor for transmitting. Talk about ground losses.
>
> Anyone can not have too many receive antennas. Found I could copy a DX
> station on the BOG with an approaching thunderstorm when there was no copy
> on other antennas that included standard Beverages.
>
> Bruce-K1FZ
>
>
>
> Does the panel think hocus pocus,or is there some truth in the design.
>
>
>
> _______________________________________________
> UR RST IS ... ... ..9 QSB QSB - hw? BK
>
_______________________________________________
UR RST IS ... ... ..9 QSB QSB - hw? BK
|