I have been looking for information on BOGs and have not been able to find any
info on measured impedance values. Other than they are low, depends on ground,
etc. I thought I would relate my recent experience.
I had tried a couple Beverages in the past at a salt marsh location and did not
have much luck. Ground too good, some say. I doubt that was the major problem.
May never know for sure. Last year I tried one at a new location ( 50 miles
inland from my salt marsh QTH) and was not impressed. However, I used unknown
wire and my old beverage transformer. The unknown wire should have worked
anyway. I now suspect my old transformer had design or construction problems!
This year at the suggestion of a friend, I simply ran a wire, on the ground,
about 220 feet in the woods. The 220 was chosen due to the property line and
other grass cutting factors. Since I could not find the old transformer, I
wound a new one. The new one was the same design as I had used in the past,
but was the same core. I got BOG up, I mean down, in time for the CQWW CW
contest. WOW! it was all I used on 160, almost all I used on 80 and I even
used it quite a bit on 40.
For the ARRL 160 contest it was used almost all the time. I probably spent too
much time switching back and forth just to see and hear the difference. I had
run it unterminated directly away from Europe. The strength of the signals from
Europe was fantastic. No pre amp needed. W5s were strong too,naturally with it
un-terminated.
This week, I ran a feedline out to the far end (SW end). I also received a
package of BN-73-202 cores and wound a 4:1 transformer. I figured that may be
in the ballpark. The one I wound in November that worked so well had a Z ratio
of 3:1 and on an old FT-140-43 core. The old one (which maybe never worked
right was a 9:1 ratio) Last night I worked eight stations from Europe, all
very good copy on the BOG. I also noticed that when I switched from the
transmit antenna to the BOG, unlike the European stations, 6Y5WJ disappeared.
So there is some directivity. More BOGs are planned before the CQ 160 contest!
So far I have not had time to play with the termination resistor or experiment
with any other transformer impedance ratio. My old MFJ seems to be close on
reading SWR but is off on the actual R value.
With my BOG terminated with 160 ohms, I am reading an SWR of 1.9 across the 160
band. The MFJ 259B is showing no reactance. That is telling me that the MFJ is
seeing either 25 or 100 ohms. I also am using close to a half wavelength of
coax to the feed point of the BOG, maybe more. If that is the case the
other side of the 4:1 transformer must be seeing 400 ohms or 100 ohms. From
what I have read, the 400 is probably too high for a BOG. The 100 ohms seems
more likely.
My intention is to wind a 2:1 transformer (Z ratio not turns ratio) which, if
my guess is right, will transform 100 down to 50 and be a good match for my 50
ohms coax, thus the MFJ would give me a 1:1 indication. Should I dare hope for
such results?
Merry Christmas to all and Good DXing on Topband.
73,
Don
N4DJ
n4dj@me.com
n4dj.com
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Stew Perry Topband Distance Challenge coming on December 29th.
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