Topband: BOG Antenna Measurements

[Don Johnson]

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 at me.com
n4dj.com