[TowerTalk] Balun question

[K7LXC at aol.com]

Steve, 
I have not taken apart my  Force 12 B-1 baluns but the size and 
shape of mine suggest they  are likely ferrite beads on a coax center. 
Perhaps RG-142 or one of the  others with a Teflon dielectric, a classic 
ferrite 
bead choke balun. I have one  on my 6 element 6m Force 12 yagi. I take it the 
leads are irretrievable (to  reach) to solder back onto the SO-239. 
Others have already  suggested a replacement. I am only a few steps  beyond 
your understanding. The ARRL  Handbook (mine is c2000) says that “A balanced 
antenna should be fed by a  balanced feeder to preserve this electrical 
symmetry (of your  yagi/dipole driven element) with respect to ground,  
thereby 
avoiding difficulties with  unbalanced currents on the line and undesireable 
radiation from the  transmission line itself. Line radiation can be  
prevented 
by a number of devices which  detune or decouple the line for currents 
radiated by the antenna back  onto the line that feeds it, greatly reducing 
the amplitude of such  antenna currents.” 
It goes on to say,”a circuit  that will isolate the balanced load from the 
unbalanced line, while  providing efficient power transfer”… is called a  
balun. 
I believe most yagis tend to  have a lower impedance than 50 Ohms 
(per the Ham Good Book) but  I also understand that height above ground, 
element sloping, the  presence of other parasitic elements all affect  this. 
So we are not talking about  “transforming” one impedance to another in this 
 case 
AND choking coax jacket  currents. 
The book shows a bead balun  attributed to M. Walter Maxwell, W2DU. If I 
understand the theory  correctly a bead “choke” balun would have most of  
the 
advantages of a wound  toroid-type balun without the inductance and stray 
capacitance that Jim, K9YC,  describes since there is no “coax outside the 
core” except at the far ends  where the coax enters and leaves. 
He does point out in his  tutorial that you do end up using a lot 
of ferrite (cost) for this  (luxury?). 
The multiple turns in fewer  cores is a lot more efficient use of the 
ferrite. 
The limiting factor is  getting coax that can wind many times in the  largest 
(2.4 inch) cores, not  exceeding the minimum bend radius and compromising the 
  
dielectric but has the specs  to work at up to 1.5KW (in some cases or more…) 
at the frequency in  question. Oh, and the jacket must be protected if  it 
cannot handle the UV. Or you  can use larger coax, a few more cores with 
fewer turns as Jim  suggests. 
So I would say you are  pretty much “right on”. It then becomes a question 
of whether you want to make  an easy to string but expensive bead balun or 
a more efficient (cost and  perhaps size-wise) and a bit more complex to 
make balun with larger cores  and multiple turns. 
I hope I have it basically  correct. I have to keep reading my several books 
on Baluns, Un-uns, and  transmission lines to keep trying to understand and 
hopefully absorb more info.  For those of us who were not formally educated 
in this the books and a  little osmosis are it (with the expert guidance 
from those on this  reflector). Jim’s tutorial is excellent and I will  be 
winding more inline chokes  with more turns on fewer cores for our 
multi-multi 
contest station, though we  have been using a long bead balun on our 160  
vertical. 
Good luck! Still learning (I  hope). 
Aloha, 
Kimo Chun,  KH7U



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