Topband: Balun for a balanced tuner ?

[Jim Brown]

On Tue, 08 Apr 2008 18:48:37 +0200, Martin wrote:

>I plan to build a balanced tuner and am not sure what balun to 
use.

Martin,

I suggest that you study the sections of my tutorial that address 
antenna balance and the section on common mode chokes (commonly 
called "current baluns."  

http://audiosystemsgroup.com/RFI-Ham.pdf  

If you think that putting twinlead on a dipole will make it 
balanced, you are badly mistaken. Nearly all ham antennas are 
UNbalanced by their surroundings (trees, buildings, sloping 
ground), so there will be common mode current on a symmetrical 
feedline. Twinlead has significant leakage flux, which causes lots 
of heat to be dissipated in a common mode choke wound with any 
form of twinlead (including a tightly spaced enameled or insulated 
pair). This makes it difficult to put a common mode choke on a 
balanced feedline without significant loss and the associated 
heat. 

This IMbalance causes two problems. First, it puts transmitted RF 
on the feedline as common mode current. That puts RF in your 
shack, and the feedline radiates. The second problem is that 
antennas work in reverse -- that is, noise picked up on the 
feedline as a common mode current couples to the antenna. A good 
common mode choke AT THE ANTENNA blocks common mode current, 
solving both problems. 

73,

Jim Brown K9YC