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
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