On Sun, 26 Aug 2007 07:42:50 -0700, W6SX Hank Garretson wrote:
>I suggest that the most important is to make sure the dipole is
>indeed "completely self-contained." If you are using coax feed, you
>need a balun at the antenna feed point or you will have current on
>the outside of the coax and the antenna will not be balanced.
Good advice, Hank.
Also, it is important to realize that there are other sources of
imbalance in antennas besides using coax to feed them. Imbalance is
defined by impedance, including the coupling between two halves of the
antenna and surrounding conductive objects -- trees, the earth, towers,
buildings, other antennas. Virtually any antenna that a ham erects is
likely to have some imbalance, even if fed by balanced line, and that
imbalance will cause common mode current to flow on the feedline.
So a common mode choke is important for ANY antenna, not only an antenna
fed with coax. From the viewpoint of decoupling the antenna from the
feedline, coax is a BETTER feedline for balanced antennas that balanced
line. That's because the field carrying the transmitter power to the
antenna is contained entirely within coax, whereas there is considerable
leakage flux (about 30%) surrounding twinlead. This fact means that the
core material for chokes wound with twinlead must be rated for at least
30% of the transmitter power to avoid overheating. Not easy if you're
running high power, and it makes the ckoke design FAR more difficult.
By contrast, common mode chokes for coax see ONLY the unbalanced current,
and if you make their impedance large enough, that current approaches
zero, so the dissiplation approaches zero. A MUCH easier design problem.
For more, see my tutorial. http://audiosystemsgroup.com/RFI-Ham.pdf
73,
Jim Brown K9YC
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