Topband: Bead balun waterproofing

[Jim Brown]

On 8/19/2013 4:50 AM, Jim Garland wrote:
> In
> other words, I believe a common mode inductance always reduces the common
> mode currents along the feedline, as one can easily visualize by drawing out
> the equivalent circuit.

Jim,

Drawing the equivalent circuit is always a key to understanding, but we 
must first get that circuit right. The common mode circuit is an 
unintentional antenna, formed by the feedline, the intentional antenna 
to which it is connected, and the connection to the transmitter (and 
from there usually to the earth).

Calling a transmission line "balanced" does not MAKE the system balanced 
-- mother nature determines that, and she does so on the basis of the 
balance of the impedances in the system. Parallel wire line still can 
carry common mode current if the circuit has imbalance -- many (most?) 
ham antennas are unbalanced by their surroundings, such as buildings and 
other conductive elements, trees, ground slope, even slope of the 
antenna itself. The difference in the equivalent circuit for parallel 
wire line is that the connection is to both sides of the antenna rather 
than one, and the Z between the feedline and earth may be higher 
depending on how it is coupled to the transmitter.

Also, because the common mode circuit is part of the antenna, and 
because antennas do not require a connection to earth to work, depending 
on the length of the feedline, there could still be substantial common 
mode current on parallel wire line even if the Z to earth was high.

W1HIS did a great job of pointing out the importance of high choking Z 
on a feedline to minimize noise pickup, correctly observing that while 
500 ohms of choking Z might be enough to prevent noticeable pattern 
distortion, it is not nearly enough if you're trying to dig a weak 
signal out of the noise, where a few dB matters. Not everything in his 
app note is correct, but he definitely got that right.

73, Jim K9YC