[RFI] Auto quieting
jim at audiosystemsgroup.com
Sun Oct 11 12:39:23 EDT 2015
On Sun,10/11/2015 5:51 AM, Howard Lester wrote:
> What am I missing here? Zip line is not coaxial; it is "balanced" line and
> requires a transmatch. With a simple transmatch you get the SWR down to 1:1
> and you're all set. ?
What you are missing is the fundamental physics of how transmission
lines work. The SWR is determined by the match between the antenna and
the line, not between the transmitter and the line. Second, the major
problem is not mismatch, it is the high loss in zip cord at RF. The
mismatch adds slightly to that loss.
You are also missing the fact that "balance" in a system is defined by
equal impedance to the reference plane, usually earth or circuit common.
Parallel wire line is not "balanced," it simply does not CAUSE
imbalance. There's nothing inherently bad about imbalance in an
emergency situation, but burning half you TX power when you're starting
out with low power CAN be a problem.
I don't object to saying that we CAN use zip cord as an improvised
feedline. I DO object to calling it a GREAT solution. That part is urban
myth, and it's wrong.
73, Jim K9YC
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