[TenTec] Model 238C Tuner Questions
k9yc at audiosystemsgroup.com
Tue Oct 16 20:57:32 EDT 2012
On 10/16/2012 5:32 PM, Robert Mcgraw wrote:
> The feed impedance on my 160M 1/2 wave wire is about 38 ohms as I recall.
Some clarifications. First, feedline loss is directly related to
feedline current and conductor resistance, and is determined both by the
feedline impedance and the SWR. What gives open wire line it's low loss
characteristic is it's IMPEDANCE, NOT the fact that it is balanced. Loss
is lower in high Z line because for a given power level and the same
conductor size, because the current is lower.
Your 38 ohm feedpoint impedance determines the degree of mismatch to
your transmission line, but the current varies along that line as a
function of the electrical length of the line. That mismatch will
increase the loss.
You may not "feel good" about connecting coax to an antenna you think of
as balanced, but that coax is a MUCH better match to the antenna than
450 ohm open wire line, the loss may turn out to be lower than in your
badly matched open wire line, and with a decent common mode choke, the
dipole will BEHAVE as a balanced antenna. This is the sort of thing that
SHOULD cause us to go to the computer and do the math to figure out
which is better.
And, of course, we should consider the antenna, feedline, and matching
box as a SYSTEM, because that's what it is. How are we providing
lightning protection for the equipment connected to it? How does it come
into our building? How noisy is it on RX? All of these factors are
important when deciding what antenna to put up and how to feed it.
73, Jim K9YC
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