[TenTec] Model 238C Tuner Questions

Jim Brown 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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