On Mon, 30 Nov 2009 11:24:01 -0500, Paul Christensen wrote:
>In many cases, keeping the line to
>load mismatch at less than 8:1 and changing the feedline length to function
>as a variable transformer can make a world of difference when high power is
>used, especially when a commonly available "T" tuner is used.
Yes. It's important to analyze everything between the transmitter input and
the radiation from the antenna as a SYSTEM. That includes the transmitter
output stage, whatever matches it to the line, the line itself, any matching
network at the antenna, and the antenna itself.
Every element of that system can contribute loss, and as Paul has noted, one
element of that system can increase the loss contributed by another element.
Most losses in HF transmitting systems are IsquaredR losses, so avoiding
high values of current throughout that system is an important key to
optimizing the transmitted signal.
The primary mechanism for loss in a transmission line at HF is IsquaredR
loss in the copper, and those losses increase with mismatch (because the
current gets very high at some points on the line, and the loss is squared).
Same deal in a tuner, where losses increase in the resistance of coils and
capacitors. In the extreme, the dissipation can fry those components. While
my Ten Tec tuners are nice tuners, there are some capacitors that fry if you
run high power at an impedance that puts too much current in them. I like
these tuners and own six of them, but one of the things I do is replace some
of the fixed caps with lower loss components.
Another contributor to loss in these systems is a "voltage balun" -- that
is, a transmission line transformer that puts all the transmitteed power in
one or more ferrite cores. When you hook up a parallel wire line to most
antenna tuners, you're running through a transformer inside that tuner. It
can be instructive to open the box and feel that transformer after you've
been transmitting for a while. :) I didn't even need to open the box to do
that with a DX Engineering matching transformer designed to be used with
mobile whips. It got pretty warm with only 100 watts on 30M in my car with
rather intermittent operation.
Another point that it's critical to understand. Open wire lines can carry
significant unbalanced (common mode) currents if what is connected on either
end is unbalanced. MOST ham antennas are unbalanced by their surroundings --
trees, houses, sloping ground -- and an off-center-fed wire is itself
unbalanced. One of the major shortcomings of a system fed with open wire
line is that you can't choke it to kill common mode current (and the RX
noise and "RF in the shack" coupled by common mode current).
73,
Jim K9YC
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