[TenTec] Station setup

[Jim Reid]

Roy, the following is long,  but I felt necessary to
explain the "why" of the correct set up. 

> Rig +short coax+ watt meter +short coax+ tuner +long 
> transmission line+ Antenna.

The above is the correct set up.  And,  your
rig includes your linear,  if you have one.

The tuner is nothing more,  nor less than an impedance
matching device.  So the input Z to the tuner at it's
input terminal will be at/close to 50 ohms at the frequency
of use;  then the rig output of 50 ohms,  and the watt
meter input and output Z of 50 ohms are all matched
(note it your rig output Z is not exactly 50 ohms real
and no reactive component,  then this value will not
be exactly 50 ohms,  but it will be at the correct
value to correctly match the impedances throughout
the output "antenna system" diagramed above).
The forward reading wattmeter will be most accurate
with this positioning.  Were you to place the wattmeter
at the output side of the impedance matching tuner,
it almost certainly would not see 50 ohm pure R
at both it's input and output terminals,  and it would
not read the true forward power of you rig being delivered
to the antenna input terminals anyway;  it would read,  in
the forward direction both the rig's forward power and the
re-reflected power back up the transmission line originally
reflected at the feedline/antenna input terminal  which is
causing the vswr which continues to exist on the long
transmission line to the antenna.

To the extent that the tuner itself has low internal loss 
(and the greatest loss is ALWAYS within the inductor,
so insure your tuner has an L with LARGE conductor
windings and is air core) and the feedline is low loss, then
the entire antenna system,  after the tuner is correctly
adjusted, and will be conjugately matched.  Why?  Because,
a total lossless network,  which is brought to Z match
between any one of the network components (rig output
network,  wattmeter,  tuner,  feedline and antenna itself)
will be "conjugately" matched and ALL the output power
from the rig will be input to the antenna feedpoint
terminals.  To be conjugately matched simply means
that if at any point in the entire system set up,  you cut
the feedline/transmission coax or ladder line,  you will
measure,  looking in either direction (back toward the rig,
or out toward the antenna) the value for the real part
of the Z,  and equal but opposite sign values for the
imaginary (or reactive) part of the Z.  And on the tuner
output side of the system,  the antenna feedline side,
you probably will not measure 50 ohm real part,  
but some other R + jX = Z which accounts for the VSWR 
which is on the transmission line because of the Z mismatch 
between the coax or ladder line and the antenna input Z
using only reactive elements.

And that is why we use these antenna tuner impedance
matching units in our stations.  They give us the ability
to QSY about the bands and "tune"  the entire antenna
feed system so that ALL of our rig output power is
input to the antenna input terminal,  and only less
a few tenths of dB loss within the tuner and within
the resistive part of the feedline.  Unless the line
vswr is very high,  4, 5 or higher,  the line loss is
going to add only tenths of an additional dB from the
multiple vswr reflections within the long transmission line
up to the antenne element itself.

Unless you have a very lousy,  lossy tuner,  and poor
coax in your long transmission line,  you are not going
to lose more that a total of a dB or so from your output
power when using a tuner in such a system.  The guy on
the other end,  be he far away DX station or not,  will
never hear the difference from you signal with such a
set up as described here,  and a perfect set up using
an absolutely matched antenna and zero loss feedline
system.

By the way,  if you know the Z of your antenna system;
that is,  at the end of the transmission line in your shack
that you are going to "screw"  onto your tuners output
terminal (and you can measure this with an MFJ-259B,
Autek or other antenna system analyzer),  you can input
this number along with your frequency of operation,  and
even your tuner component max values (if you have them
from the tuner manufacturer) into a tuner simulator set
up at the University of Chicago:

http://fermi.la.asu.edu/w9cf/tuner/tuner.html

This site was set up by Kevin, W9CF.  If you don't know the values
of you tuner C's and L,  then just accept the default values.
You will note,  that for highest tuner efficiency,  that is lowest
loss,  either the input C or the output C is to be set to its 
maximum/highest capacity value.  The simulator output tells
you exactly how to set up your tuner for the particular frequency
and antenna system Z value for best performance/highest
efficiency.

I have actually measured the Z values on both sides of my
tuner,  and to the extent that my tuner has some internal
loss,  I have actually measured the complex conjugate Z's
to be equal and conjugate,in each direction on my long coax 
transmission line. Walt Maxwell is correct!  And his 2nd Edition 
of "Reflections" which should be published and available from 
Worldradio within a very few more months,  well explains
the theory behind all of this,   hooray!  Further Steve Best
has been publishing in QEX his analytic analysis of these
impedance matching antenna tuners.  Other than the fact
that he and Walt disagree on the "why"  of it all,  for example,
see the current edition of QEX, Sept./Oct. 2000, Letters
to the Editor,  pgs. 60 thru 62,  they do come to the  same
conclusion.  I took some of the system data measurements
for Steve last Spring.  Steve presented some of his
analysis during the Antenna Forum at Dayton in May.

Don't get tangled up in their arguing in QEX about the "virtual
short" at the output of the tuner box,  just use and enjoy
the results of tuner use.  All so clearly enunciated some
years ago by Lew McCoy,  bless his resting soul,  and
his wonderful writings about these tuner boxes.  And the
tuner is not "fooling"  your rig;  it is helping the entire system
set up to operate at the highest possible efficiency under
the given circumstances,  by bringing about a conjugate
match within.

Good luck and 73,  Jim,  KH7M


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