I still don't see no matter how much people try to come up with reasons
to explain it that it could possibly improve things at the antenna
itself. I'll accept that through some magic the imbalance on the line
is matched to the rig but there's no way that the electrical length of
the antenna can be magically changed by turning a few knobs on your
antenna tuner. Imagine a fish tank on a center fulcrum being rocked
back and forth. The water(rf) is going to slosh back and forth from
side to side(rig to antenna or tuner to antenna in this case) and a
little will spill out as radiated RF through loss. An incorrectly cut
antenna is like a water faucet thats clamped down on the end of a hose.
The water cannot get out any easier just because you put a pressure
regulator on the other end.
Jim Reid wrote:
> 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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