[TowerTalk] Mismatch loss and tuners

[DavidC]

> David, I'll discuss your last question first. I don't know what you mean
by a
> '20:1 choke point' at the antenna feed point. If you mean a 20:1 mismatch
at the
> feed point, then the answer is no, there is NOT a profound real-world
loss when
> a low-loss feed line, such as open-wire, or ladder line is used with an
antenna
> tuner. The actual loss in such a case is so small that the person at the
> receiving end cannot distinguish between it and a case where the SWR is
1:1.

Thanks for trying to straighten me out here!  I am an inadequate Extra!

When the feedline, or anything other than the antenna radiates, is that
considered loss?  

Is it not common that when a tuner conpensates for a high vswr at the
antenna that it turns much of an open line feedline (or other feedline)
into part of the radiating antenna?

> David, reflected power not being lost power is not a paper theory. It is
> measured every day with SWR indicators that indicate forward and
reflected
> power.
> If the SWR indicator is in a line with a mismatched termination of 2:1,
> the SWR is 2:1, and If a transmitter were first adjusted to deliver 100
watts
> into a matched line we know the SWR is 1:1, and forward power of 100
watts will
> be indicated. If the line is now terminated with a 2:1 mismatch the
reflected
> power will be 11.1 watts. When the antenna tuner is adjusted to show a
1:1 SWR
> at the input, the impedance at the input will be 50 ohms resistive, which
> indicates that no reflected power is traveling rearward past the input of
the
> tuner. The reason is that, when the tuner is so tuned, it totally
re-reflects
> the reflected power, adding it directly to the power from the
transmitter. Thus
> the total forward power is now 111.1 watts, which will be so indicated on
the
> SWR-power meter. When the meter is reversed to read reflected power it
will
> indicate 11.1 watts. Conclusion: forward power is equal to the source
power plus
> the reflected power. The values of power I've used here are
demonstrative, but
> are overly simplified, but I will explain how the real values are
obtained
> later. (We'll discover that the difference is small.)

OK, but ... the meters tell us what they see at the moment the energy
"checks in" at the meter coupler point in the system as it is running back
and forth up and down the line, but can the meters tell us for certain if
the energy is being transformed into heat somewhere else in the system,
radiated by the feedline, or radiated by the antenna?
 
> David, using a pipe containing water is an invalid analogy, because water
can't
> flow in two directions in a pipe.

So then it becomes compressed and then looks for a weak point to break out?
 Is that what RF does?  Like lightning?

> As a result the power reflected at the mismatched antenna adds to
> the source wave at the tuner, only to be reflected again at the mismatch,
> leaving the 100 watts delivered by the transmitter to be absorbed and
radiated
> by the antenna.

This is troublesome for me to follow.  The antenna has not changed, why
would it suddenly become any more conducive to radiating than it was at
first?  I agree that some of the signal as delivered back to it (due to the
reflection from the tuner) will be radiated (as before) but it seems
logical that the same percentage (not total quantity) will again be
reflected.  I don't see yet how a tuner located at the far opposite end of
the feedline from the antenna can cause the antenna to suddenly be properly
matched (it seems that the tuner instead turns the entire feed system into
a resonant system ... however good or poor that may be).  I would
understand if we were talking about a remote adjusted tuner right at the
antenna feedpoint ... how is the feedline rendered invisible in the
equation that the tuner "sees".   Did you assume that I knew something I do
not that is critical to your process of thought?  (Sort of like when I rush
someone through explaining how to do something on a computer?)

> In other words, the difference between the values
> we obtain in the real world and those I've presented here are so small
that they
> could not be detected at the receiving end.

I understand that a 10% loss would be generally imperceptable at the
receiving end, but what if, as I have hypothesized, far more than 10% is
actually improperly radiated in the feedline (forgetting loss in the tuner,
connectors, switches, feedline) -- would not this loss be imperceptable by
the meters -- don't they "assume" that energy "gone" from the system is
energy radiated only by the antenna?  (not necessarily a true reading)

> To respond to your suggestion that we have a perpetual-motion machine,
let me
> remind you that all the power that is absorbed and radiated by the
antenna comes
> from the transmitter, with only insignificant losses in the tuner and the
> transmission line. That is why we buy the lowest loss line possible, and
keep it
> as short as possible.

It was a poor illustration upon reflection, sorry.  I was highlighting the
reality that we have to live with conneectors that cause loss, tuners that
cause loss, switches that cause loss, and feedlines that cause loss.  I am
defining loss as the transformation of RF energy to heat, unintended
radiation, and additional points of vswr ... thus creating multiple points
of energy cycling ... adding to vulnerability to additional system loss.

I don't intend to be contentious, it is just that several explanations have
been offered here for this process and all seem to rely on assumptions that
have not been measured (to my knowledge) using field strength meters that
would prove the antenna is actually radiating the energy (or not) and/or
that the other measurement systems are misrepresenting (or correctly
representing) what is truly occurring.

-- 
Respectfully submitted, DavidC  AA1FA

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