Cecil Moore wrote:
Ian White GM3SEK wrote:
If you measured the impedance of that incorrect antenna, and then
replaced the antenna with a dummy load of the same impedance (a
resistor of the correct value, in series with an inductor/capacitor
of the correct value) then your transmitter will not know the difference.
It is true that transmitters are dumb as a stump. However,
a human being should be smart enough to realize that the
virtual impedance, which is only a voltage to current ratio
has been replaced by an impedor with a resistor, inductor,
and/or capacitor.
The impedor *causes* the load conditions. That virtual voltage
to current ratio is a *result* and not the cause of anything.
At the terminals of the load, both the voltage and current are
physically real and physically measurable, as also is the phase angle
between them. Their ratio is the (complex) load impedance as seen by the
transmitter.
Any device that creates those same electrical conditions possesses the
same impedance; by definition.
The transmitter affects the magnitude of the voltage and current in the
load, but it categorically does NOT affect their ratio, or the phase
angle. In other words, the transmitter has no effect on the value of the
impedance that is connected to it as a load, That value is created
exclusively by the load.
To get down to the actual cause of the conditions, the human
being needs to know whether the load impedance is virtual or
not.
I can see your underlying point, about the difference between a lumped
impedance physically present at the transmitter output terminals, and an
impedance created by 'action at a distance' through a transmission line.
But if both kinds of load create the SAME steady-state voltage:current
ratio and phase angle at the transmitter output terminals, then by
definition they both have the SAME impedance, and the transmitter will
respond in EXACTLY the same way. There is no steady-state measurement
you can possibly make on the transmitter than can tell the difference
between those two different kinds of load.
That principle is absolutely fundamental. It underlies all steady-state
impedance measurements using bridges, network analysers etc. Regardless
of the nature of the DUT (device under test), you connect it to the
meter, measure what you find, and that IS "the impedance of the DUT".
The differences only appear if you change frequency, or if you make a
time-dependent measurement, but there is never a difference in the
steady state.
Why do you imply that a virtual impedance can *cause* the
conditions seen by a source but deny that a virtual impedance
can *cause* 100% re-reflection? Seems a contradiction.
In fact, virtual impedances cannot cause anything. The
voltage to current ratio associated with a virtual impedance
is a *result* of something physical. Choosing to ignore that
physical "something else" cause has gotten lots of folks into
logical trouble.
I invite you to consider another possibility: that the people who have
chosen to stick with the established textbook analyses are not ignoring
anything, and they are in no kind of logical trouble because those
analyses are both logical and consistent; and that the only person in
logical trouble is actually yourself, because you are making
distinctions between different varieties of impedance that do not exist.
In the huge majority of applications, both amateur and professional,
it IS possible to separate those two topics cleanly and completely. It
seems perverse to tangle them together unnecessarily.
It seems perverse to say the antenna system can be replaced
by a resistor and inductor or capacitor and nothing changes.
How about the radiation pattern? Does that change?
Nothing changes in the part of the system I was talking about, namely AT
the transmitter/load interface. (Lord, gimme strength...)
It should be absolutely no surprise that, when summed to an infinite
number of terms, this series produces exactly the same results as the
steady-state model - exactly the same pattern of standing waves, and
exactly the same load impedance presented to the transmitter.
How about the total energy in the steady-state system? The
number of joules pumped into the system during the transient
state is *exactly* the amount required to support the forward
and reflected power readings.
If you say so; but nobody else feels the need to calculate those
quantities.
The important conclusion from this more detailed time-dependent
analysis is that re-reflections at the transmitter have NO effect on
the final steady-state pattern of standing waves.
This is based on a rather glaring rule-of-thumb assumption,
that any standing wave energy dissipated in the source was
never sourced to begin with. Born of necessity, that is a
rather rash assumption. Thus some people sweep the reflected
energy dissipated in the source under the rug and forget
about it, hoping that nobody ever lifts the rug and points
out the conservation of energy principle.
All valid solutions to the problem of AC/RF generators, transmission
lines and loads will most assuredly comply with the conservation of
energy! But countless textbooks show that it isn't necessary to invoke
that principle in order to make a valid analysis.
I await the inevitable photon explanation.
None needed. If anyone wishes to introduce additional complications
where none are necessary, then of course they're at liberty to do so.
But when invited to join in, everyone else is at liberty to decline.
Optical physicists did not have the
luxury of dealing with voltages. As a result of dealing with
power densities, they learned a lot more than RF engineers
know to this very day. Optical physicists have never asserted
that reflected waves are devoid of ExB joules/sec or that
EM waves are capable of "sloshing around".
But WE DO enjoy the luxury of having complete information on voltages,
currents and phase angles, at any instant and at every point along a
transmission line. That allows us to obtain complete solutions without
dragging in unnecessary concepts from other disciplines.
--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek