The reflected wave combines with the incident wave at the source
such that the source sees a mismatched load. Whether the load
impedance presented to the source is high or lower than Zo depends
on the phase of the reflected signal relative to the incident signal at the
source point.
In some sense the reflected power reading on the meter is fictitious.
It doesn't necessarily imply that X number of watts of power are
being reflected back to the source where they are dissipated. It
merely indicates the degree to which the load impedance presented
to the source departs from 50 ohms. The actual power dissipated
by the source could actually be lower for the mismatched case than
for the matched case depending on the magnitude and phase of the
load impedance scene by the source.
When looking at the steady state case (the voltage and current
levels that establish themselves within a few microseconds after
the transients reflections and re-reflections have settled out), it is
sometimes easier to think in terms of a balanced bridge model
of the wattmeter rather than the directional coupler model. In this
view of things, the reflected power reading is just a measure of
how much imbalance exists in the bridge. Here the reflected
power reading is merely a measure of the impedance mismatch
between the load and the characteristic impedance of the meter
rather than a true reflection (pun intended) of the amount of power
flowing back into the source. This reflected power reading is
however, equivalent to the amount of power actually reflected
from the load when the leading edge of the RF waveform
envelope first encounters the load.
Of course my explanation is a bit of a handwave. Steve Best has
treated the subject thoroughly in recent issues of QEX. I think
Walt Maxwell also covers the subject in his book "Reflections".
73 de Mike, W4EF...........................
real
----- Original Message -----
From: "Gary Schafer" <garyschafer@attbi.com>
To: "Paul Christensen" <w9ac@arrl.net>
Cc: "Phil Clements" <philk5pc@tyler.net>; <2@mail.vcnet.com>;
<wb8jkr@juno.com>; <amps@contesting.com>
Sent: Tuesday, March 26, 2002 7:11 PM
Subject: Re: [Amps] Re: [Amps] Bird® 43 Manual
>
>
> Paul Christensen wrote:
>
> > > If the power reflects back and forth then that portion would never
reach
> > the
> > > load would it?
> >
> > Correct. In the case of a 50-ohm resistive source (e.g., a solid-state
> > transmitter without a pi-network, ATU, or other reactance network),
> > reflected power created by a line-to-load mis-match is absorbed by the
> > source (The PA in the transmitter), less a portion of the reflected
power
> > absorbed by line losses.
>
> If all the reflected power was absorbed by the transmitter PA then there
should
> be no increased forward power reading on the bird meter whether there is
> reflected power or not?
>
>
> >
>
> >
> > > Doesn't a bird meter read higher forward power than there really is
when
> > > there is reflected power.
> >
> > Yes.
> >
> > > So subtracting the amount of reflected power from the forward power
> > reading
> > > would not give you the amount of power reaching the load?
> >
> > Someone keep me honest here with regard to the mechanics of a
directional
> > coupler: Notwithstanding line losses, the amount of reflected power
shown
> > by a Bird Thru-line Wattmeter is the amount of power reflected solely by
the
> > line-to-load mismatch. The mismatch creates a 180-degree reversal of
the
> > reflected *wave,* with respect to the forward wave at the juncture of
the
> > line and load.
>
> I think that it would only be 180 degrees if it was an infinite mismatch.
(short
> or open) Otherwise it would be something less or greater.
>
>
> >
> >
> > Whether the source is purely resistive, which absorbs all reflected
power
> > (less power lost in the line), or if the source is reactive as in the
case
> > of a pi-network or internal ATU, the Bird will still see the effect of
the
> > reflection, in both forward and reflected readings, as a result of the
> > line-to-load mismatch. Except for line loss and assuring that the line
> > characteristic impedances matches the charcteristic impedance of the
Bird
> > directional coupler, I do not believe any other portion of the system
has
> > relevancy with respect to the indicated reflected power.
> >
>
> I think what happens at the transmitter end is that the line is no longer
the
> same impedance as the transmitter when there is reflected power. So the
> transmitter will never present a pure resistive load to the line when the
line
> is mismatched at the antenna end. (solid state transmitter)
>
> With a pi network matching system or other type of matching system, If the
> matching circuit is tuned for maximum power out then the final should be
> presenting a resistive load to the line? In this case all the reflected
power
> would end up in the plate of the final if this were true?
>
> 73
> Gary K4FMX
>
> >
> > -Paul, W9AC
>
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