The reason your tuner doesnt melt is because the reflected wave is not
absorbed by the tuner, its re-reflected and conserved to be absorbed by
the load. Not the tuner or the transmitter finals.
One must remember the tuner is a non-dissapative device that
intentionally introduces another mis-match on the line. The 1:1 match
seen at the tuner is a result of mutual cancellation to two complementry
mis-matches. One wave is from the load mis-match the other is from the
induced tuner mis-match. The two mis-matches reflected waves combine at
the tuner and are of equal magnitude and opposite phase. This
cancellation caused by the difference between the two reflected waves
basically presents an open circuit which prevents the reflected wave
from travelling any further down the line. Like the finals load
resistance. Now because the reflected wave sees this "Open Circuit" it
is re-reflected AND added to the source or forward wave, and the
reflected wave is completly conserved! this is reflection gain, this is
why the watt meter at the tuner reads higher than the output source. The
reflection gain compensates for the reflection loss at the mis-match.
This is why all the power is delivered to the load EXCEPT the power that
is lost from I2R or dielectric materials in the transmission line. This
is a dissapative loss. This is where a tremendous amount power can be
absorbed if the mis-match is big and the transmission line is lossy.
This is where care must be taken on the choice of transmission line. I
wouldnt feed my 25' (1/4 wave) center fed rotatable dipole for 30 meters
with 200' of coax because the losses from the forward, reflected and
re-reflected waves would be excessive. But I have no concerns feeding it
with the same length of ladderline because the combined losses from it
would be minor.
A tuner used with low loss transmission line can make non-naturally
resonant antennas to operate with high effiency while using one with
lossy transmission line seems like a dummy load.
73 Chuck
On Wed, 09 Sep 1998 00:18:36 -0400 "w8ji.tom" <w8ji.tom@MCIONE.com>
writes:
>
>> Steve,
>> It goes without saying that transmitted voltage/power arriving
>> back at the tuner due to a mismatch at the antenna gets absorbed by
>> the tuner...except for the small portion that gets re-reflected
>> back toward the antenna.
>
>Hi Bob,
>
>How can a reactance, or group of reactances, absorb and dissipate
>power? If
>the tuner
>really absorbs reflected power and dissipates it, shouldn't it get
>hot?
>
>You have that backwards, the BULK of reflected power is returned to
>the
>line at the tuner, with only a small portion absorbed by the small (as
>a
>percentage of reactance values) resistive losses in the tuner.
>
>I feed an open wire line that operates with a 20:1 SWR (thirty ohm
>load,
>600 ohm line) with an Ameritron ATR-30 tuner or Johnson KW matchbox.
>Neither tuner gets the slightest amount warm, and the antenna it feeds
>constantly equals or beats people with two or three element
>yagis...even
>some who run much more power than me... day after day.
>
> "Match efficiency" is 18.14 % between the line and antenna, but:
>
>1.) The tuner does not get hot.
>
>2.) My signal is competitive with others with similar antennas and
>power.
>
>3.) Current at the antenna terminals flowing into the antenna's 30 ohm
>resistive impedance at resonance is 6.7 amperes with 1500 watts
>applied at
>the tuner input (the antenna is a dipole with multiple reflectors that
>I
>feed with open wire so I can use it on multiple bands).
>
>Please tell me why my tuner, with 140 feet of 600 ohm line going
>straight
>up from a doghouse up to the 30 ohm antenna feedpoint, does not get
>hot.
>
>Why does the same 600 ohm line and tuner(s) deliver 1.3 amperes into
>50
>ohms (SWR 12:1) with only ~100 watts applied to the system? Why
>doesn't
>something melt, if 80% of 1500 watts is absorbed in the tuner?
>
>73 Tom
>
>--
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>
>
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