Agreed that adding a 9:1 balun (impedance match) will have little mismatch
loss at x and all the Tx power will be "presented" to the antenna port. If
their is a matching section of 450 to 50 (another 9:1) balanced at the
antenna then there should be no reflections due to impedance mismatches at
the antenna, and all of the power goes out the antenna.
If the antenna is not 50 ohms, or the balun from 450 to 50 at the antenna is
missing, then part of the power is reflected back into the Tx output.
What the Tx source does with that depends upon the design of the final
Amplifier stage, some change output impedance, some go unstable, many cut
back Power out. If at a higher frequency one can use a circulator, and
dump the reflected power into a 50 ohm dummy load and keep the Tx safe (also
good in suppressing transmitter IM).
If the Tx source reflects the power back out, then the Tx source can "drive
that amount of power out into a higher VSWR", which means that Tx source can
accommodate loads around 50 ohms and keep Power output to spec. Which means
that there is no problem if all the mismatches from the antenna (not being
50 ohms), antenna/line mismatch, line/Tx mismatch etc. all add up to be less
than a 3:1 VSWR or so for mobile FM. Probably higher for Ham bands.
But one needs to keep an eye on the Tx output impedance as it changes, then
the load presented to the antenna (looking back into the line) will change,
not be close to 50 ohms, which can change antenna pattern (at higher
frequencies). One could put in a resistive pad to stabilize the impedance at
the Tx output, but why waste the power? Antenna tuners make sense at the
lower frequencies, they match up the strange loads, most of the mismatches
and Tx output,and maximize power output.
The optics example is good, as the anti reflective coatings are 1/4 wave
thick, and the coating constant (speed of light in the material) is the
square root of the product of Air and Glass, it is an optical matching
section. Need a coating on both sides of the glass.
"Cecil Moore" wrote in message
...
John Smith wrote:
Checkout "Reference Data for Radio Engineers", Transmission lines,
Mismatch
and Transducer losses. There are formulas there for this case. The
simplification ignores the mismatches at the Tx and Antenna to the line,
which causes about 4.43 dB additional "mismatch" loss, in the case of
50,450, 50. This shows up as VSWR.
There is little actual loss due to the "mismatch loss" since the
system is Z0-matched. Let's add a small length of 50 ohm line.
100W XMTR--50 ohm line---x---1/2WL 450 ohm line----50 ohm balanced load.
Since there are no reflections on the 50 ohm line, the system is
Z0-matched
to 50 ohms at point x. (Assume a 9:1 balun at 'x') The 9:1 SWR on the 450
ohm
line causes very little loss and the "mismatch loss" is not really lost
because
the forward power on the 450 ohm line is 278 watts, delivering virtually
all of
the 100 watts of XMTR power to the load.
It can be illustrated that a steady-state destructive interference event
toward
the source due to wave cancellation at point 'x' feeds energy to a
constructive
interference event in the direction of the load. This has been understood
in the
field of optics for many decades and is how non-glare glass works.
--
73, Cecil http://www.qsl.net/w5dxp
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