[TowerTalk] Mismatch Loss and Tuners
Tue, 08 Sep 98 14:07:56
> Think about the receive signal. When it hits a mismatch, some
>portion will be reflected back to the antenna, to be re-radiated and
>Not really. What really happens with a tuner in line is the system is
>conjugately matched for the receive signal. Efficiency is no
>different on receive as it is on transmit.
Actually, the first statement above is more correct. When an antenna
intercepts a signal, it develops a voltage across its terminals which
then "drives" the feeder transmission line. The level of "driving"
voltage is a function of the antenna impedance and the transmission
line impedance (antenna VSWR).
This "driving" voltage will then travel towards the receiver. Arriving
at a conjugate match in the system, some portion of the voltage will be
reflected back towards the antenna. The portion not reflected will be
delivered through the conjugate match towards the receive circuit. A
conjugate match is not a "perfect termination" as stated in one of the
previous posts. Like any impedance at the end of a transmission line,
a conjugate match will reflect voltage (hence power) based upon the
reflection coefficient (due to VSWR) it presents to the transmission
The reflected signal from the conjugate match will arrive back at the
antenna where some signal will be radiated and some will be
re-reflected back towards the receiver. The level of signal radiated
and reflected at the antenna is a function of the antenna VSWR.
This process of multiple reflections between the antenna and the
conjugate match will continue until the total voltage decays as a
result of the following: Some is delivered to (or through) the
conjugate match, some is lost in the transmission line and some
re-radiated by the antenna.
I don't want to get into the "time/speed of light" discussion again,
however, if you want to determine the TOTAL power delivered to (or
through) the conjugate match and the TOTAL power re-radiated by the
antenna, you would have to add up the power contributions from all of
the individual voltage reflections.
In order to fully and accurately describe the operation of a conjugate
match "tuner", a complete reflection analysis must be done with
voltages NOT power. Once the system voltages are determined, power
delivery in the system can be determined. It must also be understood
that for any incident voltage arriving at the conjugate match output
from the antenna (transmit or receive), only a portion will be
reflected back towards the antenna. The amount of voltage/power
reflected will be a function of the conjugate match VSWR. Only if it
has an infinite VSWR will ALL voltage, hence power, be reflected.
In describing the transmit case, some posts have stated that a
conjugate match adds reflection gain by re-reflecting ALL power
reflected by the antenna. If this is true, then why in the receive
case does it not re-reflect ALL power delivered by the antenna thus
preventing it from reaching the receiver? Obviously, it does not, in
either case. It functions as described above. The conjugate match is
simply seen as an impedance at the end of a transmission line,
reflecting voltage or power simply as a function of the impedance
mismatch (VSWR) it presents.
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