13-06-2012, 01:50 PM
Seminar on The Effects of VSWR on Transmitted Power
VSWR on Transmitted Power.pdf (Size: 203.07 KB / Downloads: 199)
• What is VSWR and Why Should I Care?
An ocean wavetrain traveling toward shore carries energy toward the beach. If it
runs up onto a gently sloping beach, all of the energy gets absorbed, and there are no
waves traveling back offshore. If instead of a sloping beach a vertical seawall is present,
then the incoming wavetrain gets completely reflected, so that no energy is absorbed at
the wall. The interference between the incoming and outgoing waves in this case
produces a "standing wave" that doesn't look like it is traveling at all; the peaks stay in
the same spatial positions and just go up and down.
The same phenomenon happens on a radio or radar transmission line. In this case,
we want the waves on the line (both voltage and current) to travel one way and deposit
their energy into the desired load, which in this case may be an antenna where it is to be
radiated. If all the energy gets reflected (for example, by an open or short circuit) at the
end of the line, then none gets absorbed, producing a perfect "standing wave" on the line.
This is a bad, undesired situation. In fact, when the power meant to be radiated comes
back into the transmitter at full strength, it will usually burn out the electronics there.
Formal Definition of VSWR and SeaSonde Measurements
The voltage component of a standing wave in a uniform transmission line consists
of the forward wave (with amplitude Vf) superimposed on the reflected wave (with
amplitude Vr). Reflections occur as a result of discontinuities, such as an imperfection in
an otherwise uniform transmission line, or when a transmission line is terminated with
other than its characteristic impedance.
If you are interested in determining the performance of antennas, the VSWR
should always be measured at the antenna terminals itself rather than at the output of the
transmitter. Because of ohmic losses in the transmit cabling, an illusion will be created of
having a better antenna VSWR, but that is only because these losses damp the impact of
an abrupt reflection at the antenna terminals.