[Elecraft] TxMon and K3 (tuner bypassed) SWR missmatch

[Richard Ferch]

You are talking about SWR as if it were something real and physical that
could damage the transmitter's finals.  It's not - it's just a mathematical
construct that makes it easier to calculate various other quantities (such
as loss in the transmission line). The real physical quantities that can
damage the finals are currents and voltages.

The SWR reading on an SWR meter is back-calculated from currents and
voltages measured, for example, in an SWR bridge circuit. An analog SWR
meter is just a milliammeter with an unusual scale - the scale does the
calculation for you (like a nomograph). A digital meter does the
calculation using a formula.

The protective circuitry does not measure SWR directly - it measures
currents and voltages, and it can measure them close to the finals, not at
the end of some indeterminate length of transmission line. There is no need
for the protective circuitry to calculate an SWR from those measurements -
all it needs to know is the measured voltage at which it needs to act to
limit the voltages and currents inside the finals to safe levels.The
calculated SWR that is displayed on the front panel is there to keep the
operator happy, not because the circuitry itself needs to know it.

The SWR at which the foldback occurs in the finals actually varies with
output power. At very low output power, the SWR can be very high without
causing damage, because the currents and voltages are still low, so the
foldback circuit does not need to operate just because the SWR is high. It
only needs to operate when the voltages or currents start to threaten the
finals.

This being the case, any errors in the SWR calculation based on the
measured voltages and currents at various locations are basically
irrelevant to the issue of protection of the finals, regardless of their
origin.

73,
Rich VE3KI