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 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [email protected]
