NOTES ON TELECON WITH STEVE FLOYD, JIM BREAKALL, DUAN YEAGLEY, & ANGEL VAZQUEZ (Dana at home, all the others gathered around Jim Breakall's cellular "speakerphone" at the Dayton Hamvention, 16 May 2015). This was basically a conversation between Steve and myself, with the others at Dayton kibitzing. NOW HERE'S THE BEEF: The VSWR tolerance of a transmitter like ours depends on a number of potential problem areas, such as plate voltage swing, voltages and currents in the elements of the matching network and filter, voltage and current tolerance at nodes in the Heliax line, etc. One item in particular that Steve emphasized relates to PA output tube survival. The idea is, if the negative excursions of the plate voltage bring the plate down to near the screen grid voltage (or heaven forbid, below the screen voltage), then the screen grid begins to pull dramatically higher currents. Note that the screen grid is held at a fixed voltage, with negligible voltage excursions due to its being heavily bypassed to GND at RF. The screen grid has a finite power dissipation limit (before it starts melting!), and this limit is given on the tube datasheet. Transmitter metering should show both screen voltage and screen current. We need to monitor both and be sure to operate the tube so that the actual screen dissipation is held to no more than 50% of its maximum rating (< 25% is preferred) at all times. Steve said that the worst case tends to be when the load impedance presented to the plate is on the high side, thus leading to a larger than design value of the plate voltage swing. From the perspective of this particular risk, then, we need to establish the maximum output power level (and corresponding reflected power) that keeps screen dissipation within safe limits when operating into the actual antenna. This should be pretty easy for a single transmitter, but could be more of a problem for multiple transmitters feeding the cross-coupled antennas in the array. But first we should establish that all the tubes are being operated at reasonable and proper bias conditions per the transmitter manual. Steve also recommended that we do a sweep test across a few hundred kHz (including our operating frequency) to check transmitter flatness. This should be done with dummy loads. If we see more than about 2 dBpp unflatness, we should consider that the output network is probably somehow mistuned, and take steps to correct this (if we can figure out how to do it). This test should be run primarily on a dummy load, but doing the same on the antenna and keeping the results might not be a bad idea either. Note that the transmitters' AGC systems must be switched OFF (as they probably should be for all our operations, in fact). Here's my thought for an all-tansmitter VSWR trip point test: At severely-reduced power on all transmitters with all antennas in place, tune up the feedpoint phases and amplitudes for proper array operation. Then slowly advance the power levels on all transmitters together, while watching PA screen current indications on all the transmitters at once. When the first one approaches the danger point, we have arrived at the highest safe power. Stop there and record all the operating parameters on each transmitter, especially including the reflected power. Set the transmitters' VSWR trip points accordingly, then the tubes should be pretty safe. This procedure has a caveat, which is variations in the phase monitor indications with feedline temperature. It is likely to be necessary to re-tweak the phase settings as we approach the maximum safe operating power, with time allowed for the feedlines to reach their new equilibrium temperatures. Stevee suggests that we try to obtain the impedance of the load presented to the tube with the antenna connected, although what we do with this info is not necessarily clear (it would be nice if the TX manual were to indicate what this was meant to be. Perhaps we can work this out from knowledge of the DC plate voltage, plate standing current, and the tube characteristic curves from the tube datasheet. One way to do this is to repeat the early-2012 measurements with a VNA clipleaded into place, but with the tube installed (it wan't then) and the antenna connected (instead of doing a 2-port measurement of the internal networks as in 2012). That is, we'd be doing a 1-port (S11) test with the VNA connected from the tube's plate to GND. Actually we should do this for both the dummy load and the antenna as loads. If the impedance is higher with the antenna, I think we need to be especially vigilant for dangerous screen current levels when testing. Unfortunately, this test may not mean much for the case of all transmitters operating, becaue of cross-coupling between antennas. We might also be able to use the 2-port S-parameters recorded in 2012, by terminating the black-box networks in the antenna impedances and adding the tube's nominal plate capacitance across the input of each. If we also added the tube's parallel-equivalent plate resistance at the input of each network, along with an adjustable-phase current source, we would seem to have a complete simulation tool at hand for assessing the whole shootin' match, including plate voltage swings. The discussion turned also to the passive components of the output networks, and their voltage and current handling limitations. Here we are hurting a bit because we have no specifications on the current handling capabilities of the inductors, and probably not much info on the voltage breakdown and current handling capabilitied of the capacitors. I mentioned to Steve that some component values in the matching net- work and filter circuits are adjustable, and I opined that these might be adjusted to improve the situation vis-a-vis plate voltage swing. He expressed discomfort with this idea, pointing out that we could unwittingly disturb (redistribute) expected current and voltage stresses on the network components, as well as degrade the filter's stopband rejection. He says that we should be working by trimming feedline lengths and/or adding matching networks at the antennas. But doing this would completely scotch the present feedpoint phase monitoring scheme- we would be forced to abandon our "economy" system and do it "right" (actual current sensing devices at the antennas and separate coax lines running back to the TX building.