[Elecraft] Balanced AT

[Ron D'Eau Claire]

Another issue with a balanced AT is that they tend to be used in multiband
systems fed with open wire line where high SWRs are expected. High SWRs mean
very, very high RF voltages at a current "loop", demanding relays, etc.,
that can safely deal with thousands of volts of RF at any power level much
above QRP. 

As a practical example of what RF voltages can do, on large ships I often
encountered SGC unbalanced tuners mounted on the ship's forward stack
(funnel) feeding a 22-foot ship for the 2182 kHz emergency SSB rig on the
navigating bridge. A 22 foot antenna is very short for 2,182 kHz (just above
the Ham 160 meter band) and, in spite of the tuner being a sturdy,
commercial unit fully sealed against the elements, it wasn't a surprise to
see them 'go up in smoke' at regular intervals. Opening the case one would
find charred relays and lots of carbon. That was at less than 100 watts PEP
sideband. They tuned at low power but they were pushed to their limits at
2182 into a short whip. 

My homebrew manual balanced tuner uses a large transmitting-type variable
with plate spacing on the order of 1/4". Wires in the tuner are separated by
several inches with acrylic insulation in all critical areas. 

Ron AC7AC