I've never understood what is gained by using a balun at the feedpoint
in this application. The feedline is unbalanced as is the load.
What's the point of a balun. The wild variations in R and X over the
range of the band will likely result in heat and loss at the balun.
If the point is to keep RF off the exterior of the feedline, that
problem vanishes if you have a lot of radials which are needed
regardless. The best solution is to put the matching network at the
feedpoint but if you opt to match the line to the transmitter at the
shack, the vswr loss for 70 feet of 213 won't be terrible and the
dielectric will probably withstand the voltage maxima.
73
Rob
K5UJ
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