@David, so, we are talking about using a length of coax fed through the polythene water pipe (fairly lightweight and very rigid), either 1 or 1.5m diameter. The ends of inner conductor go to the magloop amp. Are the ends of the braid connected to each other? I guess the braid could/should be earthed, but to a ground spike? With both the outer cable insulation in place, and contained in the pipe, should be quite dry.
@Symon, to start with, can you just fix the loop loosely in the shack somewhere? Chances are you'll want to play about with the amp while listening to what comes out. I intend to put it outside eventually, but have found it very interesting to have it all at arm's reach. One little finding I've made is that my amp is not optimum at 12V
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B