Calculation of the Fresnel Zone given as r=M*sqrt( 1/F * (d1d2/(d1+d2))) uses 72.1 and 17.3 as the constant of proportionality (M) for r in feet and meters respectively. Does anyone know where this constant comes from? I've tried plugging-and-chugging with feet-to-miles, and f=c/lambda, but I can't see how M converts units and comes out from under the root.
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