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“I am referring to buried radials and I do not know about rooftop or above ground radials per se. They may well be counted in the 3 meter length. The safest way is to install buried radials and mount the transmitter at earth level with a short ground lead to the radials.”
Well, I’m quite sure there are a lot of Rangemaster installations on roofs. My next best idea for a roof-mounted system would be to mount on a metal roof, or lay out a lot of bonded radials, as many as possible, and not connect them (or the metal roof) to the TX ground, but you could certainly connect them to Earth ground. Let it serve as counterpoise.
Connect the TX ground to a roof (metal) sewer vent pipe, one which is known to go all the way into the soil (no PVC). It might be legal, since it will have bends, other pipes connected, etc. all inside the building and under it. Organic doo-doo in the sewer ought to be conductive, too 😉
However … “The conductivity of seawater is 5,000 millimhos per meter, resulting in the best propagation of AM signals.” from http://www.fcc.gov/mb/audio/m3/index.html
… works for me, no need for ground radials at all 😉