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Lead Roofs and Antennas

Hi folks, a not so simple question.
I have a flat lead roof on the top of my shack in the loft of the house and
the LEO antennas are mounted on a small rotator a few feet above it. They
work well, but all things can be improved and in the UK I cannot find any
literature to help me further.
1. Do I assume  the lead roof acts as a perfectly conducting surface and
optimise the antenna heights based on that, or do I ignore it and go for the
height above the real earth plane?
2. The roof is not specifically earthed in any way ( I do not live in a
lightning prevalent area). Should it be earthed, if so, what to? The roof
and shack is some 20 feet above ground level and rf earthing in the shack
makes no difference at either HF or VHF, even with the MFJ tunable earth kit
connected to a good earth as per marine radio station practice.There are no
operating problems and even without earthing, the lead roof provides a very
significant downward reduction in radiated field strength
so I'm well below fcc and European limits.
Mike G3LGR

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