A vertical conductor, a metal tube, running diametrically across a
large magloop with the tuning capacitor at the top, has no effect
either on radiating efficiency or the radiation pattern. It may as
well not be there apart from providing a prop or support for the
antenna itself. It will, of course, need to be well-insulated from the
high-voltage parts.

In general, provided the longest dimension of foreign metalwork is
much less than 1/2-wavelength, and there is not much of it, negligible
loss will be induced in it. This is due to the very low loss
resistance of most metals.

Metalwork in the vicinity of beam antennas may upset only the
radiation pattern.

Short lengths of foreign metalwork in close vicinity of tramsmission
lines, such as open-wire and ladder lines, will cause neglible loss
but will slightly reduce the line impedance Zo which might affect the
L and C settings of the tuner.

But beware of "semi-conductors", like damp wood such as foliage and
tree trunks, in the near-fields of antennas and lines. Their higher
impedance provide a better power match to the 'generator'.

You can have as much foreign polyethylene as you like.
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Reg.