"kl7r" wrote
Dont try to use an endfed wire antenna in an apartment.

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If the domestic plumbing system, hot and cold pipes, central heating
system and mains wiring are connected together, then the impedance to
true ground, as a whole, will be very low. The RF potential difference
between 'ground' and true ground at the lower HF frequencies will be
small.

In principle it could be measured - IF you could find a real ground.

The actual distance in wavelengths between 'ground' and true ground,
wherever that may be, is indeterminate and inconsequential.

In the extreme case the elevated ground system can be considered as a
sort of Faraday cage with no more than normal interference between the
electrical equipments in the vicinity.

By not using low-feedpoint-impedance anrtennas, such as exactly
1/4-wavelength or 3/4-wavelength in length, things can very often
continue as normal. It is, in any case, undesirable to use very low
impedance antennas even in good ground circumstances.

And antennas crudely 1/2-wavelength in length work just as well with
either good or very poor ground systems.

Appartment and flat dwellers should not be discouraged from simple
endfed antennas. In all probability they will be successful. Very
often they are unable to erect anything else.

My own experience extends from successful working with end-fed wires
from bedrooms to a 13th storey in an appartment block. The appartment
block antenna was a 0.3 wavelength sloping wire on the 160m band, open
circuit at the bottom end fed at the top.

Ground was an aligator-clip connection to the nearest hot water
radiator pipe in the domestic central heating system. But I could just
have well used the cold water pipe to the flush-tank in the toilet had
it been nearer.

Appartment dwellers - carry on as normal!
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Reg, G4FGQ.