Will a shortwave longwire antenna work if it is laid on the ground or does
it have to be hung in the air from a tree or something similar?

Pierre,
Indeed you can just lay it on the ground, actually (many already know this)
you can actually 'bury' your antenna and it will still work.

If restrictions or lack of a tree are in your way, you always have the
ground. I have one antenna that is 'on the ground' and has been for over
two years and it had always worked extremley well for myself. I started
with just 50ft. at first and now I think it is well over 700ft. pointed
'southwest.'

What is the 'makeup' of your terrain may I ask. {?}

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