David,

Try putting the dipole on the ground and earthing both ends as well as
circumstances permit.  In effect you will have a large vertical loop
although it will be underground.

This technique is the basis of much cave radio communication, and I seem to
recall that one of the earliest 73 kHz above-ground experimenters (now a
Silent Key) usesd this technique to set one of the distance records on that
band.

By the way, does the 500 kHz NoV require you to tell the authorities
whether you will be working above or below ground, as was the case on 73?

John F5VLF

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