Hello Dick,
I have been on LF for about 18 months using the antenna shown in the
attachment with up to 200W on 73KHz and 100Won 136KHz from a standard size
suburban plot without any interference problems at all on the transmit side.
Plenty of QRM on receive but no trouble on TX. The antenna and its
supporting stays are contained within the back garden which measures 18m. by
14m. with 5 houses all within 16m. of the vertical bit that does the actual
radiating.
As it happens, this coming weekend will be my last on LF as I am quitting to
go back to VHF/UHF which remains my first love, and since all the VHF/UHF
antennas had to come down to make way for the LF antenna so things have to
happen in reverse now.
My sole reason for going on to LF was to see if I could manage to fit a
useful LF antenna into the available space and make QSOs on 73 KHz and that
has been done. 136KHz is easier  and even though the actual TX output power
is only about 80W on 136, my QRSS signals have got out to UA6 and been seen
(but not well enough for a QSO) by VO1NA.
So you see it is not absolutely necessary to have lots of space and/or
power. Good luck and have fun.

73 de

Ian GI8AYZ/MI0AYZ

LF Antenna digram.doc
Description: MS-Word document