Dear Alan, LF group.

As far as antennas for LF/MF are concerned, the "lots of wire as high as
possible" principle applies, with height being the factor that has by far
the biggest effect on efficiency. Your 15m high antenna is pretty big by
most amateur standards, so should have relatively high efficiency - you
shouldn't need more  than a few watts from the TX output to reach 0.1W ERP.
The "high voltages" will only be in the hundreds of volts at most. The
presence of  the tower will have an effect on the current distribution in
the antenna, and so the antenna current required to obtain a particular ERP.
The radiated power depends on the mean current in the vertical part of the
antenna; if the horizontal section is much longer than the vertical section,
the vertical current distribution will be nearly uniform, less current will
be capacitively coupled into the tower, and the effect of the tower and/or
loading coil will be minimal. If the horizontal top-loading section is
shorter, adding an elevated loading coil as you describe will have more
beneficial effect. This rather depends on whether your inverted L is cut for
operation on the 160m band, or is 160m long. The effect also depends on the
spacing between the downlead and the tower.

Personally I am using a loop antenna for receive, as at my QTH it gives
lower noise. On 500kHz, the local noise on my TX antenna is mostly splatter
which appears to be intermod products from the Brookmans Park MF broadcast
site about 500 metres from my QTH. The intermods appear to be generated by
the antenna itself - even though it is resonanted at 500kHz, an antenna
current of about 40mA flows, mostly from the 909kHz Radio 5 signal, and I
suspect the ground rods introduce some non-linearity due to electrolytic
effects. But most QTHs don't have 400kW of assorted broadcast transmitters
across the road - many people seem to be using their TX antenna successfully
for receive. Potential benefits of seperate RX antennas depend a lot on the
local QRM conditions - I have found loop antennas on LF and MF make good
mains cable locators, but when well sited can give reduced noise, and useful
directional nulls..

I think from Ofcom's point of view, restricting power to an ERP limit makes
a lot of sense. ERP is defined by field strength at some distance from the
TX, or vice-versa. If Ofcom want to know what ERP you are using, they just
have to measure the field strength of your signal, rather than gain access
to your shack to measure the TX output. Also, the ERP defines the field
strength that potential "victims" of breakthrough, interference or overload
are subjected to as a result of amateur signals, which is one of Ofcom's
major concerns as spectrum regulators. From the amateur's perspective, it is
quite an egalitarian arrangement, since the station with a small antenna can
achieve  the same results as the big boys on transmit just by running a bit
more TX power, which for 500kHz is quite low anyway.

Cheers, Jim Moritz
73 de M0BMU