Probably using 6mm copper tubing would not be a practical approach for the loading coils of Part 15 AM/FM systems.
At 1650 kHz, an air-core loading coil using about 85 turns of 12AWG (~ 2mm OD), uninsulated, round, copper wire with a coil OD of 4" and coil length of 7" would have an inductance of about 300 µH, and an r-f resistance of about 7 Ω.
So while that coil loss is less than 1/2 of the coil in my previous example, the system radiation efficiency when using it would still be poor: 0.1/7.1 = 0.014 = 1.4%.
I sure don't understand the polarization thing.
I like the animated chart on that, shown above -
back quite a few posts. NICE JOB!! More reading
on my part is required.
On the receiving end -- or receiving Uber-Weak FM
signals - - well. There is the extraordinary case of
the FM DXer in Ireland in the summer of 2003. Who
with just himself, his low noise FM BCB receiver and
his "much more than gigantic" rotatable phased receiving array, and
a huge knowledge of propagation, accomplished what
many thought was impossible. Which was receiving 4
FM BCB stations at his receiving post - all from the
northeastern U.S. and southeastern Canada.
So - - how far away could a Part 15.239 transmitter
get from his location???? What would
the maximum distance be????? We can only
speculate.
Brooce, Part 15 Hartford CT
The polarization of radio signals can change, with ground reflections, bouncing off obstructions, etc., Which adds even more uncertainty.
That chart that was referenced earlier gives field strength values at distances in FREESPACE, and is really just showing the linear inverse relationship between distance & field strength.
In the real world, there are so many other factors that enter into the equation that you have to run simulations - hence the NEC software.
And don't forget that to induce a 1uv signal at the antenna terminals of a receiver, it takes a much larger field strength ( on the order of 2.5 to 3 uv/m). So even in free space, you'd have to cut down the distances in that chart by a factor of 3.