http://worlditc.org/h_07_meek_spiri_029_041.htm

Scroll down and see the schematic for the machine. What's interesting
is that they use 10-12 ft. antennas (which today could be replaced by
simple CB antennas).

For a more clear schematic and samples of the voice contacts using the
tones:

http://worlditc.org/k_06_spiricom.htm

Hope this helps you to get a better understanding of the equipment and
theory involved with this time-consuming research.

Jeff