There is a shortwave ISM band centered on 13.560mhz.
This is the old allocation that was given to medical diathermy machines.
You hear all sorts of stuff in this little band!
Propagation on this frequency is amazing!
I build a tiny transmitter that put out 10mw and left it on a tall hill in Griffith Park. (Los Angeles)
It had a waterproof case with bits of broken solar panel glued to the top. I placed an ATtiny84 processor on the board and programmed it to check the temp sensor and send out the reading once every 5 minutes. The ATtiny84 would send the data and then shut down power to the transmitter and go into sleep mode until time for the next transmission.
A small nimh coin-cell battery was able to gather enough charge to keep the rig going 24/7
I regularly could pick up the signal in El Paso, about 800 miles away. It worked for a little over 3 years and then met some kind of sad fate.
The antenna was a simple inverted vee fastened to a nearby tree limb and connected to the radio by a 15ft run of RG8 coax. The rig was screwed down on a small stump where it could get the sun
From the FCC regs.
15.225 (a) The field strength of any emission within the band 13.553-13.567mHz shall not exceed 15,848 microvolts/meter at 30 meters.
Thank you for that information!