From time to time you see signs near blasting sites saying, “Turn off your radio”. They mean turn off your radio transmitter, no? Or, could these warnings actually be referring to receivers too? (If yes to this second question, is it because they may be heterodynes?)
You answered your question. The Armstrong heterodyne design is still in use. Your radio can act as a transmitter.
Are radio signals used to actually detonate the explosive, or are they only used to send verbal signals that all is clear and the explosives can be safely detonated by wire?
I think the point is not that your radio will transmit the code to detonate, but that any transmission could be picked up by the 5 miles of cable they have stretched out to the blasting site and just enough noise will be introduced that it goes boom.
Note that it is not necessary for you to use a radio receiver to receive radio signals, any chunk of metal or electrical circuit that is not specially shielded can pick up signals, and if the signals are unexpected the results can be unexpected.
From the Lawrence Livermore National Laboratory’s Environmental Safety & Health Manual
And from OSHA Regulations Explosives and blasting agents.
More from OSHA:
So from what I can tell from the above those who make the rules are concerned with transmitters accidentally causing electrically activated detonating devices to activate. It leads me to think that they don’t normally use radio transmission to set off a blast, just that it can happen. I notice Livermore is happy with 15 feet while OSHA wants 100 and, in the quoted part, requires posting signs out to 350 feet; in another part of the OSHA regs that I didn’t quote, they specify signage out to 1000 feet.
Feel free to correct any mistaken impressions on my part - I knew absolutely nothing about this before I started searching about 30 minutes ago (aren’t you glad we have insomniacs on this board?).