Actually, SLF receivers can be very small.
http://en.wikipedia.org/wiki/Super_low_frequency
Yup, transmitter requires 10s of kilometers of "power line on poles"; the Project has hidden bigger things. The Project has large amounts of electrical power.
I think the biggest issue with an SLF or ELF system: the chance of being mistaken by the Soviet Union for some sort of secret backup to the Navy's communications systems. "Look, Comrade-General, the Americans think we won't notice their secret strategic transmitter system."
Any system requiring above-ground antennae or dishes isn't good for communication with boltholes.
There are a lot of systems to use once you've waked the team up. The basic PRC-70 radio isn't really a "pick up the handset and talk to any other PRC-70 in North America", but for Morse code and low-rate modem transmission it's not too bad.
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Michael B.