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It’s impossible to say exactly why the Zetas chose to build the radio network, but given their military and law-enforcement background, it seems likely that Z1 and his capos understood that a widespread communications system would provide a crucial competitive edge over other cartels. Radio was the clear choice. Unlike cell phones, which are expensive, traceable, and easily tapped, radio equipment is cheap, easy to set up, and more secure. Handheld walkie-talkies, antennas, and signal repeaters to boost transmissions are all available at a good radio shop or from a Motorola distributor. A radio network could provide communications in many of the remote areas in Mexico where the cartel operated. And, if they suspected law enforcement eavesdropping, the cartel’s drug smugglers and gunmen could easily switch frequencies or use commercially available software to garble voice transmissions.
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This is a very interesting read:
http://www.popsci.com/article/technology/radio-tecnico-how-zetas-cartel-took-over-mexico-walkie-talkies