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==Why do they call it MARS CAP Mod==Amateur radio operators that expand the frequency coverage of their transceiver often do so after researching the MARS/CAP mod (modification) for their radio. But why is it called MARS CAP? Is it like a baseball cap?*MARS is the Military Affiliate Radio Service. They use HF and VHF frequencies near ham bands to forward personal messages to and from military personnel. * CAP is the Civil Air Patrol, partially a youth group and partially a domestic search and rescue auxiliary for the Air Force. They also use frequencies near ham band with volunteer ham radio operators to coordinate search activities and relay messages.Often expanded transmit modification to a ham radio transmitter is done based on a MARS/CAP modification guide when the user is not a MARS operator or member of the Civil Air Patrol. This is not necessarily right, and often illegal to use such modified equipment, however, the terminology is deeply rooted in the owner's quest for the correct modification procedure.Performing the MARS/CAP modification of a ham radio transceiver is intended for actual MARS radio operators or Civil Air Patrol radio operators.It is not illegal to use an amateur radio transceiver or receiver with expanded receive coverage to listen to transmissions below 834MHz. Listening to radio traffic between 824-849MHz or 869-894MHz remains illegal as of 2017 due to an outdated FCC regulation regarding the AMPS phone system. See: [[Cellular blocked]].