The FC-50SP is for radios without the black 6 pin hard-wired frequency counter connector on the back. The FC50 SP will work with AM and SSB radios. The radio connects via the two SO239 coax connectors on the back. You will need a short coax jumper to connect the counter to the radio (sold separately). The FC 50 SP displays the frequency when the mic is keyed, on transmitt only. The easy to see bright blue display is visible in "direct" or "near direct" sunlight. Very nice when it's used in a vehicle. "S" designates for single side-band users Internal tone generator and speaker Includes 3' plug in speaker lead for tone generation in to mic Momentary toggle allows for single side-band reading without voice Allows Ham/Amateur users to accurately measure frequency in 1.8 MHz to 50 MHz range For any radio without hard-wiring Measures RF signal or TX channel to ant. Level from .1 to 1V Max. Has Input/Output Type SO-239 connectors DOSY Professional Architecture Printed Circuit Board Blue LED color For AC/DC Power Supply add "P" to P/N