s ight ▁propag ation ▁range ▁of ▁each ▁other , ▁to ▁communicate . ▁Re pe aters ▁are ▁found ▁in ▁professional , ▁commercial , ▁and ▁government ▁mobile ▁radio ▁systems ▁and ▁also ▁in ▁amateur ▁radio . ▁ ▁Re pe ater ▁systems ▁use ▁two ▁different ▁radio ▁frequencies ; ▁the ▁mob iles ▁transmit ▁on ▁one ▁frequency , ▁and ▁the ▁repe ater ▁station ▁receives ▁those ▁transmission ▁and ▁transm its ▁on ▁a ▁second ▁frequency . ▁ ▁Since ▁the ▁repe ater ▁must ▁transmit ▁at ▁the ▁same ▁time ▁as ▁the ▁signal ▁is ▁being ▁received , ▁and ▁may ▁even ▁use ▁the ▁same ▁anten na ▁for ▁both ▁transm itting ▁and ▁receiving , ▁frequency - select ive ▁filters ▁are ▁required ▁to ▁prevent ▁the ▁receiver ▁from ▁being ▁over loaded ▁by ▁the ▁transm itted ▁signal . ▁Some ▁repe aters ▁use ▁two ▁different ▁frequency ▁bands ▁to ▁provide ▁isol ation ▁between ▁input ▁and ▁output ▁or ▁as ▁a ▁convenience . ▁ ▁In ▁a ▁communic ations ▁satellite , ▁a ▁trans ponder ▁serves ▁a ▁similar ▁function , ▁but ▁the ▁trans ponder ▁does ▁not ▁necessarily ▁dem od ulate ▁the ▁relay ed ▁signals . ▁ ▁Full ▁du plex ▁operation ▁ ▁A ▁repe ater ▁is ▁an ▁automatic ▁radio - rel ay ▁station , ▁usually ▁located ▁on ▁a ▁mountain ▁top , ▁tall ▁building , ▁or ▁radio ▁tower . ▁ ▁It ▁allows ▁communication ▁between ▁two ▁or ▁more ▁bases , ▁mobile ▁or ▁port able ▁stations ▁that ▁are ▁unable ▁to ▁communicate ▁directly ▁with ▁each ▁other ▁due ▁to ▁distance ▁or ▁ob stru ctions ▁between ▁them . ▁ ▁The ▁repe ater ▁receives ▁on ▁one ▁radio ▁frequency ▁( the ▁" input "