▁mode ▁at ▁a ▁time , ▁altern ating ▁between ▁receiving ▁and ▁transm itting ; ▁so , ▁mobile ▁stations ▁do ▁not ▁need ▁the ▁bul ky , ▁and ▁cost ly ▁filters ▁required ▁at ▁a ▁repe ater ▁site . ▁▁ ▁Mobile ▁stations ▁may ▁have ▁an ▁option ▁to ▁select ▁a ▁" t alk ▁around " ▁mode ▁to ▁transmit ▁and ▁receive ▁on ▁the ▁same ▁frequency ; ▁this ▁is ▁sometimes ▁used ▁for ▁local ▁communication ▁within ▁range ▁of ▁the ▁mobile ▁units . ▁ ▁Fre quency ▁separation : ▁input ▁to ▁output ▁There ▁is ▁no ▁set ▁rule ▁about ▁spacing ▁of ▁input ▁and ▁output ▁frequencies ▁for ▁all ▁radio ▁repe aters . ▁Any ▁spacing ▁where ▁the ▁designer ▁can ▁get ▁sufficient ▁isol ation ▁between ▁receiver ▁and ▁transmit ter ▁will ▁work . ▁ ▁In ▁some ▁countries , ▁under ▁some ▁radio ▁services , ▁there ▁are ▁agreed - on ▁convent ions ▁or ▁separ ations ▁that ▁are ▁required ▁by ▁the ▁system ▁license . ▁In ▁the ▁case ▁of ▁input ▁and ▁output ▁frequencies ▁in ▁the ▁United ▁States , ▁for ▁example : ▁ ▁Am ateur ▁repe aters ▁in ▁the ▁ 1 4 4 – 1 4 8 ▁M Hz ▁band ▁usually ▁use ▁a ▁ 6 0 0   k Hz ▁( 0 . 6   M Hz ) ▁separation , ▁in ▁the ▁ 1 . 2 5 - meter ▁band ▁use ▁a ▁ 1 . 6   M Hz ▁separation , ▁in ▁the ▁ 4 2 0 – 4 5 0 ▁M Hz ▁band ▁use ▁a ▁ 5   M Hz ▁separation , ▁and ▁in ▁the ▁ 9 0 2 – 9 2