One of my old FM tuners (a Leak Trough Line 3 [1]) has that arrangement. It's quite simple electrically, since the impedance ratio goes as the square of the turns ratio, you need twice as many turns to match 300 ohms as to match 75 ohms. And since the 300 ohm input is balanced, it should really be centre-tapped to earth. These conditions are both met by having a centre-tapped input coil (matching to the right impedance on the secondary side of the RF input transformer) with the tap grounds. I am sure the Trough Line 3 schematic is on the web (every other old Leak schematic I've wanted is) if you want to see the circuit. [1] So called because the oscillator 'coil' is a concentric line soldered to the tuning capacitor. It is therefore very stable. -tony