"I bought a 12V to 24V Converter and have wired it up ready for a new Dab/UHf radio. The radio needs a constant feed and a switched feed. I am not keen on wiring it up through the converter on a constant feed If I use the switched feed it works OK and will feed the aerial on the switched feed. The aerial needs a constant supply as well. Because the chassis is positive it would seem logical that this would feed the radio constantly!! But I am not sure how this will work? Would it go through the Radio and to earth (the radio now being Negative)? I am not sure if this will work. I don't want to destroy my Radio".

I suppose modern radios may need a constant power feed for the digital memory. A power antenna may need a constant power feed to run it up/down when the radio or ignition is switched on/off. For this you may need two power converters, one to be constantly hot, and one to be switched. Or you could do it with one power converter and a relay, with the relay controlled by ignition switch to control power between the converter and radio switched input.

Leaving the converter live at all times could make an additional power draw on the battery when parked. This is an issue with all new cars, where the on-board computers draw a small amount of power when parked. The battery has to be sized for 30 days reserve power, after which it has to retain enough charge to start the car. After 30 days all bets are off when the battery may go too low if the car is not driven regularly. For long term storage you either have to disconnect the battery (which kills the radio memory) or plug it into a battery tender. The power converter might draw more power when idle and run the battery down in much less than 30 days.