RadioHead Library - init transmit-only

Hello!

I am just about to update a project from VirtualWire to the Radiohead library.
In VirtualWire I've configured the pin of the ASK transmitter module using
vw_set_tx_pin(pinVWTransmit);
but in Radiohead this is done using
RH_ASK(speed, rxPin , txPin , pttPin , pttInverted);

What shall I pass as rxPin if a module should be using RadioHead only for sending data?
I want to avoid that RadioHead is changing anything on a possibly used pin or using ressources like installing an intterrupt-routine that is not needed.

Thanks,
Ralf