Cable on mine is fairly long. I just stick it out a window and don't try to center it on the roof unless we put it out the SUV's sunroof :-)

As for a receive only antenna - I'm pretty darned sure that 'stumbler transmits. Might be interesting to use with Kismet or WalensomethingIcan'tpromounce or perhaps a dedicated sniffer but sure as heck I'd get stupid and blow it up :-(