I did a similar test a while back for receiving VHF on one of these dongles as an SDR. It seems that at least for VHF, the majority of the signal picked up is through the coax.
However I bought a larger antenna with some curly bits on it (!) to try with the ARF, but I tried the antenna screwed onto the DVB antenna base - and it does bring in traffic from further away. However we're talking 100-150 miles here, so it's not really needed for collision avoidance where, as noted, a wet piece of string will do the trick! Or at least a little stubby antenna.