Ant
You appear to have the same set up I experimented with. The wideband signals picked up by the discone antenna will completely overload the front end of the dongle receiver, which is why you didn't receive anything worthwhile. The antenna you used was simple too good!
Getaway Turtle
The dongles have very sensitive receivers that are easily overloaded. The best bet when starting out is to connect a couple of feet of wire to the centre connection in the antenna socket of the dongle. This should pick up local FM radio and various other stations. You may even hear some airband stuff if it is very local.
Once the system is proved to be working, connect a length of coax cable using a suitable adapter plug and adapter to the antenna socket. Mount the far end of the cable as high up as possible, preferably in the loft or near the ceiling in an upstairs room. Connect up two lengths of wire 60 centimetres (2 feet) long to the end of the coax, one wire to the centre of the coax, one wire connected to the braid. Arrange these wires vertically with the center connect wire uppermost. The wires can be stuck down to a non conducting surface like a bit of bamboo or a stick to keep them out straight and supported vertically. Use a bit of insulating tape to keep the connections from sorting out. Especially keep an eye out for stray wisps of braid that tend to get wrapped around the centre conductor of the coax. That would short out all the signals you are trying to receive! Soldered connections are great if you have a soldering iron, but twisted connections are good enough.
This arrangement will form a half wave dipole antenna tuned to approximately 125 MHz which is around the middle of the airband frequencies you want to receive.
This type of antenna arrangement will tend to be less sensitive to transmissions away from the airband frequencies, so provides a degree of filtering from unwanted signals. You should certainly pick up local transmissions with this type of setup.
Remember, airband signals are AM modulated, so make sure you select the correct receive mode in the receiving software. Airband signals travel essentially along lines of sight. You may hear aircraft many miles away at altitude, but just barely receive a ground station five miles away, or not at all if there is a hill or high buildings in the way. Hence the desirability of having an antenna as high up as possible.
Cheap TV or satellite coax cable can be used, and the wire for the antenna can be stripped mains cable, or whatever you have to hand. This project shouldn't cost any more than £20, even buying everything from scratch.
Good luck.