you stole my post!
does anybody know what the average range of a scanner with the handheld aerial?
It depends on so many factors. The power of the transmitter, the location of the transmitter antenna (many frequencies actually have more than one transmitter site), the position of the receiver, and obstacles between receiver and transmitter are probably the most influential. You can be parked next to an airport listening to the tower and not receive a transmission from the tower if the receiver isn't sited well in your car, for instance. By contrast, with a good antenna in a good geographic location and with the right atmospheric conditions, you might receive the same transmitter many tens of miles away.
Here's a few rules to go by:
Area/Approach control often use hilltop sites, whereas towers generally don't, unless there is major terrain in the area
Towers are often the curse of home scanner enthusiasts because of the previous rule
You'll receive aircraft much further away than any ground station any day, even though their transmit power is considerably lower, due to having a clear line of sight
Area control centers often use multiple transmitter sites, and sometimes even switch between them depending on where the aircraft is they are calling
Many uncontrolled airports have ground stations specifically for IFR clearances either by Christchurch information, or the nearest area control center
I live about 4NM from Palmerston North tower, and I can't get it with a handheld antenna. If I lived in the country, I possibly could at this distance. I can, however, get Ohakea Control very clearly and I'm about 6NM from their hilltop transmitter site. When I lived in the country (25NM from Ohakea's radio) I could receive it fine with just a handheld antenna. PM tower was a little more challenging and required an outdoor antenna installation, but I got it at 10NM. Not a great reception, but readable.
If you give me specific details of where you're trying to listen from and what you're trying to pick up, I could give an idea if its feasible or not. I can get details on the radio equipment used at each of the transmitters (its public information).