It will depend on your height above sea level (and Shoreham tower's).
VHF radio range is very simple: it won't normally go over the horizon. The radio horizon in miles is roughly 1.3 times the square root of your height in feet. Add your horizon and his, and if they are less than the distance between you, you're not going to hear him.
If Shoreham is 20 miles away, you'll need to have your aerial about 240 feet high. If you're on a 250 foot hill, with falling ground towards Shoreham, you'll be OK. Check the OS map for heights - I don't know the area well.
Keef is right. VHF is essentially line of sight so you have to be more or less in sight of the transmitting antenna to receive the signal.
However, you need to consider that the Shoreham Tower antenna is not located on ground level. It'll be some considerable amount of feet up in the air - maybe as high as 100 feet even. This means that your antenna doesn't have to be 240 feet up in the air but significantly less.
But that's all theoretical. Without a detailed survey of the ground inbetween you and Shoreham it'll be hopeless to make an accurate prediction. Best is simply to try it out.
What also helps a lot is to replace the dinky whip antenna of the scanner with a proper antenna, tuned for airband, placed as high up as possible.
Of course you can go all the way, buy an outdoor antenna, mount it on the roof, ground it properly and so forth. Or you can do as I do: whip up a simple dipole antenna from stuff you might have lying around in the garage, or can obtain cheaply, and mount it in the attic.
If you search for dipole on this forum (use the Advanced search) you should be able to find the description.
I struggled getting any intelligible reception at all from Shoreham and I was up on the hill near Foredown water tower (Portslade) with my scanner. Mind you, it was a cheap crap one. I reckon my recent (better) one would have less of a problem.
External aerial definitely recommended, where possible.
I'm off down to Shoreham in a few days, I'll try my new scanner (if I remember to take it) to see if it was that, or just the general geography of the area which was preventing me getting a signal. I'll post back.
Last edited by Blues&twos; 24th May 2011 at 06:46.
My new scanner gets good reception on 123.15 up near Foredown water tower, unlike my old one.
I did a bit of a test as I drove south down the M23/A23 - left the scanner switched on and tuned to 123.15/125.4 and got a few completely unintelligible transmissions just south of the M23 jct 10. Then I got nothing else until I was approaching the A27 Portslade turn-off (i.e. westbound).
Whether this was because there were no transmissions or I just wasn't getting them I can't say. So from my experience I'd say your chances of getting readable transmissions in Horsham were slim, but I was only using a handheld device with a little antenna. You might be able to get something with a large outdoor or loft mounted antenna. The only way to find out for sure would be to borrow a scanner and try it, I suppose! You might be lucky.
Having lived in that area for many years I would say you have near zero chance of hearing Shoreham from Horsham at near ground level because of the huge pile of chalk hill in the way called the South Downs.
Although I doubt you will hear the tower you will be able to hear some of the aircraft's half of the conversation, as they are a fair bit higher than the tower. Unfortunatley the South Downs will almost certainly block your signal.
At Horsham you will be able to get the Gatwick frequencies though...
Seems crazy when you think I can use my 'Live ATC' app on the iPhone to listen to hundreds of ATC's around the world, but none from the UK are featured.
I can even listen live to ATC at Fort Pierce in Florida where I did my PPL. Is cool listening to some of the other students and the instructors I knew. Just wish the app would start including some UK stuff.