Hi Covernotes, 20 mtrs is a huge run of coax, you will get lots of loss even with good coax. Try to get your antenna as close to your shack as possible, as long as it is in the clear - ie it is clear of the roof and any nearby objects. Any extra height requires more coax and more loss. So the small amount of extra distance you get with height will be lost in more coax. One thing you could do is move your radarbox up into the roof space and run shorter coax through the roof. Then use a usb extension back to your computer, there is no loss in the usb cable. I am testing another antenna, its a 1/4 wave (65mm) on an aluminium groundplane, I can get over 100nm with it and it has 6 mtrs of rg58cu coax
Good luck
Mike
Burnie, Tasmania