Roddy,
As you must have noticed by now, nobody has tried with a 450' extention and got presentable results. The reason is most likely the expected very high attenuation in such a long cable, the screen will show noise, and that's it. The longest functional cable, confirmed with images, I'm aware of is our french friend Abra's 60 m (180') "space technology cable".
Regards,
Rickard