[UK-CONTEST] Vertical radials

[Ian GM4KLN]

Hi folks

Just installing a 6BTV which will be at ground level on heavy, stony
Scottish Highlands ground. Radials will be permanent, and lain on top of the
grass with a view to them disappearing into the ground by spring next year.
Interest is DX, contesting: NVIS minimal interest. 80m is not a key goal
with this antenna - 20, 30, 40 are.  

There is enough room to get a 360 spread of 20' radials which will cover the
near field fine. 

But over and above this: am I likely to get any practical benefit from
adding additional radials of 30' (in a bow-tie) or even 60' in certain
directions  (i.e. diagonal in a rectangular plot)

If so.....can these also be random length, or will tuned for lowest band
(80) reap benefit. 

Any sources of radial wire in the Highlands also welcome! 

73, Ian GM4KLN