Owen writes:

I have been exploring models of a quarter wave monopole over a set of
radials on 80m using NEC4 models.

If my models are valid, and they use 'average ground', the indication
is that while it may require a large number of buried radials (16)
before efficiency levels off a bit, similar efficiency can be obtained
with just three radials elevated more than 100mm above the soil.

N6LF made extensive measurements and essenstially confirmed this. He
wrote a 7 part series of articles for QEX. You can download them at
his site:
http://www.antennasbyn6lf.com/2009/1...periments.html

This leaves me wondering why the popularity of extensive fields of
buried radials for the lower bands.

Practicality. In most cases, you either want to be able to walk above
the radials, (i.e. bury them or leave them on the ground) or below
them. This means at least 2 - 2.5 m up, and there will be some
sagging. Essentially, your vertical just got that much shorter. But if
what remains is tall enough, it's a great choice. If a friendly farmer
lets you borrow a field in wintertime, stringing four elevated radials
is a lot less work than rolling out 32 on the ground.

Jon LA4RT, Trondheim, Norway