Mike,
Relatively thick plastic insulated wire will do fine for shallow-buried
radials.

Funnily enough, they result in slightly lower input loss resistance. (If you
take them out of the ground entirely, so that they behave as elevated
radials, you get a very low loss of course and with strong resonance
effects.)

Thick plastic insulated buried radials will have slightly greater resonance
effects than bare or enamelled wire which have next to none. They will also
have lower attenuation in the ground. In principle they could be made
somewhat longer than the equivalent bare wires. But as the length of bare
wires is all wishy-washy anyway you can forget about it. Go ahead and lay
it.

By the way, on 160 and 80 meters, if the shape of your property is suitable,
include your domestic plumbing system in the radial system. The best radial
you have is the incoming metal water main pipe. ;o)

At present I have a 160m inverted-L. Had it for years. The ONLY radial is
the domestic incoming main water pipe. At one time there were 7 other
radial wires of various lenghts around the garden but these were all
disconnected when part of the garden was paved with concrete slabs. I
noticed no difference in performance with the solitary but very good
water-pipe radial.
---
Reg.