Owen Duffy wrote in
:

Top loading helps increase radiation resistance, but the radiation
resistance of a 10' radiator on 7MHz will be low relative to practical
ground system resistances, and typical system efficiency with a
minimal ground system is unlikely to be greater than about 25%.
However, that might be a quite acceptable compromise to many users.

Deep in the manual, I find:
1. the expectation that VSWR on 40m is likely to be around 1.2, and
2. that the unloaded part of the radiator is adjustable from 5' to 8'.

Following from 2, modelling indicates that a top loaded 8' vertical
probably has a radiation resistance around 5 ohms on 7MHz.

From 1, assuming that VSWR 2 is at the feed point and is achieved with a
feed point resistance of 42 ohms, then efficiency at the feedpoint = 5/42
or 12%.

The VSWR is so good that line loss with 100' of RG8/X for example should
give an antenna system efficiency of around 10%.

Owen