Rees,

The use of a device such as this that purports to protect the radio from
a differential signal impressed across one interface could lure you into
a false sense of security.

It does not eliminate the effects of elevation of the 'grounded' sided of
the interface above real ground, and especially relative to other
equipment. Of course, if the radio has more than one interface (eg a
power lead), the problem is much greater than protecting it against a
differential transient on that interface.

It is entirely possible that you do such a good job of protecting the
input interface, that the receiver is destroyed by a transient from the
power cord (if it uses one). You must take the bigger picture into
account.

Then there is a question of the damage threshold (current / time profile)
for the interface, the expected current waveshape, and the response
characteristic of the device.

Whilst a high performance FET might be destroyed long before a gas tube
fires, even one doped with radioactive isotope, on the other hand, the
input stage of a HF receiver with the input attenuator in circuit might
survive without protection.

It is a really complex problem, and the risk is that spending some small
money migth delude you into a false sense of security.

I am with Richard, don't take less than competent measures for possibly
worse outcomes.

Owen