Hi Andy and All

The catch diodes clamp over-voltage spikes extremally well, in fact I
don't see any spikes at all on the breadboard design.

Yes, the "totem-pole" configuration is sort of self-clamping if MOSFETs
are used. Did you add external (low Vf) diodes or do you rely on the
intrinsic diodes in the FETs?

As far as I can see, the only potential device destroyer is a low
impedance load or short circuit.

<snip...>

By the look of the network, A detuned antenna will reflect back as an
increased load impedance giving an inherent fail safe mechanism.

The lowpass filter between the amp' and the antenna can turn things upside
down when a severe mismatch occurs. For example, a simple pi filter with
a Q of 1 (C1 across, L along, C2 across, all reactances equal to line Z)
will "invert" the impedance like a lambda/4 length of transmission line.
In case the filter output is open (antenna way off resonance), L and C2 forms
a series resonant circuit which makes the filter input look like a dead short at
the design frequency. On the other hand, A short on the filter output causes the
input to look like an open circuit (C1 and L in parallel resonance)...
The impedance inverting properties of a simple pi filter can be useful when the
antenna is fed via a tap or link on the loading coil but for series fed antennas
I'd choose two cascaded pi filters (like the one on G3YXM web site).

73
Johan SM6LKM