[Radio Wrangler]

If something is getting overloaded, protecting its input with back-to-back diodes will protect it from damage, but the clamping effect of the diodes will generate terrific amounts of harmonic and intermod distortion, crossmodulation and also blocking.

If you still want to use the signals afterwards, then there are some other routes to go down.

An attenuator between the antenna and the amplifier will reduce the amount of the big signal hitting the amplifier. It will also reduce your wanted signal. This might still be a good deal. It will also reduce local environmental noise picked up by the antenna, so you may have more sensitivity which you can use from your receiver. However, doing an attenuator in the lead of an antenna of awkward impedance can be a challenge.

A preselector filter between the antenna and the amplifier can do a bit better if the strong and wanted signals are spaced enough for it to pull the big one down more than the small one. This is heading towards mechanical variable capacitors remotely operated by a servo or stepper motor. Varactor diodes distort quite a lot. For the full HF range, you're also into relay switched coils.

Finally, you can womph up the active antenna amplifier to something more linear through being substantially more powerful than you'd think the signal levels need. All amplifiers are non-linear to an extent. As your signals, even the big one, now use a smaller part of the amplifier's swing capability, they see less curvature of its characteristic, and so less distortion. This will run warm and take a fair amount of quiescent current, but it is broadband. One disadvantage is that the full dynamic range of all the signals hitting your antenna gets passed straight on to your receiver.

The NRD525 was one of the better amateur receivers of its day in terms of large signal handling, but it was not as much better as legend has it.

David