That's similar to mine, but mine has two 1N4148 diodes in series,
back-to-back, total of four diodes.
However, after I did that, I got to thinking that it would be better with
only two. Or that there was no need for four. (Don't ask me to explain,
it's been a long time since I've thought about that.)
You probably used four diodes for a good reason.
20 dB is 2.73 volts RMS across 75 ohms. That is 3.86 volts peak.
If you use back-to-back diodes, the system clamps at .7 volts peak and mixes
at lower levels. That's like 5-10 dBm or less for the sum of all signal
power.
Remember that clamp is seeing the vector sum of voltages from ALL the
signals on the RX antenna, not just signals on the band you are using or the
signal you are listening to.
I can't use diodes here because of IMD, so I use a hard limiting clamp that
is preset by a Zener diode.
Why would anyone want a clamp system that reduces the IM DR of a modern
receiver? If you pay all that money for an RX, why not use it? I would think
your 4 diodes are the absolute minimum to use almost any receiver's full
dynamic range.
73 Tom
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