Hi Fred,
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I agree with the points you are raising, with some caveats.
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In a 50 ohm system, a pair of 1N4148's or similar small signal silicon diodes will limit at somewhere around +10dBm, and second and third order (two tone) IMD, at average signal levels, (peaking around-30dBm max) will typically be at, or below, the noise floor of an average receiver, in an average location.
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E-Probe, high impedance, antennas are tricky, and it's usually necessary to and more diodes to a string, or provide some reverse bias, to offset the point of conduction. Loops and relatively short E-Probe type antennas generally don't develop a high enough voltage across the diodes for IMD to become noticeable. Plus, many designs are pretty poor in terms of IMD performance anyway, and I suspect that additional IMD produced by the protection diodes, would probably go unnoticed.
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Sometimes we have to trade one problem for another, and it's a user's choice which is preferable.
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Regards,
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Martin
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On Thu, Feb 27, 2025 at 11:35 AM, Fred M wrote:
It is a common misconception that there is a sharp forward voltage threshold for diodes and that there is no current flow below this "forward voltage". A diode is an amplitude-dependent, non-linear resistor, described by the Shockley equation. The non-linearity also exists at small voltages and is the reason why IM Products occur even with samll RF levels below the "forward voltage".
