The S-Meter can give exact values as long as you don't relate to the scale. I've measured dB values with a professional Rohde & Schwarz receiver. So here is what you may call a conversion table. The values depend on the IF filters used and the frequency.
Wider IF-filters will lower the signal level. Values in dB:
There are at least four ranges with significant differences. The first one leads up to 230 MHz. There seems to be a pre-amplification centered on the 144 MHz amateur radio band, indicated by "AMP" in the display.
The second area ranges from 230 MHz to 400 MHz. ATT20 does not make any difference to ATT10 anymore. The "AMP"- preamplifier can't be switched off. It seems like it's still the one that is focussed on 144 MHz because values decrease with higher frequencies.
From 400 to 1 GHz another pre-amplifier seems to be on. This one is centered at 600 MHz. ATT10 attenuates by around 9 dB in the whole range.
Above 1 GHz the attenuators do not work. The AR5000 switches to another pre-amplifier. The signal at 1 GHz was +7 dB compared with the reference. As my professional oscillator stops at 1 GHz I could not measure anything above.
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