SNR linearity

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SNR linearity

Following wide local measurements campaign on 2m we found some differences between attenuation applied at reception and SNR reported. Being in a configuration where fading effect was insignificant, we found that all SNR values reported beow 0 dB were following perfectly the attenuation applied but not over this limit with a kind of compression effect : 13 dB change on SNR for 20 dB applied.
Is there a reason for this compression or not ?
In any case below 0 dB the reliability seems excellent and since we went down to -31 dB it is already a good dynamic range for traditional measurements. With about 10 nanowatts on GPA antenna both sides at 15 kms, decoding was still possible !
73' to all,
Jacques / F2MM