My friend Ricardo, EA4GMZ, has a strong interference in several portions of the HF spectrum. Yesterday he was taking notes about interference levels on 28 MHz band using his Yaesu FT-817. He found out that pressing the IPO button or ATT was mandatory for having a useful curve with diferent strenght levels. If not pressed, interference level values were "compressed around S8". We suspected that FT-817 smeter readings were not linnear.
Test setup was like this:
The available display SMETER readings are:
The results are shown in the next graph:
At levels below S8, we have a S-unit per dB curve. After that, we have an interval of 16 dB input level range that produces a S8 reading. Next three additional dB produces a S9 reading. Following 5 dB read as S+. 12 dB more read as S+#. 17 dB more read as S+##. The last dB increment triggers the S+### level indication.
Smeter calibration is far from the 6dB per S unit standard!
Copyright (c) 2013 Ramiro Aceves . Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled "GNU Free Documentation License".