# [amsat-bb] Re: FT-847 - relating dbm to S units

Sun Feb 7 22:11:55 PST 2010

At 07:36 PM 2/7/2010, Phil wrote:
>
>I'm still trying to determine if the aerial I have is working correctly or
>not.
>
>If -120 dbm for a 12 db SINAD is considered the minimum usable signal,
>what is the likely signal level required to indicate an S1 reading (at 435
>MHz) on an FT-847 with the internal pre-amplifier turned on?
>
>The Yaesu manual quotes 0.2 micro volts for a 12 db SINAD. How many S
>points is likely to be?
>
>--
>Regards,
>Phil
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0.2 uV is directly convertable to dBm (assuming 50-ohms
impedance).  The answer is -121 dBm.  I do not know where you found
-120 dBm as MDS (min. detectable signal) but the 0.2 uV spec it for
MDS on the FT-847 (for FM).  For SSB it is 0.125 uV (-125 dBm).  You
will not read any s-meter reading at MDS because the signal is equal
to noise (again by definition).

S-1 is (by definition) +6 dB over MDS (i.e. -115 dBM for FM and -119
dBm for FM).  The FT-847 s-meter is not linear so it reads a bit off
from that.  I calibrated my FT=-847 for FM some years ago, and can
recall that S-1 was something like +8 dB over MDS (but each radio
will differ a little so if you absolutely need to know then measure
it with a calibrated signal generator and a SINAD meter which was
designed for FM).  (Note: by this reasoning S-9 = +54 dB over MDS,
but most cite that S-9 is +50 dB; see that reality is a bit murky here).

Do not be confused by the standard 12 dB SINAD.  It is a standard
amount of signal quieting for making comparable measurements (very
over simplified definition).  After 30-years experience making SINAD
measurements on radios I can estimate within about 3 dB by ear when a
signal is at 12 dB SINAD (obviously the meter makes more precise
measurements possible).

I hope this helps,

73, Ed - KL7UW, WD2XSH/45
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