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

Edward Cole kl7uw at acsalaska.net
Sun Feb 7 22:11:55 PST 2010

At 07:36 PM 2/7/2010, Phil wrote:
>Thank you for reading this.
>I'm still trying to determine if the aerial I have is working correctly or
>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?
>Sent via AMSAT-BB at amsat.org. Opinions expressed are those of the author.
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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
  BP40IQ   500 KHz - 10-GHz   www.kl7uw.com
500-KHz/CW, 144-MHz EME, 1296-MHz EME
DUBUS Magazine USA Rep dubususa at hotmail.com

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