[AMPS] Power Meter Readings Question

Jim Reid jreid@aloha.net
Wed, 28 Apr 1999 10:25:37 -1000





Peter wrote,  in part:

> Power/SWR meters are somewhat better than S meters (where 
>'S' stands for 'suspicious'), but especially in cases like the one 
>Vic outlines, they can lie nearly as much as a personnel manager!

Hi to all,

Some comments on power levels,

Out here in KH6 land,  many rcv'd signals  are S0 on my FT1000 
S meter,  and the same on my Omni VI+.

The ARRL handbook refers to an old standard of  S9 calibrated at
50 microvolts RF input and 6 dB per S unit up/down from that 
reference point.

The section also clarifies this by stating in reality there are seldom
two rigs that follow the same absolute calibration curve considering
different designs and gain variations from band-to-band.

It is stated that the S meter is useful for relative comparisons on
the same rig operating on the same band.

It is interesting to play with the numbers considering the sensitivity 
of a typical modern rig and the calibration reference of S9 for 
50uV RF input.   One result,  using 6 dB of power change per 
S unit ( or a doubling of the signal voltage, or four times increase 
in signal power):

Signal in  microVolts  vs S-Units

Based on the accepted standard of 50uV = S9, with the signal
on a transmission line with Z=50 ohms,  resistive,  into the rig:

MicroVolts      S-Units
   0.099........S0
   0.2............S1 (uV rounded to nearest .1 from here,)
   0.4............S2
   0.8............S3
   1.6............S4
   3.2............S5
   6.3............S6
  12.5...........S7
  25.0...........S8
  50.0...........S9
 158.0..........S9+10dB

And,  certainly we have all "read" signals when our S meter was
sitting on 0,  no deflection,  so the signal had to be above the
noise and the S meter, in fact,  not reading down to the rcvr's
minimum detectable signal level.

I have the ARRL's very detailed, 28 page, test report about the
Ten Tec Omni VI+ rig.  This test report lists the following
interesting numbers:

Minimum discernable signal, CW, 500 Hz  BW:...-135 dBm@ 3.5 MHz.
and  -133 dBm@14 MHz,

or these would be the numbers of -dBm above which one could
just hear a CW note,  depending upon the operator,  perhaps one
or 2 dB above the noise (mds) level.  Now,  maybe the ARRL used an
oscilloscope,  and took the measurement where they could
just see or notice a signal pip just above the noise peaks,  not
sure.

Similar numbers reported for the Kenwood TS-950SDX are
around -137,8 dBm,  and for the Yaesu FT-1000,  in the -136 dBm
area.  I suspect that the ARRL used the 250 Hz CW filters
available for each of these rigs.  No two rigs off the mfg line are 
going to measure exactly the same,  probably to within 2 or 3 dB  
for such difficult measurements,  and the results  are also
going to depend upon the 500/250 Hz bandwidth mfg accuracy 
of the IF filter chain being used,  or in both IF's are being filtered,
and cascaded,  or not??

For the Omni VI+, the ARRL engineers reported initially that
to get the S meter to read S9,  required a signal level of  
82 microvolts, or,  the S meter was reading roughly a half S 
unit lower than the presumed "standard".  ARRL returned
the rig to Ten Tec for adjustment,  and upon return, S9 was
read with 50 microvolts input,  I assume on 20 meters,  was
not explicitly stated in the report,  or I missed it.

Someone should check my arithmetic,  but I believe I calculate
that 1 microvolt standing across a 50 ohm resistor is going to
dissipate a power of 2 X 10 to the minus 14 watts,  or  -107 dBm.

So,

1.0 microvolt..........-107 dBm
0.5 microvolt..........-113 dBm
0.25microvolt.........-119 dBm
0.125microvolt.......-125 dBm
0.1 microvolt...........-127 dBm,  and S meter reads,  0,  zero.
and so forth.

And,  thus you can see why we can "read" or detect or hear
a signal from our rigs,  even when the S meter is reading 0
if it were absolutely accurate at 50 microvolts being an S9 signal,
where S=0 would be about a 0.09+ microvolt (about -127 dBm)
signal,  and this would then still be several dB above the rcvr's 
ARRL   measured noise level (minimum detectable signal levels) 
in a stated 500/250 Hz bandwidth in the IF strip of the rig being 
tested in the 20 meter  CW band.

With the FT1000,  250 Hz BW,  USB around 14070,  I get solid PSK
print out here all the time with the signal below 0.1 uV,  or -127 dBm
or weaker.  Have to center the narrow filter using th shift knob,  but
works fine.

So all very interesting,  I guess,  hi.

73,  Jim,  KH7M
On the Garden Island of Kauai












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