"Ian Jackson" wrote in message
...
In message , JB
writes
"Roy Lewallen" wrote in message
m...
ve2pid wrote:
Hi to all,

Hope that my question has some sense...: If we double the power
radiated by an antenna (+3 dB), how does it translate on the S-meter
at a receiver 'far' away.. i.e. is the difference in dB on that meter
has some mathematical relation to the 3 dB change at the transmitter?

73 de Pierre

No, none at all. S-meters almost always just measure the AGC voltage,
which is only approximately logarithmic. And manufacturers adjust the
sensitivity of the meter to please the customers, who don't like "dead"
meters. So the sensitivity of S-meters varies greatly from rig type to
type and from one end of the scale to the other. I measured the
sensitivity of the S-meter on my Icom 730 on one band. Here's how big
an
S-unit is on my rig:

S1 - S2 1.4 dB
S2 - S3 1.3 dB
S3 - S4 1.6 dB
S4 - S5 2.3 dB
S5 - S6 1.8 dB
S6 - S7 3.2 dB
S7 - S8 3.1 dB
S8 - S9 4.0 dB
S9 - "S9 + 10 dB" 5.6 dB
"S9 + 10 dB" - "S9 + 20 dB" 7.3 dB
"S9 + 20 dB" - "S9 + 30 dB" 6.6 dB
"S9 + 30 dB" - "S9 + 40 dB" 10.5 dB
"S9 + 40 dB" - "S9 + 50 dB" 11.3 dB
"S9 + 50 dB" - "S9 + 60 dB" 13.5 dB

Now let's suppose that you built a new 5 element Yagi antenna and I
honestly reported that your signal went from S2 to S6 when you switched
to it from your dipole. The gain is really 7 dB, about par for the
beam,
but you read the postings on the Web and decide that an S-unit is
"defined" as 6 dB, so the gain improvement must be 24 dB. Wow! Your
modest beam has the same gain as a beam with a 25 WAVELENGTH boom and
more than 50 elements! By making that incorrect assumption about the
sensitivity of my S-meter, you've overestimated the gain by a factor of
63.

S-meter sensitivities vary all over the map, so any assumption you make
about how many dB in an S-unit is very likely wrong, and often grossly
wrong.

Roy Lewallen, W7EL

Nail on the head. A step attenunator in line will be the fastest way to
determine the linearity of your scale if you can't afford a calibrated
generator. FM rigs are all over the map on this because some radios have
better limiting than others, and it might be actual limiter reading or
some
other derived reading.

You'll need an adequately-screened signal generator to do this test. If
it isn't, the signals which leak out can bypass the attenuator and enter
a not-too-well-screened receiver. If this happens, the results you get
will be false.
--
Ian

Good point. If the signal is off the air it is less likely to be an issue.
You need to seriously work on your setup to get better than 60 db of
resolution. I homebrewed the 3, 6, 10, 20 step attenuator from one of the
ARRL publications and find it quite useful. Output cable is double
shielded.