ve2pid wrote in news:753fee20-bfec-4b9a-81b2-
:

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

Pierre,

There is a convention that S meters are calibrated for S9 at 50µV (CW) at
the receiver terminals, and the other S units are spaced out at
6dB/Sunit. (Some of the other conventions mean the same thing, eg 100µV
EMF as one manufacturer is want to specify).

So, if S9 is -73dBm, then S0 ought to be -127dBm.

In a typical receiver with a sensitivity of around -135dBm for 10dB S/N,
the AGC will be delayed until the signal is about 20dB higher than that,
or about -115dBm... so an AGC derived solution cannot indicate less than
S2 on that scale. Nevertheless they do.

At the high end, extremely strong signals are a challenge for nice scale
shape.

In my experience, modern transceivers that provide the facility for three
point calibration of the S meter are reasonalby good between around S6
and S9+20dB... give or take. Reasonably means withing a dB or two per
Sunit.

S meter calibration is invariably dependent on a particular setup of
preamps and front end attenuators. Some of the outrageous 'measurements'
one hears of are done in an 'uncalibrated' state (eg preamp ON when it is
meant to be OFF.

The S meter behaves fairly similarly to a quasi peak detector, responding
to most of the peaks, but not all, and much higher than average power. So
applying the S meter to impulse type signals introduces another issue.

http://vk1od.net/software/fsm/ details another method of making more
accurate measurements of signal level and field strength using a comms
receiver. The system compared favourably in a field comparison with a
commercial EMC receiver and active loop on a BPL measurement exercise.

Owen