RE: S-Meters and Measuring Signal Strength with Ham Receivers

---------

From: Nick Kennedy (nkennedy@tcainternet.com)
Date: Thu Feb 21 2002 - 19:03:04 CST


-----Original Message-----
From: James R. Duffey [SMTP:jamesd1@flash.net]

>With all due respect to Paul, I think that the S-meters on most receivers
>are pretty worthless. George enumerated the reasons. There isn't much a
>S-meter tells me that my own ears can't. ;^)

I guess I agree Doc, and I think you've proven it with the info in a
separate post. But I think S-meter (S-unit) discussions always take two
paths that seem to disagree but don't necessarily--

Proposition #1: Typical S-meters are pretty worthless because they aren't
consistent in terms of dB/unit and also don't even show the same dB/unit
response throughout their range. I'd suppose that one is true.

Proposition #2: The concept of a standard 6 dB S-unit is useful in
discussing the effects of increasing/decreasing power (changing antenna
gain, etc), meter or no meter. Joe Six-Pack Everyham (that's me) finds this
kind of description more meaningful than mere "-xxx dBm" descriptions. It
think this one is true too and doesn't contradict #1.

>But you can make pretty good quantitative measurements if you can turn off
>your receiver's AGC. Get a VU meter or old analog multimeter that has an
AC
>scale calibrated in dBs. Its that atrange scale in red that you never use.
>VU meters are usually available surplus pretty cheap.

Now you tell me. I think I've finally junked all my meters like that.
(Well, busted 'em or burned 'em up, more likely.)

>They are calibrated
>when used with a 600 Ohm source impedance, usually from -20 dB to +3 dB.
>Although some new recevers (and some very old ones) have 600 Ohm outputs,
>most have a typical 8 Ohm audio output impedance. Get an 8 Ohm to 600 Ohm
>transformer. Radio Shack carries them. Connect the 8 Ohm side to the
>receiver and the 600 Ohm side to the meter.

Minor quibble--related to the harangue on transmitter output impedance I got
involved in a while back--Would you really need that transformer? Except
possibly as a step-up to get the voltage on scale for your meter? The
difference in dBs between two readings would still be the same, transformer
or not--right? A receiver with an "8 ohm output" won't really have an 8 ohm
audio amplifier internal impedance; or not likely. Probably about an ohm or
two. If you wanted to measure actual dBm relative to 600 ohms on your
meter, I guess you'd hook up a 600 ohm load. If you wanted to do that AND
get maximum available power out of the amp, you'd do that AND use the Radio
Shack transformer.

Those of use cursed with modern DVMs and no dB scale can still use your
method: Record the voltage at power level #1, then at #2. Calculate V2/V1,
take the log of that, multiply by 20. If you want the absolute (dBm)
level--hook up the 600 ohm resistor, measure the voltage, square it, divide
by 600, take the log, multiply by 10.

The extra cautious will want to hook up a scope to the audio amp and make
sure the output is in it's linear range (i.e. --not flat topping). When
flat topping starts, as you said--start using your built-in attenuator.

Your initial statement, "There isn't much a S-meter tells me that my own
ears can't. ;^)", reminded me of an ear based system described in JF10ZL's
excellent homebrew page & quoted below:

***********************

[Sending RST without an S-Meter]

How to send ear S
Mr. Shimura made a question on my friend's home page that how do we send the
ear S with our S meter less machine. I made answer as follows. I sand ear S
for more than ten years on this way.

59 plus : When I receive his signal, fire burning on my eyes. The fan noise
of linear amplifier can be heard. Also the click noise of the clock is
heard. His voice over drives the inter frequency (IF) amplifier of my
receiver. Therefore the voice has distortion. So hard to hear.
59 : Not hard to hear. Strong enough. I feel hearing his voice, like face to
face. I can hear his voice with no electric noise.
57: Enough to strong to make conversation. I can hear his signal with slight
electric noise.
55: Slightly week signal. Not to hard top hear. I can hear his signal always
with electric noise. I can copy his call sign. But I do not want to make
long QSO, because it will make me tired.
53 : weak signal. Hard to hear. I could copy his signal with hearing three
times.
51 : I want to stop QSO with him. Weak! Only call sign , we made QSO enough.
I do not be interested about your local weather.

***********************

JF0OZL also has some fine homebrew experiments
and projects and more great ham philosophy on his page. Check it out.

72--Nick, WA5BDU


Search QRP-L Archives

[ QRP-L Archive | ]
[ 1993 | 1994 | 1995 | 1996 | 1997 | 1998 | 1999 | 2000 | 2001 | 2002 ]

---------

This archive was generated by hypermail 2b29 on Tue Mar 19 2002 - 16:04:13 CST

kd4ab@kd4ab.org