[600MRG] Uncalibrated S-Meter

Mon Jul 18 13:18:10 CDT 2016

Edward,
No confusion, it's called an analogy. I was saying that the "variable resistor" was analogous to propagation conditions which will change the measurements in both cases.Calibrating the s-meter to s-9/ 50uv with a calibrated generator is all well and good, but in practice of receiving a distant station ( other than ground wave) the s reading can vary minute to minute, day to day. due to propagation. All you can say for sure is that the signal was s- something at some given moment tin time.

You must be looking to measure the ground wave in a special case.situation.  I don't know...  
Being such a sticker for accuracy, you still concede that the MW ground wave is 'NEARLY" invariant, meaning slightly variable just like the s-meters!

 Even if the remote station has an s-meter error, assuming it's linear, an improvement in ground wave signal can still be  noted.  Also, you are transmitting to HAM stations not commercial stations, so it is what it is.  Life is too short to stress over trivia. 

    On Monday, July 18, 2016 1:49 PM, Edward R Cole <kl7uw at acsalaska.net> wrote:

 Mr. Mcvey,

Think about what you are saying.  You are confusing reception reports 
with making lab measurements of your equipment.  There is no 
"propagation" involved in measuring receiver performance when 
connected to test equipment for providing a test signal.  If you are 
looking for high accuracy (say 0.1 dB) then you should also measure 
the loss in your test cables and compensate your measurements.

First step in making accurate on-air signal measurements is 
calibrating the receiver.  Then your readings will be comparable with 
other stations with likewise calibrated equipment.  Otherwise just 
use the big big signal - big signal - average signal - weak signal - 
weak weak signal report system which is meaningless to anyone but 
yourself.  I lump S-meter readings in with that report system.  Ever 
notice contest signal reports are all "5x9".

Even WSPR is dependent on station components (antenna, feedline, 
receiver).  Most hams consider that its near impossible to know all 
factors precisely.  Reports are subjective.

On air reception is somewhat useful for determining your station 
performance if you take into consideration that propagation is 
changing the signal.  Much better to use good test equipment to take 
out the uncertainty of propagation.

Pretty hard to make to measure "conditions" of on-air signals for 
tweaking your equipment.  One exception it for near-distance 
ground-wave signals on MW which have been shown to be "nearly" 
invariant in strength.  But for most ham radio signals the 
uncertainty of propagated signal level makes it unusable for accurate 
system adjustments.  Sure you can guessimate what's going on and 
maybe get lucky.

Don't ever think about trying eme with that philosophy or setting up 
commercially reliable communications.  Those require precise 
engineering.  This is why so many professionals disdain hams.

73, Ed - KL7UW
600m-10GHz

At 08:52 AM 7/18/2016, J Mcvey wrote:
>You can't really ever get an "accurate" reading of how your 
>equipment is doing because it is propagation dependent.
>For example: You can have a lab calibrated voltmeter, but if there 
>is an unknown high resistance between the meter and the point being 
>measured, the calibration is compromised. The resistance can create 
>a much greater error than any calibration slippage. ( assuming that 
>it was calibrated once upon a time.)
>
>The effort may be better served to plot wspr reports from various 
>areas vs a propagation map conditions in between you and station X.
>This way you will know for a given condition, station X can hear 
>you. You can determine if your equipment improvements have born 
>fruit if station X can hear you in worse conditions than before.
>
>
>
>
>On Monday, July 18, 2016 11:35 AM, Edward R Cole <kl7uw at acsalaska.net> wrote:
>
>
>Frank,
>
>At 04:07 AM 7/18/2016, Frank Lotito wrote:
> >In discussing the measurement of noise some of us have used the
> >phrase "uncalibrated S-Meter."  What is an "uncalibrated S-Meter?"
> >What makes us think that without challenge, S-Meters are unreliable,
> >while signal strength meters reading out in microvolts or dBm are
> >any more accurate?
>
>Most ham radio s-meters are not an accurate measure of signal
>power.  But do not take my word for that - measure it.  Inject a
>signal of known level (say from a signal generator of recent
>calibration) and read what the meter indicates.  An awful lot will
>not show S9 with 50 uv carrier signal.  Then lower the signal in 6-dB
>increments noting what the meter indicates.  My FT-847 was not too
>bad but S2 to S3 was 7-dB and S7 to S8 was 5-dB so the meter was not
>even linear with power.  That's OK if handing out signal reports, I
>suppose, but hardly a reliable indicator if you are making technical
>measurements.  My K3 appears much better in both regards (I'd say its
>within 1/2 dBm and quite linear).
>
>The difference for the Elecraft and other SDR's is that signal level
>is measured in the digital domain and displayed digitally (LCD screen
>has no magnetic movement that requires precise alignment).  Or if the
>signal is displayed on a computer screen it represents the digital
>information obtained from DSP and not a simple micro-ammeter with
>physical movement.  Note I'm not saying all receivers but most
>receivers.  Laboratory instrument meters can be calibrated and
>measure accurate ac levels (e.g. a good audio voltmeter).
>
> >  If for example, if S9 means 50 uV at the antenna terminal of a
> > receiver having a nominal 50 ohm input impedance, wouldn't the
> > microvolt readout option indicate 50 uV?  The dBM readout option
> > indicate -73 dBm? (For those receivers where we can select the
> > signal strength unit, S-unit, dBm, or microvolt.)
>
>If you have not tested it with a calibrated signal then the s-meter
>is uncalibrated.  If it passes both accuracy and linearity tests then
>you are good.
>
>
> >Granted, the glow-in-the-dark, heavy-iron boat anchor's S-Meter in
> >all likely hood  has a signal strength metering circuit that lacks
> >not only accuracy, but also "repeatability." Maybe some of the newer
> >design solid state receives suffer the same ills?  Ditto for RF
> >spectrum analyzers, RF level meters, RF signal generators with
> >built-in output meters / step attenuators.
>
>Depends on mfr quality and how recent the instrument was
>calibrated.  That's what the calibration sticker is all about.  Most
>hams have surplus stuff which is mostly out of date for
>calibration.  But the instrument may not be that far off and probably
>provides original linearity.  But we do not know for certain unless
>they are recertified.  Example is I have a HP432A (circa 1970's)
>which I depend on for mw level power measurements.  It could be off a
>dB or so, but I have no access to calibrate it so I just decide its
>my standard against which all my other equipment is measured.  Same
>for frequency; I have a surplus Rubidium source of 10-MHz which is
>supposed to be within +/- 5 E-11 = +/- 0.000000000005 Hz.  It might
>not be dead on but good enough for my use.
>
>
> >At our individual stations, repeatability is more important than
> >accuracy. I submit even linearity is more important, e.g. each
> >S-Unit really is a step of "X" dBm? When we compare the measurement
> >results between stations, accuracy and repeatibility are both important.
>
>See my comments , above.
>
>
> >There are ways to determine a signal measuring device's "accuracy
> >and repeatibility."  Can you suggest one or more methods  of doing
> >that other than "using a calibrated signal generator that has not
> >seen the inside of a bona-file RF calibration lab" in the past 10 to
> >20 years?  I don't believe we can for accuracy.  However, within the
> >confines of our own shack, our "A" vs "B" measurements, especially
> >when we can do the comparison by flipping a switch in a fraction of
> >a second, should not be clouded by accuracy concerns.
>
>Yes and no.  Granted our test equipment is probably not lab-accurate,
>but some items are much better than others.  The typical s-meter is
>very low in accuracy in that universe.  I trust my SDR displays and
>my surplus HP spectrum analyzer over s-meters (unless I have
>rigorously checked the s-meter against a more trustworthy instrument).
>
>An example is measuring noise figure.  I don't own a NF meter but I
>can verify my system performance by measuring sun noise.  Of course
>that varies but one can get an accurate daily solar flux reading from
>one of the solar observatories.  That probably gets you within +/- 1
>sfu which I believe is measured in Janksies.  I don't have to do the
>math to determine my solar reception as I have sw to do that.  The
>performance figure of merit is a comparison of cold sky noise to
>solar noise called a Y-factor which is expressed in dB (and
>convertible to dBm).
>
>But I agree that for hams its all relative; some hams are more
>exacting than others.  If I really need to get an accurate
>measurement or calibrate an instrument I can ask one of them to do
>that.  May not be cal-lab level but I'll bet its much closer than I
>can get by myself.  I take my preamps to VHF conferences which offer
>NF measuring with equipment borrowed from commercial labs that are
>certified.  Helps me know how my stuff is doing.
>
> >
> >
> >73 Frank Lotito  K3DZ / WH2XHA
> >
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>73, Ed - KL7UW
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