[TowerTalk] Speaking of S units

[Roderick M. Fitz-Randolph]

>> I dont understand what you are griping about, Pete.  An S unit IS 6 db
>> and a properly aligned radio will bear this out.  I've checked my MP and
>> it's S meter is dead on.  73, Ty K3MM
>
>I'll stand by everything I wrote. If Yeasu got religion and arranged for
>the MP's S units to be 6db each, that's great but it puts that rig into a
>very select group. Based on all I've seen, the statement that an S unit is
>always 6 db is about as useful in this hobby as the statement that "a tank
>of gas in a car is always equal to 300 miles."
>
>Regards,
>Pete
>KS4XG
___________________________________________________________________________

Pete, back in the mid to late 1940's, when I first became interested in
ham radio, virtually everyone you could have met would have told you that
an "S" unit was 6 dB.... that is, by definition, not practice.  Since all
of the S-meters of that era ran off of AVC (or later AGC) voltage, an
S-meter was only as accurate and as linear as the circuit parameters/design
allowed it to be.  Component values would change, tube gain varied from tube
to tube, etc., and all you knew was that an S-unit was SUPPOSED to be 6 dB.
Most folk knew that the limitation was the components/tubes, etc.

Nothing has really changed.  Some of the manufacturers in later years began
to use 5 or even 4 (and I think one went to 3) dB per S-unit.  But for the
most part, it has been a standard that an S-unit was SUPPOSED to be 6 dB.
Now, I've never seen an S-meter that would linearly represent signal strength
across its signal strength meter as 6 dB per unit: the Collins 75A4 I owned
came as close as any of them and its S-meter required 100 microvolts for an
S9 setting.... but you could adjust the sensitivity control and zero control
so that you get almost anything you wanted - within a very narrow range on the
S-meter.  As an example, if you set it equal to 6 dB between S7 and S9, I could
almost guarantee you that the same would not be true from S1 to S3 or from 15
over 9 to 27 over 9!  The non-linear properties of AGC voltages, etc., simply
won't allow a completely accurate and linear presentation of received signal
strength.  It is a RELATIVE thing.

My present TS-940S/AT is a prime example.  I can diddle with the S-meter
circuit parameters and get the S-meter to represent, at S9, 50 microvolts
of signal as received from a calibrated signal generator but I guarantee
that it won't be linear up and down the scale.

I've digressed a little.  S-meter readings started out as 6 dB per unit.
That is still understood to be the "standard" by most hams.  Younger hams
that have come in and learned about ham radio on a receiver/transceiver
that had a different setting (because, in some cases, I think the
manufacturers were engaged in the "my car has more horsepower than your
car", "my hi-fi set has more watts than your hi-fi set", and "my receiver
has a higher deflection on it S-meter than your receiver for the same
signal" race) have not had the advantage of learning or knowing that an
S-unit is equal to (supposed to, that is) 6 dB.

Anyhoooo, that's the facts of it as I have always known them.  An S-unit
WAS and still should be, IMHO, 6 dB.  Not 5 dB, not 4 dB, and not 3 dB.

The curmudgeon has written, and having writ, moves on. All thy piety and
wit will not call back one word of it.......

That sure does feel better now.  Not near as much gas pains.

Rod, W5HVV



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