[Elecraft] K3/P3 issue

[Guy Olinger K2AV]

3 dB?

I've seen an S unit described as 6 dB -- double the *voltage*, or four
times the power. Or in recent years some radios do 5 dB -- keep a linear
string of LED's where over S9 is in 5 dB increments and below S9 is in 5 dB
S units.

Your assertion is the first time I have ever heard an S unit described as 3
dB.

But to be truthful, there is no metric system authority that sets an S
unit. There are recomendations from radio organizations, all stating 6 dB.
To see S units way back in the beginning, here is a photo of the front of a
1936 RME-69 receiver.

http://www.virhistory.com/ham/spkrs/RME_007.jpg

If you look at the meter labeled "Carrier Level Indicator," below the scale
marks you will see S1 through S9. Above the scale marks you will see dB,
originally spelled DB, from 0 to 72, in 6 dB steps. The S1 through S9 use
the same marks as 0 through 48 on the dB scale. S1=0dB, S2=6dB, ...
S9=48dB. Very clearly S units on this pre-WW2 classic radio are 6 dB.

This predates a 1939 National HRO receiver manual that explains S-units as
6 dB.

So history says S units are 6 dB, with each S unit a doubling of the
voltage of the prior S unit.

An earlier version of the signal strength meter on the RME-69 was
calibrated in dB on one side of the scale and microvolts on the other,
without S units.

Most S meters are notoriously inaccurate and might not tell you any more
than signal A is louder than signal B. Whether the RME-69 could function as
a frequency selective microvolt meter with a decent degree of accuracy is
anyone's guess. I suspect that they could not make the old tube circuits
and the manufacturing tolerances actually match a microvolt scale across
manufactured units, and so they went relative with S units.

Everything you never wanted to know about an S-unit.   :>)

73, Guy K2AV



On Fri, Oct 2, 2015 at 2:43 AM, David G4DMP <david at g4dmp.fsnet.co.uk> wrote:

> In a recent message, Alan <n1al at sonic.net> writes
>
>> So you would expect it to read 3 dB lower (half the power), which is half
>> an S unit.
>>
>
> This is something that has always puzzled me, Alan. I agree that -3dB is
> half the power (in watts), but S-meter calibration is related to
> (micro)voltage.   It is often assumed that doubling ones power increases
> the signal at the receiving end by one S-point.  But what is the technical
> explanation of this?
>
> 73
>
> David G4DMP
>
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