For a continuous CW tone, the level should be the same on the P3 and the
external monitor. For SSB, the SVGA display should read 1/2 S-unit lower.
The reason is that, unlike CW, an SSB signal is spread out in frequency.
Since the SVGA monitor has twice the frequency resolution than the P3
main screen, each display point only covers half the bandwidth. So you
would expect it to read 3 dB lower (half the power), which is half an S
On 10/01/2015 01:11 PM, David Cole wrote:
> Perhaps I missed the answer to this...
> My SVGA external monitor shows signals about 1/2 to one S unit lower
> than my P3 does on the native screen... What am I missing?
> If I use Peak Hold, there is a clear 1/2 to one S-Unit difference in teh
> display height...
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In a recent message, Alan <[hidden email]> 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?
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
Your assertion is the first time I have ever heard an S unit described as 3
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.
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
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. :>)
> In a recent message, Alan <[hidden email]> 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?
> David G4DMP
> + - - - - - - - - - - - - - - - - +
> | David M Pratt, Kippax, Leeds. |
> | Website: http://www.g4dmp.co.uk |
> + - - - - - - - - - - - - - - - - +
> Elecraft mailing list
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