[Antennas] S units, dB's, and power in wtts

[Jim Reid]

Jerry, K�EJF, asked:

> Excuse me!  I forgot!  How many db's does it take to equal one
> S unit?  In the far field?

Alleged to be 6 dB per S unit;  however,  this is only a "theory".
In actual practice,  it is different for every rig,  and also is
different at various places on the rig's S meter scale.  Most
rig manufacturers today will only say that they calibrate
the S meter to read S=9 at 50 microvolts (-73dbm) at the rig
antenna input terminal.  It is said that Collins Radio started
this decades ago.  If your question was what is an S unit
reading of 1 in the far field equal to,  the answer is 0.2 micro
volts,  or -121 dbm (if the decrease per  S unit were actually
exactly 6 dB down per unit down on the meter).

> How much less is the guy on the other end apt to hear me
> when I am running my linear at 600 Watts v/s barefoot at
> 100 Watts?

Each time your double your output power,  or the input rcvd
signal changes by double,  the change is 3 dB.  So doubling
100 watts to 200 watts is +3dB; double again to 400 watts,
and you have a total change of +6dB (1 S unit increase), and
were you to double again to 800 watts,  the increase would be
9dB,  or just a bit over 1 S unit.

But increasing from 100 to 600 watts is a power  change of
+6 times,  or, on the dB scale an increase of +7.8 dB.  Should
decrease your power from 600 down to 100 watts,  the other
end would "hear" a drop of just 1.8dB more than a 6 dB drop
of one S unit.

If you watch your S meter while you are operating,  you will find
very few occasions when a signal decrease of 1 or even 2
S units will make much of a difference in your ability to copy.
The only time the issue becomes important is when you are
attempting to copy signals very near the band noise/QRM level
at the time,  then the 1 or 2 S units (or 6 to 12 dB) of siganl strength
change can make all the difference in whether you copy
100%, or even at all,  hi.

Note that often, the typical noise level on a band hovers around
the -125 to -120 dBm,  and does change when you change the
IF bandwidth of your rig.  Some examples:

Here at mid-day,  the noise level on 20 meters (14195 just to pick
a frequency) with the BW set to 2400 Hz (very usual for SSB mode),
the noise level hovers about -128 dBm;  for CW BW,  of say
500 Hz,  the noise dropped a bit to around -131 dbm or so.

On 15 meters,  21295 area,  the SSB BW noise level appears
to hover,  maybe right around -130 dBm;  CW BW of 500 Hz,
about -133 dbm,  or so.

On 75 meters,  3800,  the 2400 BW noise level is hovering
right around -120 dBm.  Down on 160 metes,  the mid-day
"natural" noise is about -118 dBm.


Of course,  on the lower bands the noise
level is often higher,  but these numbers are fairly typical from
40 meters.  10 meters is often our most quiet band,  as far
as "natural" noise is concerned.

I suppose this is more info than you wanted to know,  hi.
Anyway,  the above noise readings were all taken just now
using a Ten Tec commercial/military spec RX-340 rcvr
which has a very acurately calibrated meter dbm scale;  the
S meter scale on this particular unit does track the 6 db delta
change per S unit accurately between S 1 and up to S9.  I
have checked the calibration using a high precision attenuator.

But it should be accurate; the RX340 is an expensive rcvr!

73,  Jim  KH7M