1. Robert Orndorff, W4BNO RATS, April 2015
What’s a dB
Robert Orndorff, W4BNO
RATS, April 2015

2. What’s a dB It’s one tenth of a Bel Named for Alexander Graham Bell
Originally used in telephone system measurements in the early 20th century
Based on MSC (Miles of Standard Cable) and TU (Transmission Unit)
Bel is seldom used without “deci”

3. What’s a dB dB is a ratio Measurement is compared to a reference level
dB alone is not meaningful
Generally used to measure power or power changes
Many uses, only discussing how it is used in Amateur Radio

4. What’s a dB dBm – dB referenced to 1 milliwatt
dBV – dB referenced to 1 volt
dBu – dB referenced to 1 milliwatt into a 600 ohm load, or around millivolts. Typically used in audio applications
Have a calculator handy, check my work!

5. Formula for dB when referencing Power
What’s a dB
Formula for dB when referencing Power

6. Formula for dB when referencing voltage
What’s a dB
Formula for dB when referencing voltage

7. Why use dB?
When dealing with very small or very large measurements, dB is useful
You could say: mW
-90 dBm

8. Why use dB?
A negative dBm reading indicates powers of 10 on the right side of the decimal.
Negative dBm is less than the reference value.
0 dBm = 1 milliwatt
-30 dBm = milliwatts
-50 dBm = milliwatts
See a pattern here?

9. Why use dB?
A positive dBm reading indicates powers of 10 on the left side of the decimal. Positive dBm is more than the reference value.
100 milliwatts is equal to +20 dBm.
1 watt (1000 milliwatts) equals +30 dBm
10 watts equals +40 dBm
Power doubles every 3 dB

10. Why use dB? Power doubles every 3.010299956 dB dB=10*log(P2/P1)
Close enough to say that power doubles every 3 dB change

11. 3 dB changes dBm mW V at 50 ohms 1.00000000 0.22360679775 3 1.99526231 3 6 9 12 15 18 21 24 27

14. Industry specific use
dBsr is a Scaled Reading. A scaled reading is obtained when using equipment intended for 600 ohm systems to read voltages on a 50 ohm system.
Telephone and communications techs doing RF (power line carrier) work.

17. S units
The amount of signal strength required to move an S meter indication from one marking to the next.
S meter is a microammeter connected to detector or in the IF stage, full scale 50 – 100 µA

18. S units
S9 originally defined as 50µV at the input of the receiver (1930s). Input impedance was not standardized, so this was not necessarily a measure of power.
In 1981 the IARU defined S9 as -73 dBm (50µV at 50 ohms) on HF. For VHF S9 is equal to -93 dBm, or 5µV at 50 ohms
1 S unit is equal to 6 dB
1 S unit is equal to a voltage ratio of two, or a power ratio of four

19. S unit, dBm, milliWatt S dBm mW mV at 50 ohms S0 -127 S1
-121 S2
-115 S3
-109 S4
-103 S5
-97 S6
-91 S7
-85 S8
-79 S9
-73

20. What about antenna gain?
My antenna has 3 dB gain.
What does that mean?
(group discussion)
Voltage gain?
Power gain?

21. What about antenna gain?
My antenna has 3 dB gain.
Did you mean 3 dBi?
Did you mean 3 dBd?

22. What about antenna gain?
dBi = Gain relative to an isotropic radiator
An isotropic antenna is an ideal antenna that radiates its power uniformly in all directions.
dBd = Gain relative to a dipole
dBd is 2.15 lower than dBi
3 dBi = 0.85 dBd
Antenna sales brochures and advertisements say ???.

23. Top of the Rock, NYC

24. Questions / Discussion