Presentation on theme: "Robert Orndorff, W4BNO RATS, April 2015"— Presentation transcript:
1 Robert Orndorff, W4BNO RATS, April 2015 What’s a dBRobert Orndorff, W4BNORATS, 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 centuryBased 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 meaningfulGenerally used to measure power or power changesMany 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 voltdBu – dB referenced to 1 milliwatt into a 600 ohm load, or around millivolts. Typically used in audio applicationsHave a calculator handy, check my work!
5 Formula for dB when referencing Power What’s a dBFormula for dB when referencing Power
6 Formula for dB when referencing voltage What’s a dBFormula for dB when referencing voltage
7 Why use dB?When dealing with very small or very large measurements, dB is usefulYou 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 = milliwattsSee 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 dBm10 watts equals +40 dBmPower 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 369121518212427
14 Industry specific usedBsr 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 unitsThe 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 unitsS9 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 ohms1 S unit is equal to 6 dB1 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 S1-121S2-115S3-109S4-103S5-97S6-91S7-85S8-79S9-73
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 radiatorAn isotropic antenna is an ideal antenna that radiates its power uniformly in all directions.dBd = Gain relative to a dipoledBd is 2.15 lower than dBi3 dBi = 0.85 dBdAntenna sales brochures and advertisements say ???.