Presentation on theme: "What’s a dB Robert Orndorff, W4BNO RATS, April 2015."— Presentation transcript:
What’s a dB Robert Orndorff, W4BNO RATS, April 2015
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 20 th century Based on MSC (Miles of Standard Cable) and TU (Transmission Unit) Bel is seldom used without “deci”
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
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!
What’s a dB Formula for dB when referencing Power
What’s a dB Formula for dB when referencing voltage
Why use dB? When dealing with very small or very large measurements, dB is useful You could say: mW -90 dBm
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?
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
Why use dB? Power doubles every dB dB=10*log(P2/P1) dB=10*log(2) 10*log(2) = …… Close enough to say that power doubles every 3 dB change
3 dB changes dBmmWV at 50 ohms
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. Industry specific use Telephone and communications techs doing RF (power line carrier) work.
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
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
S unit, dBm, milliWatt SdBmmWmV at 50 ohms S S S S S S S S S S
What about antenna gain? My antenna has 3 dB gain. What does that mean? (group discussion) Voltage gain? Power gain?
What about antenna gain? My antenna has 3 dB gain. Did you mean 3 dBi? Did you mean 3 dBd?
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 ???.