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**Robert Orndorff, W4BNO RATS, April 2015**

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

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**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”

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**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

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**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!

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**Formula for dB when referencing Power**

What’s a dB Formula for dB when referencing Power

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**Formula for dB when referencing voltage**

What’s a dB Formula for dB when referencing voltage

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

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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?

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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

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**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

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**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

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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.

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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

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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

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**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

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**What about antenna gain?**

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

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**What about antenna gain?**

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

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**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 ???.

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Top of the Rock, NYC

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**Questions / Discussion**

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