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Amateur Radio Quiz: A Log of dBs

TAGS: mw, power
01/29/2013

By H. Ward Silver, N0AX
n0ax@arrl.net

If there is a single unit of measurement beyond volts and amps that can be considered nearly ubiquitous in ham radio, it would have to be our friend, the dee-bee or dB. Key to computing values in dB is the logarithm, so this quiz also combines the two -- are you ready? If dBs aren’t your cup of tea, the ARRL website contains a tutorial on decibels that may help you out.

1) What is represented by a negative value of dB?
a. Power gain
b. Oscillation
c. Power loss
d. Ratios of imaginary numbers

2) What is the reference value for calculating dBW?
a. 1 Ohm
b. 1 mW
c. 1 W
d. 1 V

3) Which of the following can be used to specify a receiver’s noise floor?
a. dBuV
b. dBV
c. dBm
d. any of those choices

4) What is the abbreviation for “natural logarithms”?
a. Ln
b. Log(n)
c. Ll
d. Lnat

5) Which of the following represents a power level of 1500 W?
a. 1.5 dBm
b. 1500 dBW
c. 3.1 dBW
d. 61.8 dBm

6) What must always be included when stating a value of antenna gain?
a. Ground conductivity
b. Reference antenna gain
c. Feed line loss
d. Front-to-side ratio

7) If 10 is the base used to calculate log values for dB, what is the base for natural logarithms?
a. e
b. π
c. Boltzmann’s constant
d. Avogadro’s number

8) Which is larger: 10 dBm or -30 dBW?
a. 10 dBm
b. -30 dBW
c. They are equal
d. These two values are not directly comparable

9) Which of the following is another way of expressing SWR?
a. Nepers
b. h parameters
c. ERP
d. Return Loss

10) When making power comparisons in dB between signal levels measured in volts, what must be kept the same for all measurements?
a. Current
b. Frequency
c. Impedance
d. Resolution

Bonus Question: The dB is named for what famous inventor?

 

Answers:
1) C -- Ratios of less than one have a negative logarithmic value.
2) C -- The W indicates that the reference power is 1 watt.
3) D -- All three can be used to specify signal levels.
4) A – Ln or ln
5) D – log10 (1500 W/1 mW) = 61.8
6) B -- To have meaning, antenna gain values must be specified with respect to a reference antenna, such as a dipole (dBd) or isotropic radiator (dBi).
7) A -- e is also known as Euler’s number and has a value of 2.71828.
8) A -- 10 dBm = 10 mW and -30 dBW = 1/1000th of a watt, or 1 mW.
9) D -- Higher positive values of Return Loss (RL) in dB indicate less power returning from a load, indicating a lower SWR.
10) C -- Since power equals V2/Z, both V and Z must be accounted for when calculating power. If power calculations are based only on voltage, the impedance at the point of measurement must be the same.

Bonus Answer -- The Bel -- a unit of sound intensity -- is named for Alexander Graham Bell who invented the first practical telephone.



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