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Wireless Communication Basics

Taking the speed of light as 3×10⁸ m/s, which is closest to the wavelength in vacuum of a 2.4 GHz radio wave?

  1. A1.25 cm
  2. B12.5 cm
  3. C125 cm
Show answer and explanation

Answer: B. 12.5 cm

Wavelength is found from λ = c/f. λ = 3×10⁸ ÷ (2.4×10⁹) = 0.125 m, or 12.5 cm. A frequency ten times higher gives one tenth the wavelength, so 24 GHz gives 1.25 cm and 240 MHz gives 125 cm. Antenna size scales with wavelength, so a half-wave dipole antenna at 2.4 GHz is about 6 cm long.

  • A. This is the wavelength at 24 GHz, the value obtained by taking the frequency ten times too high.
  • B. Correct. 3×10⁸ ÷ (2.4×10⁹) = 0.125 m = 12.5 cm.
  • C. This is the wavelength at 240 MHz, the value obtained by taking the frequency ten times too low.
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  1. A radio transmits 20 dBm. Which is this power expressed in mW?
  2. Which describes the antenna gain unit dBi?
  3. With transmit and receive antenna gains of 0 dBi, which is closest to the free-space path loss when a 2.4 GHz radio wave (wavelength 0.125 m) travels 100 m through free space?
  4. With a transmit power of 20 dBm, a transmit antenna gain of 2 dBi, a receive antenna gain of 2 dBi, a propagation loss of 100 dB and cable losses on the transmit and receive sides totaling 4 dB, which is the received power?
  5. In radio communication, which is the phenomenon in which waves reflected by buildings and other objects arrive late because of differing path lengths, interfere with the directly arriving wave, and cause the received power to drop sharply at certain locations or frequencies?