Wi-Fi moves data by sending radio frequency (RF) energy through the air. Before you can design, troubleshoot or pass any part of the CWNA (Certified Wireless Network Administrator) exam, you need a clear picture of what an RF wave is. An RF signal is an alternating current (AC) in the transmitter that the antenna turns into an electromagnetic wave. That wave has four properties you must be able to name and explain: wavelength, frequency, amplitude and phase.
Frequency is how many complete cycles the wave makes each second, measured in hertz (Hz). One cycle per second is 1 Hz, and Wi-Fi works in gigahertz (GHz), billions of cycles per second: the 2.4 GHz, 5 GHz and 6 GHz bands. Wavelength is the physical distance the wave travels during one cycle, from one crest to the next. Amplitude is the height, or strength, of the wave; a transmitter with more power produces a wave with greater amplitude, and amplitude falls as the signal travels and is absorbed. Phase describes where a wave is in its cycle compared with another wave of the same frequency, measured in degrees. Two waves that are in phase (0 degrees apart) add together; two waves 180 degrees out of phase cancel each other.
Wavelength and frequency are tied together by the speed of light, about 300,000,000 meters per second. The formula is wavelength = speed of light / frequency. Because the speed is fixed, a higher frequency always means a shorter wavelength. At 2.4 GHz the wavelength is about 12.5 cm (roughly 5 inches). At 5 GHz it is about 6 cm, and at 6 GHz about 5 cm. This inverse relationship is the single most important fact in this lesson.
Why does wavelength matter in practice? Antennas are sized in proportion to wavelength, so 5 GHz antenna elements are smaller than 2.4 GHz ones. More importantly, a receiving antenna built for a shorter wavelength captures less of the passing energy, so higher-frequency signals arrive weaker over the same distance. That is why a 5 GHz or 6 GHz cell is usually smaller than a 2.4 GHz cell from the same access point (AP) at the same transmit power, and why higher frequencies are generally more affected by walls and other obstacles.
Phase matters because of multipath: when copies of the same signal arrive over different paths, they arrive at different phases. Copies that arrive in phase strengthen the signal (upfade), while copies arriving out of phase weaken or cancel it. Modern radios also use phase deliberately: modulation schemes such as QPSK and QAM encode bits by shifting phase and amplitude, and transmit beamforming adjusts the phase at each antenna so signals combine constructively at the client.
For the exam, be ready to identify each property from a description or a drawing, and to reason about relationships. If asked which band has the longest wavelength, the answer is the lowest frequency band, 2.4 GHz. If asked what happens to wavelength when frequency doubles, it halves.
Key terms
- Frequency
- The number of cycles an RF wave completes each second, measured in hertz (Hz).
- Wavelength
- The distance a wave travels during one complete cycle; it equals the speed of light divided by the frequency.
- Amplitude
- The height or strength of a wave, which relates to signal power.
- Phase
- The position of a wave in its cycle relative to another wave of the same frequency, measured in degrees.
- Hertz (Hz)
- The unit of frequency, one cycle per second; Wi-Fi uses GHz, or billions of cycles per second.
An office moves a group of laptops from 2.4 GHz to 5 GHz on the same AP at the same power. Users near the AP see faster speeds, but a few at the far end of the floor now see weak signal. The shorter 5 GHz wavelength gives a smaller effective coverage cell, so the design needs APs placed closer together.
Check yourself
What happens to wavelength when frequency increases?
It decreases, because wavelength equals the speed of light divided by frequency and the speed of light is constant.
Two copies of the same signal arrive 180 degrees out of phase. What is the effect?
They cancel or severely weaken each other, which is the destructive result of multipath.
Which wave property is most directly related to transmit power?
Amplitude, the height or strength of the wave.