Electromagnetic Waves and Frequency

• Electromagnetic waves expand in all directions from the antenna, not just in a straight line.
• Visualize this expansion like dropping a pebble in a pond, creating cyclic waves that move outward.
• In free space, electromagnetic waves expand outward in three dimensions.
• A simple idealistic antenna is a single point connected to a wire, producing waves in a spherical shape.
• This idealistic antenna is a reference point for understanding wave propagation.
• Real-world antennas come in various shapes.

Frequency

• Frequency measures the number of complete cycles per second, with hertz (Hz) as the unit (1 Hz = 1 cycle/second).
• Frequency can vary widely, so units like kilohertz (kHz), megahertz (MHz), and gigahertz (GHz) are used for larger values.
• kHz is 1,000 Hz.
• MHz is 1,000,000 Hz.
• GHz is 1,000,000,000 Hz.
• The continuous frequency spectrum ranges from 0 Hz to 10^22 Hz, including audible sounds, light, X-rays, gamma rays, and cosmic rays.

Frequency Bands

• The AM band, used by AM radio stations, ranges from 530 kHz to 1710 kHz.
• Wireless LAN communication uses the 2.4 GHz band (2.400 to 2.4835 GHz), an industrial, scientific, and medical (ISM) band available without a license.
• Another wireless LAN range is the 5 GHz band (5.150 to 5.825 GHz), comprised of four separate Unlicensed National Information Infrastructure (U-NII) bands:
• 5.150 to 5.250 GHz (U-NII-1)
• 5.250 to 5.350 GHz (U-NII-2A)
• 5.470 to 5.725 GHz (U-NII-2C)
• 5.725 to 5.825 GHz (U-NII-3)

Signal Bandwidth

• Channels need spacing due to RF signal limitations, electronics constraints, and data overhead.
• RF signals are not infinitely narrow and spill into neighboring frequencies.
• The center frequency defines the channel location.
• Signal bandwidth is the frequency range needed for transmission, bounded above and below the center frequency.
• Wireless LANs define signal bandwidth as part of a standard.
• Devices use a spectral mask to ignore signal parts outside bandwidth boundaries.
• In the 2.4 GHz band, channels are 5 MHz wide, but Wi-Fi signals have a 22 MHz bandwidth, causing overlap.
• 5 and 6 GHz bands have non-overlapping channels spaced every 20 MHz to support signals around 20 MHz wide, maximizing channel use.

RF Signals

• RF signals depend on timing due to the varying electrical and magnetic forces.
• The phase is a measure of the time shift relative to a complete cycle.
• Wavelength, designated by lambda (Λ), relates to the dimensions of a wireless LAN signal.
• 2.4 GHz signal: 4.92 inches.
• 5 GHz signal: 2.36 inches.
• 6 GHz signal: 1.97 inches.
• Wavelength decreases as frequency increases.
• RF waves travel at the speed of light in a vacuum; slightly slower in air.
• Wavelength is vital in antenna design and placement

Signal Strength

• Amplitude can be measured, which is the height from top peak to bottom peak of a signal's waveform
• Signal strength can be measured by its power, in watts (W).
• AM radio: 50,000 W.
• FM radio: 16,000 W.
• Wireless LAN: 0.1 W (100 mW) to 0.001 W (1 mW).

Decibels

• Logarithms convert exponential ranges into linear ones.
• The base-10 logarithm (log10) indicates how many times 10 must be multiplied by itself to equal a number.
• log10(10) = 1
• log10(100) = 2
• Decibel (dB) turns exponential values into a more manageable range.

Laws of 3s and 10s

• Power doubles, it increases by 3 dB.
• If power is cut in half, it decreases by 3 dB.
• A value of 10 dB means the power is 10 times the reference value.
• A value of -10 dB means the power is 1/10 of the reference.
• Multiplying power values results in a positive dB value that can be added.
• Dividing power values results in a negative dB value that can be subtracted.