Signals, Frequency, and Radio: Practice Questions & Explanations

Frequency = number of cycles per second of a periodic wave. Unit: HERTZ (Hz), named after Heinrich Hertz who demonstrated electromagnetic waves in 1886. 1 Hz = 1 cycle/second. Multiples: 1 kHz = 1,000 Hz; 1 MHz = 1,000,000 Hz; 1 GHz = 1,000,000,000 Hz; 1 THz = 10¹² Hz. Wavelength: λ = c/f, where c is the wave's speed and f is frequency. For electromagnetic waves in vacuum: c = 3 × 10⁸ m/s; for sound in air at room temperature: ~340 m/s. Higher frequency = shorter wavelength. Common frequencies: (1) HOUSEHOLD AC POWER: 60 Hz (US), 50 Hz (much of world); (2) HUMAN HEARING: 20 Hz - 20 kHz (declines with age); audio frequencies; (3) AM RADIO: 535-1605 kHz (medium wave); (4) FM RADIO: 88-108 MHz; (5) TV: VHF and UHF bands; (6) WIFI: 2.4 GHz, 5 GHz, 6 GHz (Wi-Fi 6E); (7) CELL PHONES: 700 MHz - 3.5 GHz typical; (8) GPS: ~1.5 GHz; (9) BLUETOOTH: 2.4 GHz; (10) MICROWAVES (oven): 2.45 GHz; (11) X-RAYS: 10¹⁶-10¹⁹ Hz; (12) GAMMA RAYS: above 10¹⁹ Hz. The ELECTROMAGNETIC SPECTRUM (lowest to highest frequency): radio waves → microwaves → infrared → visible light (red to violet) → ultraviolet → X-rays → gamma rays. Visible light is a tiny slice of the spectrum: ~430-770 THz, corresponding to wavelengths 400-700 nm. ASVAB EI may ask about RADIO bands and applications (AM, FM, VHF, UHF, microwave). AM vs FM: AM (Amplitude Modulation) varies wave amplitude with signal — older, longer range, more susceptible to noise; FM (Frequency Modulation) varies wave frequency with signal — higher quality, less noise, shorter range. SIDEBAND: in AM, audio signal creates upper and lower sidebands around the carrier frequency; SSB (single-sideband) uses only one for efficiency. Bandwidth: range of frequencies a signal or system occupies.

DIRECT CURRENT (DC): electrons flow in one direction only. Constant polarity. Examples: batteries (chemical reactions provide constant voltage), solar cells, DC generators, USB power, automotive 12V system, electronic devices internally. ALTERNATING CURRENT (AC): electrons reverse direction periodically. Voltage cycles between positive and negative peaks. In US household wiring, AC alternates 60 times per second (60 Hz). Most of the world uses 50 Hz. Why AC dominates electrical distribution: (1) TRANSFORMERS work with AC, allowing efficient voltage transformation; (2) HIGH VOLTAGE TRANSMISSION at low current minimizes I²R losses in wires; (3) Stepping down to safer voltages near use; (4) Easier to generate with rotating machinery (generators naturally produce AC). 'War of currents' (1880s-90s): Edison championed DC; Tesla/Westinghouse championed AC; AC won due to transformer-enabled distribution. Modern situation: AC for transmission and distribution; DC for electronics and increasingly for HVDC long-distance transmission (less loss, easier underwater/underground). Conversion: RECTIFIER (diodes) converts AC to DC — used in power supplies, chargers; INVERTER converts DC to AC — used in solar systems (panels produce DC, grid uses AC), UPS, electric vehicle drives. AC waveforms: typically SINUSOIDAL (smooth sine wave); also SQUARE WAVE (digital, switches between high and low); SAWTOOTH; triangular. AC voltage specifications: PEAK voltage (maximum positive or negative); PEAK-TO-PEAK (peak positive to peak negative); RMS (root mean square — equivalent DC voltage that delivers same power; for sine wave, RMS = peak / √2 ≈ 0.707 × peak); 120V US household is RMS — peak is about 170V. AC frequency in different countries: 60 Hz (US, Canada, much of Americas, parts of Asia); 50 Hz (Europe, Africa, most of Asia, Australia). Voltage: 120V (US, Canada, Mexico); 230V (Europe, much of world); 100V (Japan). International travelers need adapters and possibly voltage converters.

Antennas are bidirectional transducers between guided (wire) and unguided (free space) electromagnetic waves. RECEIVING: incoming radio waves induce voltages in the antenna; amplifier extracts and processes the signal. TRANSMITTING: oscillating current in the antenna radiates electromagnetic waves into space. Antenna types: (1) DIPOLE — simplest, two conductive elements; length typically 1/2 wavelength of operating frequency; common in radio; (2) MONOPOLE — single vertical element above ground plane (which acts as the second element via reflection); typical 1/4 wavelength; whip antennas on vehicles, AM tower antennas; (3) YAGI-UDA — directional antenna with driven element and parasitic reflectors/directors; high gain in one direction; classic TV antennas, ham radio; (4) PARABOLIC DISH — focuses signals from a focal point reflector; very high gain in one direction; satellite TV, radio telescopes, deep space communications; (5) LOOP — circular conductor; small loops are magnetic antennas; (6) PATCH/MICROSTRIP — flat antennas printed on circuit boards; common in GPS, WiFi, cell phones; (7) HELICAL — wound in helix shape; satellite communications; (8) PHASED ARRAY — multiple antenna elements with controlled phasing; can electronically steer beam direction; radar, modern 5G base stations. Antenna characteristics: (1) GAIN — directional concentration of energy compared to omnidirectional reference; (2) POLARIZATION — direction of the electric field (vertical, horizontal, circular); transmit and receive antennas should match; (3) BEAMWIDTH — angular width of main lobe; (4) IMPEDANCE — typically 50 or 75 ohms; must match feed line for efficient transfer; (5) BANDWIDTH — range of frequencies the antenna efficiently handles. Antenna length: roughly proportional to wavelength; lower frequencies need bigger antennas (AM radio antennas are huge, GHz cell phone antennas are tiny). Cell phones have multiple antennas: cellular, WiFi, Bluetooth, GPS, NFC. Modern smartphones cleverly use the device frame and internal structures as antennas. Antenna theory: an electron oscillating produces electromagnetic waves at the oscillation frequency; the wire 'pumps' electrons; the changing electric/magnetic fields propagate outward at light speed.

Human hearing range: roughly 20 Hz to 20,000 Hz (20 kHz). The upper limit declines with age (presbycusis) — most adults can't hear above 15-16 kHz by age 40-50. Infants and children have the full range; teenagers can typically hear ringtones around 17-19 kHz that adults cannot. Sound frequency relates to PITCH: low frequencies (low pitch — bass) at 20-250 Hz; mid (voice fundamental, instruments) 250-4000 Hz; high (cymbals, high voice, hi-hat) 4000-20000 Hz. Specific examples: deep bass guitar fundamental ~40-50 Hz; piano ranges 27-4186 Hz (its 88 keys); human speech fundamentals 85-255 Hz (men ~85-180, women ~165-255), with overtones extending much higher; soprano voices reach ~1000+ Hz on top notes; violin ~196-3000+ Hz; piccolo ~600-5000+ Hz. SOUND INTENSITY measured in decibels (dB) — logarithmic scale: 0 dB = threshold of hearing; 20 dB = whisper; 60 dB = conversation; 80 dB = busy traffic; 100 dB = lawn mower; 120 dB = rock concert (pain threshold approached); 140 dB = jet engine (immediate hearing damage). Each 10 dB increase = 10x sound INTENSITY (but only ~2x perceived loudness because of how hearing works). Hearing damage: prolonged exposure above 85 dB causes gradual hearing loss; brief exposure above 140 dB can cause immediate damage. Hearing protection (earplugs, earmuffs) is essential in noisy environments. Sound waves: longitudinal pressure waves in air (or other media). Travel at ~340 m/s in air (varies with temperature, humidity, altitude); 1500 m/s in water; ~5000 m/s in steel. WAVELENGTH = speed / frequency. 1000 Hz sound has wavelength 0.34 m in air. ULTRASOUND: frequencies above 20 kHz, inaudible to humans but heard by some animals (dogs hear to ~45 kHz, bats use 14-100+ kHz for echolocation); medical ultrasound 2-15 MHz; cleaning ultrasound 20-40 kHz. INFRASOUND: frequencies below 20 Hz; produced by earthquakes, large machinery, some animals (elephants and whales communicate with infrasound over long distances).

Frequency measures how many cycles of a wave occur per second, and it is expressed in hertz (Hz). One hertz equals one cycle per second; a kilohertz (kHz) is a thousand cycles per second, and a megahertz (MHz) is a million. Frequency is important for AC power (60 Hz in North America) and for radio, where different stations broadcast at different frequencies. Higher frequency means more cycles per second and shorter wavelengths. Distinguishing frequency (Hz) from voltage (volts), resistance (ohms), and charge (coulombs) is common on the Electronics subtest.

Wavelength and frequency are inversely related: as the frequency of a wave increases, its wavelength decreases, and vice versa. This is because the speed of the wave (the speed of light for radio waves) equals frequency multiplied by wavelength, so if the speed is constant, raising one quantity lowers the other. High-frequency signals have short wavelengths; low-frequency signals have long wavelengths. This relationship matters in radio, where different bands use different frequencies and corresponding wavelengths. Remembering the inverse relationship between frequency and wavelength is common on the signals portion of the Electronics subtest.