Radio Electronics Honor

Vocational Activities

Requirements

1. Explain, with diagrams when necessary, the input devices used in radios, for example, antenna, ground; or electronic devices such as an FM transmitter, photocell, phono cartridge, and microphone.

   Answer: The antenna picks up electromagnetic waves; the ground drains static charges and stabilizes the signal. The microphone converts sound into an electrical signal. The photocell turns light into current. The phono cartridge picks up the vibrations of records. The FM transmitter modulates audio in frequency for transmission. — Antennas can be dipole, loop, or Yagi; the ground uses rods or a metal plane. Microphones (dynamic, condenser, electret) use a diaphragm and a coil or capacitor. Photocells (LDR, photodiode) vary their resistance with light. Cartridges (crystal, magnetic) follow the grooves of the vinyl record. FM modulator transmitters operate at 88-108 MHz.

2. Build a radio receiving antenna; OR build a record player, photocell, or microphone.

   Answer: Stretch 10-30m of insulated copper wire between two high points with insulators at the ends. Connect one end to the radio's antenna terminal and leave the other free. Add a grounding to a buried metal rod. Orient the wire horizontally, away from power lines. — The ideal length varies according to the band: 1/4 or 1/2 of the wavelength. For shortwave, 10-20m works well. Use AWG 18-22 wire, ceramic or PVC insulators. Grounding improves SNR and protects against surges. Avoid crossing electrical wiring to reduce interference (RFI).

3. Explain the use and operation of several important components of a simple receiver (coils, variable capacitors, fixed capacitors, resistors, transistors or tubes, diodes, and transformers).

   Answer: Coils store magnetic energy and select the frequency. Fixed capacitors filter and couple; variable ones adjust the tuning. Resistors limit current and divide voltage. Transistors or tubes amplify the signal. Diodes rectify and detect. Transformers match impedance and isolate stages. — The LC resonant circuit defines the station. The detector (diode) extracts the audio from the modulated wave. BJT or FET transistors amplify in RF and AF. Resistors set the bias (e.g., the base divider). Transformers match impedances between stages. Electrolytic capacitors filter the power supply; ceramic ones couple high frequencies.

4. Identify the value of resistors by the color code.

   Answer: Common resistors have 4 or 5 colored bands. The first 2-3 bands are significant digits, the next is the multiplier (power of 10), and the last is the tolerance. Colors: black=0, brown=1, red=2, orange=3, yellow=4, green=5, blue=6, violet=7, gray=8, white=9. — Tolerance: gold ±5%, silver ±10%. Example: brown-black-red-gold = 1-0-×100=1000 Ω, ±5% (1 kΩ). In 5 bands, three digits + multiplier + tolerance give greater precision. SMD resistors use a numeric code (e.g., 472 = 47×10² = 4700 Ω). The reading always starts from the band closest to the end.

5. Know Ohm's law and how it is applied.

   Answer: A Lei de Ohm afirma que a corrente (I) que passa por um condutor é diretamente proporcional à tensão (V) aplicada e inversamente proporcional à resistência (R): V = R x I. Dela derivam: I = V / R e R = V / I. Aplicações: 1) Calcular a corrente em um circuito conhecendo tensão e resistência; 2) Dimensionar resistores para limitar corrente (ex.: proteger um LED); 3) Calcular quedas de tensão em cada componente em série; 4) Determinar a resistência necessária para obter uma corrente desejada; 5) Verificar a potência dissipada combinando com P = V x I. Em um receptor de rádio, é usada para calcular polarização de transistores/válvulas e os valores corretos de resistores nos estágios do circuito. — Georg Simon Ohm published the law in 1827 in his work 'Die galvanische Kette, mathematisch bearbeitet', making it a fundamental principle of electricity applied in all linear circuits.

6. Build a simple radio (with a tube or transistor), including the power supply, which should tune in stations 160 kilometers away, or build a high-fidelity amplifier.

   Answer: A long external antenna (10-30m), an LC tuning circuit with a coil and variable capacitor, a detector with a germanium diode or transistor, an audio amplifier with a transistor or tube, a stable power supply (batteries or rectifier), and a loudspeaker. Proper grounding is essential. — For distances of 160 km, on medium wave, a long antenna at height is critical. A regenerative or superheterodyne receiver offers better sensitivity. Stages: antenna → LC tank → detector → AF amplifier → loudspeaker. Key components: tuning coil, 365 pF variable capacitor, 1N34 diode, BC548 transistor. A power-supply filter with an electrolytic capacitor reduces noise.

7. Draw from memory, using appropriate symbols, a complete schematic of the device built in the previous item.

   Answer: Standardized symbols for each component (resistor, capacitor, coil, transistor, diode, transformer, loudspeaker), numeric values, connections in lines, power terminals (+V, GND), references (R1, C1), and identification of the stages (tuning, detector, amplifier, output). — The IEC 60617 standard standardizes the symbols. The schematic should show the signal flow from the antenna to the loudspeaker. Each component receives a designator (R = resistor, C = capacitor, L = coil, Q = transistor). Indicate values (kΩ, µF, mH), the polarity of electrolytics, and the transistor type (NPN/PNP). Lines should avoid unnecessary crossings, with black dots marking junctions.

8. Show fluency in troubleshooting simple tuners or electronic devices with 3 to 6 tubes or transistors, demonstrating the ability to follow routine tests such as: removing tubes or transistors and testing them, replacing them correctly, checking whether the capacitors are shorted and whether there are other problems.

   Answer: Visually inspect for burned components. Test the active components (tubes and transistors) by replacing them one by one. Check for shorted capacitors with a multimeter or ohmmeter. Measure the voltages at the terminals with a voltmeter. Check the continuity of coils and resistances. Replace suspect components one at a time, testing the effect. — Procedure: 1) visual inspection; 2) measuring the voltage at the power supply (B+); 3) signal injection (AF/RF generator) stage by stage; 4) measuring the current in transistors (Ie, Ic); 5) testing electrolytic capacitors. Dry or shorted capacitors cause noise or silence. Stage-by-stage replacement allows you to isolate the fault without swapping out good components.