Trade-specific Technical Knowledge: Radio/Wireless, Electronics and Tele-printer Operation
The trade paper tests applied radio and electronics plus tele-printer operation: wave propagation, transmitter and receiver blocks, antennas and feed lines, basic electronics and digital logic, and the start-stop signalling used by teleprinters.
Radio fundamentals
- v = fλ, so λ (metres) = 300 / f (MHz). HF is 3–30 MHz (skywave, long range), VHF 30–300 MHz (line of sight, the usual band for police static and mobile sets), UHF 300 MHz–3 GHz (short range, better penetration).
- AM varies amplitude and is prone to noise; FM varies frequency, keeps amplitude constant, and has a capture effect that gives cleaner audio.
- Simplex uses one frequency for one direction at a time; half-duplex allows both directions but not simultaneously; duplex needs two frequencies or a duplexer.
- A repeater extends coverage by receiving and re-transmitting; trunking shares a few channels among many users.
Electronics essentials
Ohm's law; series and parallel resistors; a capacitor blocks DC and passes AC; a diode rectifies; a transistor amplifies or switches; power gain in dB = 10 log(P2/P1), voltage gain = 20 log(V2/V1); VSWR 1:1 is ideal.
Tele-printer operation
- The Baudot / ITA-2 code uses 5 bits, giving only 32 combinations - fewer than 26 letters plus 10 figures - so LTRS and FIGS shift codes are needed.
- Each character is framed by one start (space) bit, five data bits and one or 1.5 stop bits; 50 baud corresponds to roughly 66 words per minute.
- Data travels as a current loop (commonly 20 mA or 60 mA); a teleprinter combines keyboard, printer and distributor.
Worked example
Find the wavelength of a 150 MHz signal and the length of a half-wave dipole for it. Reasoning: λ = 300 / 150 = 2 m. A half-wave dipole is λ/2 = 1 m. Answer: 2 m wavelength, 1 m dipole.
Traps
Using 3 × 10^8 with a frequency already in MHz without converting. Equating baud with bits per second while ignoring start and stop bits. Confusing 5-bit Baudot with 7-bit ASCII. Using 10 log instead of 20 log for voltage gain.