Radio Theory and Practice
This domain tests the practical theory behind transmitters and receivers: decibels, resonant circuits, mixers, amplifier classes, modulation, receiver performance, and spurious emissions. Advanced questions often require a calculation or a judgement about why a real circuit behaves differently from an ideal one.
Decibels and power relationships
Power gain in dB is 10 log10(Pout/Pin); voltage or current gain uses 20 log10. dBm is power relative to 1 mW, so 100 W is +50 dBm and 1 kW is +60 dBm. A +3 dB change doubles power; +10 dB multiplies it by ten. Cascaded stages add in dB, which makes system calculations straightforward.
Resonant circuits, mixers and filters
A resonant circuit has f = 1/(2π√(LC)); Q = f0/BW, so higher Q gives narrower bandwidth. A mixer multiplies RF and local-oscillator signals, producing sum and difference frequencies; the unwanted image lies at fLO ± fIF and must be filtered before the mixer. Low-pass, high-pass, band-pass and notch filters are specified by cutoff, insertion loss and rejection.
Transmitters and modulation
Class A is the most linear but least efficient; class AB is a compromise; class B is more efficient; class C is highly efficient but nonlinear, so it suits FM or CW but not SSB. SSB is amplitude modulation with the carrier suppressed, and power is quoted as PEP. FM has constant envelope, with deviation and modulation index setting occupied bandwidth. Harmonics and spurious emissions must be filtered.
Receivers
Sensitivity is the minimum signal for a usable signal-to-noise ratio; selectivity is adjacent-channel rejection; dynamic range describes overload behaviour; image rejection and noise figure are separate specifications. Front-end filtering, AGC and a low-noise first stage all matter.
Worked example
A 10 W driver feeds a 10 dB gain amplifier, a cable with 3 dB loss, then a 6 dB gain stage. Work in dB: 10 W = +40 dBm. Net gain = +10 − 3 + 6 = +13 dB, so output = +53 dBm. Since +50 dBm is 100 W and +3 dB doubles it, +53 dBm is 200 W. In watts: 10 W × 10 = 100 W; 3 dB loss halves to 50 W; 6 dB gain quadruples to 200 W. The two methods agree because dB gains add while power multipliers multiply.
Traps
Using 20 log10 for power ratios; forgetting cable loss; treating a class C stage as linear for SSB; confusing dBm with dBW; ignoring image frequency; assuming sensitivity and selectivity are interchangeable. Also check whether a quoted gain is power gain or voltage gain - the decibel formula differs.