ACMA Advanced Amateur Radio Theory common mistakes
ACMA Advanced Amateur Radio Theory · Understand the traps and how to avoid them.
Treating dB gains and losses as linear and adding them arithmetically to power or voltage ratios (e.g., saying a 3 dB increase doubles voltage, or combining 10 dB and 3 dB as 13x).
Candidates memorise '3 dB = double' without tracking whether the ratio is power or voltage, and without converting back to linear when the question asks for an output level in watts.
What to do instead
Always write the definition first: power dB = 10 log10(P2/P1), voltage/current dB = 20 log10(V2/V1). Convert each stage to dB, add the dB values, then convert the total back with P2 = P1 x 10^(dB/10). Drill 3, 6, 10 and 20 dB pairs until the power-versus-voltage distinction is automatic.
Computing resonant frequency or reactance with the wrong unit prefix, mixing MHz with Hz or pF with F, and then choosing an answer that is off by a factor of 1000 or 10^6.
Formula recall is usually fine, but candidates substitute '7' for 7 MHz and '100' for 100 pF, so the arithmetic silently produces a wrong decade under time pressure.
What to do instead
Before substituting, write every quantity in base SI units with powers of ten (7 MHz = 7 x 10^6 Hz; 100 pF = 100 x 10^-12 F). Use f = 1/(2 pi sqrt(LC)) and check the magnitude: a 7 MHz circuit with 100 pF needs roughly 5 microhenry, so an answer in millihenry should be rejected on sight.
Assuming a bipolar transistor is in forward active mode and applying beta-based gain formulas without first checking whether it is saturated or cut off.
Candidates learn IC = beta x IB as a universal rule and forget that it only holds while the collector-base junction is reverse biased; base drive beyond IC(sat) = VCC/RC changes the answer completely.
What to do instead
For every bias question, compute the maximum possible collector current VCC/RC first, then compare with beta x IB. If beta x IB exceeds IC(sat), the device is saturated and IC = VCC/RC. State the mode explicitly (cut-off, active, saturation) before calculating anything else.
Confusing class of operation with conduction angle and efficiency, for example treating class AB as 360-degree conduction or claiming class C can be used for linear SSB amplification.
The classes are memorised as a list rather than as a trade-off between conduction angle, efficiency and linearity, so questions that combine two of those properties defeat rote recall.
What to do instead
Build one table: A = 360 degrees, low efficiency, most linear; B = 180 degrees, push-pull; AB = between 180 and 360 degrees, compromise for SSB; C = under 180 degrees, high efficiency, only for constant-envelope or CW/FM. Then answer by reading which property the question is testing.
Mixing up the causes of spurious emissions: attributing key clicks to the final amplifier, splatter to the oscillator, or harmonics to the modulator, instead of matching each defect to its stage.
The symptom vocabulary (splatter, clicks, parasitics, intermodulation) is learned separately from the transmitter block diagram, so candidates cannot map symptom to source.
What to do instead
Anchor each defect to a stage: key clicks come from hard on/off keying of an early stage and are fixed by shaping the keying waveform; splatter comes from overdriving or excessive modulation in the RF chain; harmonics come from nonlinearity in the final and are fixed by a low-pass filter; parasitics come from layout and neutralisation. Rehearse the block diagram until each stage's failure signature is known.
Treating receiver sensitivity, selectivity and dynamic range as interchangeable, and choosing 'better sensitivity' as the cure for strong-signal overload or intermodulation.
All three are quoted in microvolts or dB and appear in similar-looking specifications, so candidates pick the most familiar term rather than the one the scenario describes.
What to do instead
Define each: sensitivity is the weakest usable signal (noise floor and noise figure), selectivity is rejection of adjacent signals (filter shape factor), dynamic range is the span between noise floor and overload (blocking and IMD). If the scenario mentions a strong nearby signal degrading reception, the answer involves dynamic range, filtering or attenuation, never sensitivity.
Matching impedance by simply adding a 50-ohm resistor, or ignoring the difference between characteristic impedance, load impedance and SWR when computing feedline loss.
Candidates conflate 'the line is 50 ohm' with 'the load must be 50 ohm', and forget that a resistor match dissipates half the power and that SWR adds loss rather than changing the line's characteristic impedance.
What to do instead
Separate the three concepts in writing: characteristic impedance is a property of the line geometry, load impedance is the antenna, and SWR is the mismatch ratio. Use a transformer, stub or matching network, not a dissipative resistor, and remember matched-line loss plus additional mismatch loss when SWR is above 1:1.
Describing an SSB or CW signal as if it occupied the same bandwidth as an FM signal, or assuming a narrower receiver filter improves signal-to-noise for all modes equally.
Bandwidth is taught per mode in isolation, so candidates fail questions that ask how filter bandwidth interacts with mode, noise and intelligibility across modes.
What to do instead
Memorise approximate occupied bandwidths and the reason: CW about 100-500 Hz (filter can be very narrow), SSB about 2.4-3 kHz, narrowband FM about 10-16 kHz (Carson's rule: 2(delta f + fm)). Then reason that noise power falls with bandwidth, so narrower is better until intelligibility or deviation is compromised.
Reading an oscilloscope or spectrum-analyser result without accounting for probe attenuation, reference level, resolution bandwidth or true RMS versus peak detection, and reporting a value that is 10 or 20 dB off.
Candidates focus on the trace and ignore instrument settings, and they apply the 20 log rule to a power measurement or forget the 10x probe factor.
What to do instead
Before quoting any number, list the settings that scale it: probe ratio, input coupling, reference level, resolution bandwidth and detector type. Convert peak to RMS for a sine wave (divide by sqrt 2) and use 10 log for power, 20 log for voltage. Sanity-check against an expected order of magnitude.
Answering RF exposure and EMC questions from habit ('stay away from the antenna') rather than applying the actual framework: averaging time, near-field versus far-field, controlled versus uncontrolled environment, and the distinction between conducted and radiated interference.
These domains are treated as general knowledge, so candidates miss the specific distinctions the exam tests, such as duty cycle averaging for CW versus FM, or whether a fix belongs at the source, the coupling path or the victim.
What to do instead
Learn the exposure model as a set of variables: power density, distance, frequency, averaging time and environment classification. For EMC, always classify the problem as source, path or victim and pick the remedy accordingly (filtering and shielding at the source, ferrites and layout on the path, bypassing at the victim). Cite the applicable limit category rather than a vague safety margin.