ACMA Standard Theory Exam common mistakes
ACMA Standard Theory Exam · Understand the traps and how to avoid them.
Converting a voltage or current ratio to decibels with 10 log10(...) instead of 20 log10(...) - for example calling a 2:1 voltage ratio '3 dB' instead of '6 dB', or getting a filter's stopband rejection wrong by a factor of two.
The 10 log rule is memorised first from power ratios and then applied to every ratio without checking whether the quantity is a power or a field quantity. Because the decibel is defined on power and power goes as the square of voltage, field quantities carry a factor of 2 that gets lost.
What to do instead
Before pressing log, ask 'power or volts/amps?'. Write 10 log10(P2/P1) = 20 log10(V2/V1) on your scratch sheet at the start, and check every dB answer against fixed anchors: twice the power is 3 dB, twice the voltage is 6 dB, ten times the voltage is 20 dB.
Substituting MHz and pF straight into f = 1/(2π√(LC)) and getting an answer out by orders of magnitude, then choosing whichever option looks 'reasonable'.
The formula needs hertz and farads, but the numbers in real questions are given in the units people actually use; under time pressure the conversion step is skipped rather than written down.
What to do instead
Convert to base units in a written intermediate step before substituting, or use the unit-built-in form f(MHz) = 159.15 / √(L in µH × C in pF). Sanity-check against a known tank: 1 µH with 100 pF resonates near 16 MHz, not 16 kHz or 160 MHz.
Adding reactance and resistance as plain numbers (treating 30 Ω + j40 Ω as 70 Ω), then missing questions about impedance magnitude, phase angle or resonance.
Resistance arithmetic is learned first and is intuitive; complex impedance comes later and looks identical on paper, so the j is quietly dropped.
What to do instead
Keep the j with every reactance, add series elements as complex numbers, and compute magnitude as √(R² + X²) only as the very last step. Test resonance by setting X_L = X_C rather than by looking at an impedance magnitude.
Using a transmitter's peak envelope power figure where the question asks for average power (or the reverse), then carrying the wrong number into a duty-cycle, thermal or exposure calculation.
Manufacturers quote the flattering number, and SSB, CW and data modes have a PEP-to-average ratio well above one, so the two figures are genuinely different.
What to do instead
Name the mode before touching the number. For SSB and CW use PEP for licensing and ERP figures but average power for thermal and exposure calculations, applying the stated duty factor; for FM and continuous carriers PEP is close to average. Write the mode beside the figure.
Reading a bench power or SWR meter without doing the set-zero and set-full-scale steps and without applying the range multiplier, then reporting a value that is 10 dB or 100 W out.
The zero and calibrate controls are separate from the read step, and the multiplier is a small figure printed on the scale rather than on the pointer, so it is easy to skip both under exam pressure.
What to do instead
Adopt a fixed ritual every time: select the range, zero, calibrate full scale against the known reference, read, then multiply by the range factor. State which scale the reading came from (forward or reflected) before using it in a calculation.
Working out a quarter-wave stub or phasing-line length from the free-space wavelength (λ = 300/f) and forgetting to apply the cable's velocity factor.
λ = 300/f is the memorable formula and velocity factor only enters when the medium is not free space; the VF may be buried in a table or in a suffix rather than in the sentence that asks the question.
What to do instead
Do the λ = 300/f step (f in MHz, λ in metres), then explicitly multiply by VF for any length of real coax that must be a specific electrical fraction. Anchor the values: solid-polyethylene coax is about 0.66 and foam about 0.8, so a 20 m band quarter wave is roughly 2.5 m in air but about 1.7 m in 0.66 VF cable.
Confusing ERP with EIRP and dropping the 2.15 dB dipole-to-isotropic factor when converting between them or when adding antenna gain to transmitter power.
Both are called 'effective radiated power', and a question often gives gain in dBd while asking for an answer referenced to an isotropic source, so the two references get mixed without anyone noticing.
What to do instead
Write the reference next to every gain figure (dBd = dipole, dBi = isotropic) and remember dBi = dBd + 2.15. Convert every gain to one reference before adding it to the transmitter power expressed in dBW or dBm.
Assuming an HF skywave path is available at any hour and picking an operating frequency above the maximum usable frequency without checking the time of day or the sunspot cycle.
F-layer ionisation that supports skywave changes with time of day and the solar cycle, and D-layer absorption reverses the intuitive 'lower bands are better' rule between day and night, so the reasoning is easy to invert.
What to do instead
Anchor two facts: higher bands (10–15 m) work by day and in high sunspot years; 80 and 160 m are night bands because D-layer absorption is far lower after dark. For any path question check the chosen frequency sits between the LUF and the MUF for the stated time of day, remembering the MUF is higher by day.
Treating every interference problem as one problem and reaching for a ferrite clamp on the mains lead first, even when the unwanted signal is arriving on the antenna feeder.
Mains chokes are the most familiar fix, and candidates skip the diagnostic step of sorting interference into conducted versus radiated, and into which direction the path runs.
What to do instead
Classify before you fix: is the source inside or outside the station, and does the signal arrive on the feeder, the mains, or by direct radiation? A clamp on the mains only attenuates common-mode current on that cable; feeder-borne interference needs a choke or balun at the antenna, and direct radiation into the receiver needs screening, re-siting or distance.
Judging RF exposure by 'it's only low power' or by applying one remembered safe distance to every band and mode, instead of using frequency-dependent limits and time-averaged power.
Exposure limits vary with frequency because the body absorbs RF differently at different wavelengths, and the limits are averaged over time and over body dimensions, so a figure learned for one band does not transfer to another.
What to do instead
Calculate using average power for the mode and duty cycle, look up the limit for that specific frequency band rather than reusing a single number, and apply the stated averaging period - a short burst at high power is not equivalent to a continuous carrier.