Correct answer: B — 7050 kHz

A harmonic is a whole-number (integer) multiple of the fundamental frequency. The second harmonic is simply 2× the fundamental, the third harmonic is 3×, and so on. Harmonics are the most common form of spurious emission and are why transmitters must be filtered to prevent interference on other bands.

\[ f_n = n \times f_0 \]

For the second harmonic (n = 2):

\[ f_2 = 2 \times 3525\ \mathrm{kHz} = 7050\ \mathrm{kHz} \]

• 3573 kHz — this is only 48 kHz above the fundamental, which is not a harmonic relationship; it would correspond to some form of intermodulation or mixing product, not a simple harmonic.
• 14025 kHz — this is 4× 3506.25 kHz, not 4× 3525 kHz (4 × 3525 = 14100 kHz), so it does not correspond to any integer harmonic of 3525 kHz.
• 21050 kHz — this would need to equal 6× 3525 kHz = 21150 kHz; 21050 kHz is not an integer multiple of 3525 kHz.

Therefore, 7050 kHz is the second harmonic of 3525 kHz and is the expected frequency at which a harmonic would occur.

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Correct answer: 21 MHz

Harmonics are integer multiples of the fundamental frequency.

For the third harmonic:

\[ f_3 = 3 \times f \]

Given:

\[ f = 7\ \mathrm{MHz} \]

So:

\[ f_3 = 3 \times 7 = 21\ \mathrm{MHz} \]

• 10 MHz and 14 MHz are not correct multiples.
• 28 MHz is the fourth harmonic.

Therefore, the third harmonic is 21 MHz.

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Correct answer: D — 35 MHz

A harmonic is an integer multiple of the fundamental frequency. The fifth harmonic is simply the fundamental frequency multiplied by 5.

\[ f_n = n \times f_0 \]

Given:

• Fundamental frequency f₀ = 7 MHz
• Harmonic number n = 5

\[ f_5 = 5 \times 7\ \mathrm{MHz} = 35\ \mathrm{MHz} \]

• A — 12 MHz: This is not a simple integer multiple of 7 MHz (approximately 1.7×), so it is not a harmonic.
• B — 19 MHz: This would imply roughly the 2.7th harmonic, which is not an integer multiple and therefore not a harmonic.
• C — 28 MHz: This is the fourth harmonic (4 × 7 MHz), not the fifth.

Therefore, the fifth harmonic of 7 MHz is 35 MHz, found by multiplying the fundamental frequency by 5.

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Correct answer: D — overdriven amplifier stages

When an amplifier stage is driven beyond its linear operating region, the output waveform is clipped or compressed. This non-linearity generates harmonic frequencies — integer multiples of the fundamental frequency — which appear as unwanted signals in the transmitter output. Overdriving is one of the most common causes of excessive harmonic radiation.

• A. A linear amplifier — a properly operated linear amplifier preserves the waveform shape and is specifically designed to minimise harmonic distortion, not cause it.
• B. A low SWR — a low SWR indicates a well-matched feedline and antenna, which is a desirable condition and has no tendency to generate harmonics.
• C. Resonant circuits — resonant circuits are used in transmitter output stages to suppress harmonics by selecting only the fundamental frequency; they are part of the solution, not the problem.

Therefore, overdriving amplifier stages causes non-linear operation that produces excessive harmonic output, which must be filtered before the signal reaches the antenna.

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Correct answer: the modulation level is too high

If the modulation level is too high (over-modulation), the RF power amplifier may be driven into a non-linear operating region.

Non-linear operation causes distortion of the waveform, which produces harmonic frequencies that are multiples of the fundamental transmission frequency.

• Low modulation level does not create harmonics.
• Oscillator instability causes frequency drift, not harmonics.
• Modulating more than one stage does not inherently produce harmonics.

Therefore, harmonics may be produced if the modulation level is too high.

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Correct answer: using tuned circuit coupling between stages

Harmonics generated in earlier stages can be filtered out in later stages by using tuned circuits.

A tuned circuit:

• is resonant at the desired (fundamental) frequency
• presents low impedance at that frequency
• attenuates frequencies away from resonance (including harmonics)

Thus, when used for coupling between stages, it helps:

• pass the wanted signal

• suppress unwanted harmonic components

• Increasing input may worsen distortion.

• Using FETs does not inherently remove harmonics.

• Larger capacitors do not provide frequency selectivity.

Therefore, harmonics may be reduced by using tuned circuit coupling between stages.

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Correct answer: D — a sine wave is distorted

When a sine wave passes through a non-linear or overdriven stage, the waveform is distorted — it is no longer a pure sinusoid. Fourier analysis shows that any periodic, non-sinusoidal waveform is composed of the fundamental frequency plus integer multiples of that frequency (2nd harmonic, 3rd harmonic, etc.). Distortion is therefore the direct cause of harmonic generation in RF circuits.

• A. a resonant circuit is detuned — Detuning a resonant circuit shifts its centre frequency or reduces its selectivity, but does not by itself introduce non-linearity or create harmonics.
• B. negative feedback is applied to an amplifier — Negative feedback reduces distortion and therefore reduces harmonic content; it is the opposite of what generates harmonics.
• C. a transistor is biased for class A operation — Class A biasing keeps the transistor in its linear region for the full cycle, producing minimal distortion and therefore minimal harmonic output. Classes AB, B, C, and D are progressively more prone to distortion and harmonic generation.

Therefore, harmonics arise specifically because distortion of a sine wave introduces frequency components at integer multiples of the fundamental.

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Correct answer: at multiples of the fundamental frequency

Harmonics are frequencies that are integer multiples of a fundamental frequency:

\[ f_n = n \times f \]

where \(n = 2, 3, 4, \dots\)

• They are higher, not lower, than the fundamental.
• Not all unwanted frequencies are harmonics.
• TV interference can be caused by harmonics, but that is not the definition.

Therefore, harmonic frequencies are at multiples of the fundamental frequency.

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Correct answer: second harmonic of a 10 metre transmission

The harmonic frequency of a transmitter is an integer multiple of its fundamental frequency:

\[ f_n = n \times f_0 \]

The 10 metre band is around \(28\text{–}29\ \mathrm{MHz}\).
The second harmonic of a 10 m signal is therefore:

\[ 2 \times 28.5\ \mathrm{MHz} \approx 57\ \mathrm{MHz} \]

This matches the interfering signal frequency of 57 MHz, making a second harmonic from a 10 m transmitter the most likely source.

• seventh harmonic of an 80 metre transmission
  The 80 m band is around \(3.5\text{–}4.0\ \mathrm{MHz}\).
  \[7 \times 3.7\ \mathrm{MHz} \approx 26\ \mathrm{MHz}\]
  This does not match 57 MHz.

• third harmonic of a 15 metre transmission
  The 15 m band is around \(21\ \mathrm{MHz}\).
  \[3 \times 21\ \mathrm{MHz} = 63\ \mathrm{MHz}\]
  This is significantly higher than 57 MHz.

• crystal oscillator operating on its fundamental
  A fundamental crystal frequency of 57 MHz is uncommon and does not explain why the signal is interfering with a transmitter harmonic scenario.

Therefore, a signal at 57 MHz could reasonably be the second harmonic of a 10 metre transmission.

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Correct answer: A — wave trap in the transmitter output

A wave trap is a parallel LC (tank) circuit tuned to resonate at the specific harmonic frequency you wish to suppress. When placed in the transmitter output, it presents a very high impedance at that frequency, effectively blocking that harmonic from reaching the antenna and being radiated. Because it is tuned to one particular frequency, it is the right tool when a single harmonic needs to be eliminated without disturbing other parts of the signal.

• B. Resistor — a resistor has no frequency-selective properties; it would attenuate all frequencies equally and dissipate power as heat, making it useless for harmonic suppression and wasteful of transmit power.
• C. High pass filter in the transmitter output — a high-pass filter passes frequencies above its cut-off and attenuates those below. This would actually pass harmonics (which are higher in frequency) more readily, doing the opposite of what is needed.
• D. Filter in the receiver lead — filtering at the receiver protects the receiver from interference but does nothing to stop the transmitter radiating the harmonic in the first place; the unwanted emission is still present on air.

Therefore, a wave trap — a parallel resonant circuit tuned to the offending harmonic — placed in the transmitter output is the correct method to minimise radiation of one specific harmonic.

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Correct answer: D — to reduce radiation of harmonics

A low-pass filter placed in the antenna feed line passes signals below its cut-off frequency (the fundamental transmit frequency) while attenuating higher frequencies. Because harmonics are integer multiples of the fundamental frequency, they fall well above the cut-off point and are significantly suppressed before reaching the antenna. This is one of the most common and effective methods of keeping harmonic emissions within regulatory limits set by MBIE/RSM.

• A — to reduce key clicks developed in a CW transmitter: Key clicks are a shaping/rise-time problem in the keying circuit, not a harmonic issue. They are reduced by proper keying waveform shaping, not by a low-pass filter on the antenna lead.
• B — to increase harmonic radiation: This is the opposite of what a low-pass filter does; it attenuates, not amplifies, frequencies above the cut-off.
• C — to eliminate chirp in CW transmissions: Chirp is a frequency instability caused by the transmitter oscillator being pulled by the keying process. It is corrected in the oscillator or power supply stage, not by filtering the antenna lead.

Therefore, a low-pass filter in the antenna lead is used specifically to reduce the radiation of harmonics, keeping the transmitted signal spectrally clean and within regulatory requirements.

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Correct answer: B — a low-pass filter

A low-pass filter passes all frequencies below its cut-off frequency and attenuates frequencies above it. Because harmonics are exact multiples of the fundamental transmit frequency, they are always higher in frequency than the desired signal. Placing a low-pass filter in the transmission line — with its cut-off point set just above the operating frequency — allows the fundamental to pass while suppressing the 2nd, 3rd, and higher harmonics before they reach the antenna and cause interference.

• A — a middle-pass filter: This is not a real filter category. No such standard filter type exists.
• C — a high-pass filter: This would do the opposite — attenuating the fundamental and passing the harmonics, which is exactly what must be avoided.
• D — a band-reject filter: A band-reject (notch) filter suppresses a single narrow band of frequencies. It could only target one harmonic at a time and would not provide broad harmonic suppression across all multiples.

Therefore, a low-pass filter installed as close as possible to the transmitter output is the standard and correct method for reducing harmonic emissions on HF.

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Correct answer: reduce harmonics

A low-pass filter allows signals below a certain cutoff frequency to pass while attenuating higher-frequency components.

In a transmitter, this means:

• the desired fundamental frequency is passed
• higher-frequency harmonic components are reduced

Harmonics occur at integer multiples of the fundamental frequency and can cause interference if radiated.

• Sub-harmonics are lower than the fundamental and would not be suppressed.
• A filter cannot eliminate all interference.
• A low-pass filter reduces, not improves, harmonic radiation.

Therefore, a low-pass filter will reduce harmonics.

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A spurious transmission from a transmitter is an unwanted emission unrelated to the output signal frequency

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Correct answer: is an unwanted signal developed in a transmitter

Parasitic oscillations are unintended oscillations that can occur in amplifier stages due to stray capacitance, inductance, or feedback within the circuit.

They may occur at frequencies unrelated to the desired operating frequency and can:

• reduce amplifier efficiency

• cause instability

• produce unwanted emissions and interference

• They are not generated by parasitic antenna elements.

• They can cause interference.

• They are not limited to the oscillator stage.

Therefore, a parasitic oscillation is an unwanted signal developed in a transmitter.

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Correct answer: placing suitable chokes, ferrite beads or resistors within the amplifier

Parasitic oscillations are unwanted high-frequency oscillations caused by stray feedback and reactances.

They can be suppressed by adding components that:

• damp high-frequency resonances
• reduce unwanted feedback paths

Common methods include:

• ferrite beads on leads

• RF chokes

• small resistors for damping

• Pulsing the supply is unrelated.

• Screening helps but does not directly suppress oscillation.

• Split-stator capacitors are for tuning.

Therefore, parasitic oscillations are suppressed by placing suitable chokes, ferrite beads or resistors within the amplifier.

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Correct answer: at high or low frequencies

Parasitic oscillations occur when unintended feedback paths within an RF power amplifier create conditions for oscillation outside the desired operating frequency. These oscillations can arise due to stray capacitance, inductance, or layout effects.

Such oscillations are not limited to a specific frequency range and may occur:

• At high frequencies, often well above the intended operating frequency.

• At low frequencies, due to power supply feedback or biasing instabilities.

• Limiting them to low frequencies only is incorrect.

• Restricting them to harmonic frequencies is incorrect; they may occur at completely unrelated frequencies.

• Limiting them to high frequencies only is also incorrect.

Therefore, parasitic oscillations may occur at either high or low frequencies.

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Correct answer: frequencies unrelated to the transmitter's output frequency

Parasitic oscillations in transmitter power amplifiers are unintended self-oscillations caused by stray capacitance, lead inductance, device internal feedback, and layout effects. These oscillations are not driven by the intended RF signal and can occur at whatever frequency the unintended feedback path supports.

They often appear at much higher frequencies, sometimes well into the VHF or UHF range, and are not harmonically related to the transmitter’s operating frequency.

• the transmitter's output frequency is the desired operating signal, not a parasitic oscillation.
• harmonics of the transmitter's output frequency are normal byproducts of non-linear amplification and are handled by output filtering, but they are not parasitic oscillations.
• VHF frequencies only is too restrictive, parasitic oscillations can occur at many unrelated frequencies depending on circuit conditions.

Therefore, parasitic oscillations are generated on frequencies unrelated to the transmitter's output frequency.

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Correct answer: high gain amplifier stages

Parasitic oscillations are unwanted self-generated oscillations that can arise due to stray capacitance, inductance, or feedback within a circuit.

High gain amplifier stages are especially prone to this because:

• small amounts of unintended feedback can be greatly amplified
• stray reactances can form unintended resonant circuits

This can result in instability and unwanted emissions.

• Rectifiers are not designed to amplify signals.
• Antenna matching circuits are passive networks.
• SWR bridges are measuring devices.

Therefore, parasitic oscillations tend to occur in high gain amplifier stages.

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With parasitic signals there is no simple mathematical relationship between the operating frequency and the interfering frequency. The effects may be the same as with harmonics - a VHF receiver being interfered with by a HF transmission. The cause is an additional and undesired oscillation from an oscillator or amplifier for which it was not designed. The circuit functions normally but the parasitic oscillation occurs simultaneously. Parasitics are suppressed by adding additional components to the circuit to suppress the undesired oscillation without affecting the primary function of the circuit. A typical solution is to add a VHF choke (an inductor) or a small-value resistor (a "stopper") somewhere close to the active component in the offending circuit.

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