Correct answer: B — the ability of equipment to function satisfactorily in its own environment without introducing intolerable electromagnetic disturbances

Electromagnetic Compatibility (EMC) is the discipline concerned with ensuring that electronic and electrical equipment can operate correctly in its intended electromagnetic environment, while at the same time not generating electromagnetic disturbances that would interfere with other equipment. EMC has two complementary aspects: immunity (a device must tolerate disturbances present in its environment) and emissions (a device must not itself produce excessive disturbances). In New Zealand, EMC requirements for equipment are administered by the Ministry of Business, Innovation and Employment (MBIE) under the Radiocommunications Act.

• A — two antennas facing each other: This describes a physical antenna arrangement, not a compatibility standard or discipline. It has no bearing on EMC.
• C — more than one relay solenoid operating simultaneously: This describes a mechanical switching scenario, unrelated to the concept of electromagnetic compatibility.
• D — the inability of equipment to function satisfactorily together and produce tolerable electromagnetic disturbances: This is essentially the opposite of EMC; it describes incompatibility, not compatibility, and contradicts the definition by pairing inability with tolerability in a self-contradictory way.

Therefore, EMC is correctly defined as the ability of equipment to function satisfactorily in its electromagnetic environment without itself causing intolerable interference to other equipment.

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Correct answer: radiation from a nearby TV line oscillator

A frequency of 15.625 kHz corresponds to the horizontal line scanning frequency used in analogue television systems (625-line standard).

Nearby television receivers or monitors may radiate signals at this frequency and its harmonics, which can be picked up by an amateur receiver as unwanted interference.

These harmonics can appear across the tuning range at intervals of:

\[ 15.625\ \mathrm{kHz} \]

• A government station would not produce regularly spaced harmonics like this.
• Radar systems operate at much higher frequencies.

Therefore, the unwanted signals are likely caused by radiation from a nearby TV line oscillator.

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Correct answer: transmitter harmonics

Narrow-band interference is caused by signals that occupy a small range of frequencies.

Transmitter harmonics are discrete frequencies at integer multiples of the fundamental frequency, and therefore produce narrow-band interference.

• Neon signs, motors, and lightning typically produce broadband noise.
• Broadband noise spreads over a wide range of frequencies, not a narrow band.

Therefore, narrow-band interference can be caused by transmitter harmonics.

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Correct answer: D — poor commutation in an electric motor

Commutation is the process by which current is switched between the brushes and segments of a DC motor's commutator. When this switching is poor — due to worn brushes, a dirty commutator, or mechanical imbalance — it produces rapid, repetitive sparking. Each spark generates a broad-spectrum burst of radio frequency energy. Because sparking occurs continuously as the motor runs, the interference is both broadband (covering a wide range of frequencies simultaneously) and continuous in nature.

• A. An electric blanket switch — switching thermostats in electric blankets produce interference, but only brief, infrequent clicks as the temperature cycles; this is intermittent, not continuous.
• B. A refrigerator thermostat — a fridge thermostat switches on and off occasionally (typically minutes apart), producing short, isolated transients — intermittent interference, not continuous broadband noise.
• C. A microwave transmitter — a microwave transmitter radiates on a specific frequency or narrow band, not broadband continuous interference across the spectrum.

Therefore, poor commutation in an electric motor is the most likely source of broad-band continuous interference because the rapid, repetitive sparking at the commutator generates RF noise across a wide range of frequencies for as long as the motor is running.

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Correct answer: B — outside overhead power lines

Overhead power lines are susceptible to rain because water alters the electrical characteristics of insulators, conductor surfaces, and any corroded or loose fittings along the line. Moisture can cause corona discharge, arcing across dirty or cracked insulators, and increased leakage currents — all of which generate broadband RF noise. Because this effect varies directly with rainfall, the interference level rises and falls with the weather, giving a clear clue that the source is an overhead line exposed to the elements.

• A. Underground power cables — buried cables are shielded from rain by the ground; moisture does not significantly alter their noise output, so interference from them would not vary with rainfall.
• C. Car ignitions — ignition interference is impulsive and correlates with traffic patterns and engine operation, not with rain.
• D. Your antenna connection — a poor antenna connection may cause intermittent noise, but it is internal to your installation and unaffected by external rainfall in a consistent, repeatable way.

Therefore, broadband noise that changes with rainfall is the classic symptom of wet-weather arcing or corona on outside overhead power lines.

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Correct answer: C — make sure that there is no interference on your own domestic equipment

Before attributing a neighbour's interference problem to poor immunity in their equipment, you must first verify that your own station is not causing the issue. This means checking that your own domestic appliances and electronic equipment are not affected by your transmissions. If your own equipment is also experiencing interference, the problem likely originates with your station (e.g., excessive RF, spurious emissions, or poor filtering) rather than a lack of immunity in the neighbour's devices. Only once you have confirmed your own equipment is unaffected can you reasonably suggest the neighbour's equipment lacks adequate immunity.

• A — disconnect all your equipment from their power sources: Disconnecting from the power source is unnecessary and impractical as a first step; checking for interference on your own equipment is the appropriate initial action.
• B — write a letter to the MBIE: Contacting MBIE may be appropriate later if the dispute cannot be resolved, but it is premature before you have even investigated your own station's potential role.
• D — ignore all complaints and take no action: Ignoring complaints is irresponsible and contrary to good amateur practice and the obligations of a licence holder under New Zealand radiocommunications regulations.

Therefore, the correct first step is to confirm that your own domestic equipment is free of interference before suggesting the fault lies with your neighbour's equipment.

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Correct answer: D — use screened wire for the loudspeaker leads

RF breakthrough into audio equipment typically occurs because unscreened wiring — such as loudspeaker leads — acts as an unintentional antenna, picking up RF energy that is then rectified by semiconductor junctions inside the amplifier and heard as interference. Replacing those leads with screened (shielded) cable prevents the RF from being coupled into the audio circuitry in the first place. This is a fix applied at the affected equipment, which is the correct approach when the neighbour's device is susceptible.

• A. Put a ferrite bead on the transmitter output lead — this is on the wrong side of the problem; it would slightly reduce RF at the transmitter end but does nothing to prevent the neighbour's unshielded wiring from acting as a receiving antenna. Ferrite beads are more useful for suppressing common-mode currents on feed lines, not curing receiver-end susceptibility.
• B. Put a capacitor across the transmitter output — placing a capacitor directly across the transmitter output would load and detune the transmitter, potentially causing damage, and would not address the susceptibility of the stereo's wiring.
• C. Use open-wire feeders to the antenna — changing the transmitter's feeder type does not cure the stereo's susceptibility and open-wire feeders can actually increase RF in the surrounding environment compared to well-shielded coaxial cable.

Therefore, the most effective cure for RF breakthrough into a neighbour's stereo is to screen (shield) the loudspeaker leads so they no longer act as receiving antennas for the transmitted signal.

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Correct answer: B — use the minimum transmitter output power necessary

In a densely-populated area, neighbouring households are close together, which increases the risk of transmitted interference (TVI/RFI) affecting televisions, radios, and other electronic equipment. Using only the power needed to make the contact keeps your signal at a level that gets the job done while minimising the chance of causing interference to nearby residents. This is both good amateur practice and consistent with New Zealand's Radiocommunications Regulations, which require operators to avoid causing unnecessary interference.

• A is wrong — always using maximum power dramatically increases the likelihood of causing interference to neighbours and is contrary to good operating practice.
• C is wrong — timing transmissions around television schedules does not reduce RF interference; it merely tries to avoid people noticing it, which is not an acceptable approach.
• D is wrong — pointing a beam toward television antennas would maximise the interference potential, the opposite of what a responsible operator should do.

Therefore, using the minimum transmitter output power necessary is the correct and responsible approach when operating in a densely-populated area.

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Correct answer: D — check your log to see if it coincides with your transmissions

TVI (Television Interference) complaints from neighbours require a methodical and cooperative response. The first step is to determine whether your amateur transmissions are actually the cause — and your station log is the best objective evidence available. If the times of reported interference do not coincide with your operating periods, your transmissions are unlikely to be the source. If they do coincide, further investigation (checking for spurious emissions, over-deviation, harmonic radiation, etc.) is warranted.

Good amateur practice — and the spirit of the NZART licence conditions — requires operators to take interference complaints seriously and investigate them in good faith before drawing any conclusions.

• A — deny all responsibility: Denial without investigation is both poor practice and potentially misleading. You cannot know your equipment is innocent until you have checked.
• B — immediately blame the other equipment: Blaming the neighbour's TV or other devices before any investigation is premature and unhelpful; the fault may genuinely lie with your station.
• C — inform all the other neighbours: Involving other neighbours before establishing the facts is unnecessary and risks damaging community relations without cause.

Therefore, the correct first action is to cross-reference the complaint with your station log to establish whether a correlation exists before taking any further steps.

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Correct answer: rectification of strong signals in overloaded stages

Cross-modulation occurs when a receiver stage is driven into non-linear operation by a strong signal. The overloaded device partially rectifies or mixes the strong signal with a weaker signal, causing the modulation from the strong signal to appear on the weaker one.

This is a receiver overload effect and indicates that an RF or IF stage is being driven beyond its linear range.

• key-clicks generated at the transmitter produce broadband interference, not cross-modulation inside a receiver.
• improper filtering in the transmitter may cause spurious emissions or splatter, but it does not create cross-modulation in another receiver.
• lack of receiver sensitivity and selectivity affects weak-signal reception and adjacent-channel rejection, not non-linear mixing effects.

Therefore, cross-modulation is usually caused by rectification of strong signals in overloaded stages.

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Correct answer: can be heard irrespective of where the receiver is tuned

If a strong transmitted signal overloads the audio stages of a nearby broadcast receiver, it may bypass the normal tuning circuits and be detected directly by non-linear components in the receiver.

This causes the transmitted signal to be heard even when the receiver is tuned to other frequencies.

• Appearing only when a broadcast station is received is not characteristic of overload.
• Distortion on voice peaks is more typical of transmitter over-modulation.
• Overload effects are not limited to a single frequency.

Therefore, the transmitted signal can be heard irrespective of where the receiver is tuned.

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Correct answer: B — the undesired signal in the background of the desired signal

Cross-modulation occurs when a strong nearby transmitter overloads the early (RF) stages of a receiver, causing the receiver's amplifier or mixer to operate non-linearly. In this non-linear region, the modulation from the unwanted transmitter is transferred onto the carrier of the desired station — so the listener hears the wanted programme with the interfering signal audible in the background. The effect is most pronounced when a powerful transmitter is physically close to the receiver.

• A — a lack of signals being received: This describes receiver desensitisation (blocking), a different overload effect where the strong signal reduces the receiver's gain, suppressing wanted signals. Cross-modulation does not silence the desired signal.
• C — interference only when a broadcast signal is received: Cross-modulation actually requires a desired signal to be present (the interfering modulation is transferred onto it), but the description here is misleading — cross-modulation is not limited to only broadcast reception; it is caused by front-end non-linearity and can affect any tuned signal.
• D — distortion on transmitted voice peaks: Distortion on transmitted peaks is a transmitter-side problem (e.g., over-modulation or amplifier clipping), not a receiver cross-modulation symptom.

Therefore, cross-modulation from a nearby transmitter is heard as the unwanted signal audible in the background of the desired received signal, due to non-linear mixing in the receiver's front end.

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Correct answer: D — harmonic and other spurious signals

When a transmitter operates, it ideally radiates only on its assigned frequency. In practice, non-linearities in the transmitter stages can produce additional unwanted outputs — most commonly harmonics (integer multiples of the fundamental frequency) as well as other spurious emissions such as intermodulation products and parasitic oscillations. These unintended signals can fall on frequencies used by other services and cause harmful interference, which is why regulations strictly limit their level.

• A. Rectified signals — rectification is a process of converting AC to DC, not a description of unwanted transmitter outputs.
• B. Re-radiation signals — re-radiation refers to signals that are picked up and re-emitted by a nearby conductor or circuit, not to emissions generated internally by a transmitter.
• C. Reflected signals — reflection describes RF waves bouncing off surfaces (e.g. in a transmission line or the ionosphere), not spurious transmitter outputs.

Therefore, unwanted transmitter outputs that can cause interference to other users are correctly termed harmonic and other spurious signals.

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Correct answer: low-pass filter

Harmonics are unwanted signals at multiples of the transmitter’s operating frequency.

A low-pass filter placed in the transmission line near the transmitter allows the desired fundamental frequency to pass while attenuating higher-frequency harmonic components.

• A wave trap is used to reject a specific frequency.
• A high-pass filter would pass the unwanted harmonics.
• A band reject filter removes only a specific band, not all harmonics.

Therefore, harmonic output can be reduced by fitting a low-pass filter close to the transmitter.

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Correct answer: C — high-pass filter

Television reception in New Zealand uses VHF and UHF frequencies (roughly 174 MHz and above for digital terrestrial TV). HF amateur transmissions occupy frequencies below 30 MHz. A high-pass filter placed in the TV antenna lead passes the high-frequency TV signals while blocking the much lower HF frequencies, preventing the transmitter's energy from entering the receiver's front end and causing interference.

• A. Active filter — an active filter uses amplifying components and is not a standard interference-suppression device for this purpose; it adds complexity and potential noise rather than simply blocking unwanted HF energy.
• B. Low-pass filter — a low-pass filter passes low frequencies and blocks high frequencies; fitting one in the TV lead would block the TV signals themselves, which is the opposite of what is needed. (A low-pass filter fitted at the transmitter output is a different, complementary measure.)
• D. Band reject filter — a band reject (notch) filter suppresses a specific narrow band of frequencies. While theoretically targeted, it would need to be tuned to the exact offending frequency and would not provide general HF suppression across the whole HF spectrum.

Therefore, a high-pass filter in the TV antenna lead is the correct solution, as it passes VHF/UHF TV signals freely while attenuating HF transmitter energy before it can reach the television receiver.

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Correct answer: output of the amateur transmitter

A low-pass filter used to suppress unwanted harmonic radiation must be placed where those harmonics are about to be radiated.

Harmonics are generated in the transmitter’s RF stages and appear at the output before being sent to the antenna.

Placing the filter at the transmitter output:

• allows the desired fundamental frequency to pass

• attenuates higher-frequency harmonic components before they reach the antenna

• The balanced modulator operates at an earlier stage.

• The stereo system is unrelated to RF transmission.

• The mixer input is before RF amplification.

Therefore, the filter is connected to the output of the amateur transmitter.

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Correct answer: C — attenuate frequencies each side of a band

A band-pass filter allows a specific range (band) of frequencies to pass through with little loss, while attenuating (reducing) signals both below and above that band. The band is defined by a lower and upper cut-off frequency, and signals outside this range are suppressed. Band-pass filters are widely used in receivers to select the desired signal and reject interference from other frequencies.

• A. pass frequencies each side of a band — This describes a band-stop (notch) filter, which is the opposite of a band-pass filter; it rejects a specific band while passing frequencies above and below it.
• B. attenuate low frequencies but not high frequencies — This describes a high-pass filter, which passes frequencies above a set cut-off point and blocks lower frequencies.
• D. attenuate high frequencies but not low frequencies — This describes a low-pass filter, which passes frequencies below a set cut-off point and blocks higher frequencies.

Therefore, a band-pass filter specifically attenuates frequencies on both sides of a chosen band, allowing only that band to pass.

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Correct answer: pass frequencies each side of a band

A band-stop filter (also called a notch filter) is designed to attenuate a specific range of frequencies while allowing frequencies outside that range to pass.

This means:

• frequencies within the stop band are reduced

• frequencies above and below the stop band are passed

• Stopping frequencies each side of a band describes a band-pass filter.

• Allowing only one spot frequency through describes a very narrow band-pass filter.

• Passing frequencies below 100 MHz is unrelated to the definition of a band-stop filter.

Therefore, a band-stop filter will pass frequencies each side of a band.

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Correct answer: A — attenuate frequencies above 30 MHz

A low-pass filter passes signals below its cut-off frequency and attenuates (reduces) signals above it. Fitted at the output of an HF transmitter, a low-pass filter with a cut-off around 30 MHz allows the desired HF transmission to pass while suppressing harmonic energy and other spurious signals that fall above 30 MHz. This is a standard requirement to prevent interference to other services from transmitter harmonics.

• B — pass audio frequencies below 3 kHz: A filter at the RF output of a transmitter operates in the HF range (up to 30 MHz), not at audio frequencies. An audio low-pass filter is an entirely different circuit used in the audio chain.
• C — attenuate frequencies below 30 MHz: This describes a high-pass filter, not a low-pass filter. Attenuating the desired HF signal below 30 MHz would defeat the purpose of the transmitter.
• D — pass audio frequencies above 3 kHz: Passing audio above 3 kHz describes a high-pass audio filter. Again, the transmitter's RF output filter operates at HF, not audio frequencies.

Therefore, a low-pass filter on an HF transmitter output attenuates unwanted harmonic and spurious energy above 30 MHz while allowing the intended HF signal to pass.

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Correct answer: permit lower frequency signals to pass to the antenna

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

In a transmitter system:

• the desired fundamental frequency (lower frequency) is passed

• higher-frequency harmonics are reduced

• It does not pass higher frequencies.

• It does not control SWR directly.

• It does not regulate transmitter power output.

Therefore, a low-pass filter will permit lower frequency signals to pass to the antenna.

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Correct answer: B — to attenuate signals higher in frequency than the transmission

A low-pass filter passes frequencies below its cut-off frequency and attenuates (reduces) frequencies above it. In an amateur radio installation, a low-pass filter is commonly placed between the transmitter and the antenna feedline to suppress harmonics and other spurious emissions, which occur at frequencies higher than the intended transmission frequency. This helps prevent interference with television receivers and other services that operate at those higher frequencies.

• A is wrong because attenuating signals lower in frequency than the transmission is what a high-pass filter does, not a low-pass filter.
• C is wrong because filters are passive networks that only attenuate signals; they cannot boost or amplify power.
• D is wrong for the same reason — filters do not boost power at any frequency.

Therefore, a low-pass filter is correctly used in an amateur installation to attenuate unwanted signals (such as harmonics) that are higher in frequency than the desired transmission.

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Correct answer: a low-pass filter in the transmitter output

Harmonics are unwanted signals at multiples of the transmitter’s operating frequency.

These higher-frequency harmonics may fall within television broadcast bands and cause interference if radiated by the transmitter.

A low-pass filter at the transmitter output allows the desired RF signal to pass while attenuating higher-frequency harmonic components.

• A filter in the TV receiver would not stop the transmitter from radiating interference.
• A high-pass filter would pass the unwanted harmonics.
• A speech amplifier operates at audio frequencies and cannot affect RF harmonics.

Therefore, television interference caused by harmonics can be eliminated by fitting a low-pass filter in the transmitter output.

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Correct answer: prevent interference to a TV receiver

A high-pass filter allows higher frequencies to pass while attenuating lower frequencies.

In a transmitter system, it can be used to:

• block lower-frequency signals (such as HF transmissions)
• allow higher-frequency signals (such as TV broadcast signals) to pass unaffected

This helps prevent interference from lower-frequency transmissions entering a TV receiver system.

• It is not used for preventing overmodulation.
• Passing a band of speech frequencies would require a band-pass filter.
• Telephone interference is typically addressed differently.

Therefore, a high-pass filter can be used to prevent interference to a TV receiver.

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Correct answer: at the antenna terminals of a TV receiver

A high-pass RF filter allows higher-frequency signals to pass while attenuating lower-frequency signals.

In practice, it is used at a TV receiver input to:

• pass TV broadcast frequencies (VHF/UHF)

• block lower-frequency signals (e.g. HF transmissions) that could cause interference

• A low-pass filter is used at a transmitter output.

• The Morse key is not part of RF filtering.

• Audio stages use audio-frequency filters, not RF filters.

Therefore, a high-pass RF filter is fitted at the antenna terminals of a TV receiver.

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Correct answer: D — low frequencies but not high frequencies

A high-pass filter passes signals above its cut-off frequency while attenuating (reducing) signals below that frequency. The name describes what it passes, not what it blocks — so a high-pass filter lets high frequencies through and rejects low frequencies.

Typical amateur radio applications include suppressing low-frequency interference or harmonic content below the desired band, or blocking DC and audio-frequency signals from an RF circuit.

• A — Describes behaviour centred on a specific VHF band, which is a band-pass or band-stop filter, not a high-pass filter.
• B — Passing only a band of VHF frequencies while rejecting everything else describes a band-pass filter.
• C — This is the opposite of correct; a high-pass filter attenuates low frequencies, not high ones. A filter that attenuates high frequencies is a low-pass filter.

Therefore, a high-pass filter attenuates low frequencies while allowing high frequencies to pass.

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Correct answer: negative feedback

An operational amplifier used as a filter always relies on negative feedback to control gain, stability, and frequency response. The feedback network, typically made from resistors and capacitors, determines the filter characteristics such as cutoff frequency, bandwidth, and response shape.

Negative feedback keeps the amplifier operating in a stable, linear region and prevents unwanted oscillation.

• positive feedback to reduce oscillation is incorrect, positive feedback promotes oscillation and instability.
• random feedback has no defined electrical meaning and is not used in circuit design.
• inductors and resistor circuits only is incorrect, op amp filters normally use resistors and capacitors, and inductors are rarely required.

Therefore, an operational amplifier connected as a filter always utilises negative feedback.

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Correct answer: very high but purposely reduced using circuit components

An operational amplifier (op-amp) has an extremely high open-loop voltage gain at low frequencies, often:

\[ 10^5 \text{ to } 10^6 \]

In practical circuits, this gain is deliberately reduced using negative feedback components such as resistors.

This provides:

• stable operation

• predictable gain

• improved linearity

• The gain is not inherently low.

• It is not less than one.

• It is well-defined.

Therefore, the voltage gain is very high but purposely reduced using circuit components.

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Correct answer: very high

An operational amplifier is designed to draw almost no input current so that it does not load the signal source. This results in a very high input impedance, often in the megaohm to gigaohm range for modern op amps.

A high input impedance allows accurate voltage amplification and prevents signal attenuation caused by loading effects.

• very low is incorrect because a low input impedance would heavily load the source and distort measurements or signals.
• capacitive and inductive may exist as small parasitic effects, but the overall input impedance is dominated by a very high resistance.

Therefore, the input impedance of an operational amplifier is generally very high.

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Correct answer: an operational amplifier, resistors and capacitors

An active low-pass filter requires an amplifying device along with frequency-selective components.

An operational amplifier used with resistors and capacitors can provide:

• frequency-dependent filtering
• gain
• improved performance compared to passive filters

The resistors and capacitors determine the cutoff frequency, while the op-amp provides amplification and buffering.

• Zener diodes and resistors are used for voltage regulation.
• Electrolytic capacitors and resistors alone form a passive filter.
• A transformer and capacitors do not provide active filtering.

Therefore, an active audio low-pass filter can be constructed using an operational amplifier, resistors and capacitors.

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Correct answer: D — a notch filter

A notch filter (also called a band-stop or band-reject filter) is designed to attenuate a very narrow range of frequencies while passing all others. It is the opposite of a band-pass filter. Notch filters are commonly used in amateur radio to suppress a specific interfering signal or unwanted frequency — in this case, sharply rejecting signals centred on 3.6 MHz.

• A. A band-pass filter is wrong — a band-pass filter passes a selected band of frequencies and attenuates everything outside it; the opposite behaviour is needed here.
• B. A high-pass filter is wrong — a high-pass filter passes all frequencies above a cutoff point and attenuates those below it; it does not target a narrow centre frequency.
• C. A low-pass filter is wrong — a low-pass filter passes all frequencies below a cutoff point and attenuates those above it; again, no narrow-band rejection.

Therefore, a filter that sharply attenuates only a very narrow band of frequencies centred on a single frequency is correctly called a notch filter.

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