6.2 How Interference Enters Television and Broadcast Receivers

ACMA Amateur Radio Standard Recognition Certificate - Theory Exam

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1 Chapter 1: Electrical Principles and Components

1.1 Mathematics, Units and Metric Prefixes 1.2 DC Circuits, Voltage, Current, and Ohm's Law 1.3 Power, Energy, and Resistance Calculations 1.4 Resistor Colour Code and Component Symbol Identification 1.5 Capacitors: Construction, Ratings and Safety 1.6 Inductors: Construction, Factors Affecting Inductance and Combinations 1.7 Capacitor and Inductor Combination Calculations 1.8 AC Circuits, Frequency, Wavelength, and RMS Values 1.9 Reactance and Impedance in AC Circuits 1.10 Resonance, Q Factor and Tuned Circuits 1.11 Transformers: Turns Ratio, Impedance Matching and Eddy Currents 1.12 Rectifiers, PIV and Power Supply Smoothing 1.13 Active Components: Semiconductors, Diodes, and Transistors 1.14 Transistor and FET Symbols, Configurations and External Behaviour

2 Chapter 2: Transmitters and Receivers

2.1 Superheterodyne Receiver Architecture and Block Diagrams 2.2 Frequency Converters, IF Amplifiers and Transceiver Sharing 2.3 Receiver Performance: Sensitivity, Selectivity, and AGC 2.4 Signal-to-Noise Ratio and Receiver Noise Performance 2.5 Transmitter Architecture, Oscillators and Frequency Synthesisers 2.6 Transmission Quality: Frequency Drift, Parasitic Oscillation and Over-Modulation 2.7 Modulation Techniques: AM, FM, and SSB 2.8 Amplifier Classes and Power Amplifiers (Class A, B, C, and AB) 2.9 Modulation, Duty Cycle and Rated Transmitter Power 2.10 Transceiver Control: ALC, Squelch, and Digital Modes 2.11 Digital Transmission Modes and Bandwidth

3 Chapter 3: Transmission Lines and Antennas

3.1 Transmission Lines: Coaxial, Twin-Lead, and Characteristic Impedance 3.2 Standing Wave Ratio (SWR) and Reflection Loss 3.3 Impedance Matching, Baluns, and Antenna Tuning Units (ATUs) 3.4 Resonant Antennas: Dipoles, Verticals, and End-Fed Antennas 3.5 Angle of Radiation, Antenna Height and Current Distribution 3.6 Directional Antennas: Yagis, Gain and Radiation Patterns 3.7 Effective Radiated Power: System Gains and Losses

4 Chapter 4: Radio Wave Propagation

4.1 Electromagnetic Waves: Velocity, Wavelength and Polarisation 4.2 Ground Wave, Direct Wave and Line-of-Sight Propagation 4.3 Ionospheric Layers (D, E, F1, F2) and Skywave Propagation 4.4 Critical Frequency, Maximum Usable Frequency (MUF), and Fading 4.5 Solar Activity, Sunspot Cycles, and VHF Anomalous Propagation 4.6 Seasonal and Diurnal Effects on Band Selection

5 Chapter 5: Electrical and RF Measurements

5.1 Multimeter Operation: Voltage, Current, and Resistance Measurements 5.2 RF Power and SWR Measurement (PEP vs. Average Power) 5.3 Frequency Counters, Dummy Loads, and Test Equipment 5.4 Frequency Standards, Crystal Calibrators and Standard-Frequency Transmissions

6 Chapter 6: Interference and Electromagnetic Compatibility (EMC)

6.1 Harmonic Emissions, Spurious Signals, and Transmitter Filtering 6.2 How Interference Enters Television and Broadcast Receivers 6.3 Masthead Amplifier Overload and Audio-Stage Demodulation 6.4 Electromagnetic Interference (EMI), Breakthrough, Ferrites, and EMC 6.5 Filter Types, Response Curves and EMC Filter Selection 6.6 EMC Good Practice: Minimum Power, Balanced Antennas and Feedline Routing 6.7 Vehicle EMC and Resolving Community Interference Complaints

7 Chapter 7: Safety and Environmental Hazards

7.1 Mains Voltage Safety, Earthing, and High-Voltage Capacitors 7.2 Equipment Approval, Mains Wiring and Fuse Practice 7.3 Station Layout, Cabling and Emergency Shut-Off 7.4 Lightning Protection and Station Security 7.5 Electric Shock: Prevention, Rescue and Emergency Response 7.6 Battery, Chemical and Hearing Safety in the Amateur Station 7.7 RF Exposure: EMR Hazards and Safe Distance 7.8 Antenna Erection, Siting and Power Line Safety

6.2 How Interference Enters Television and Broadcast Receivers

Key Takeaways Explain This Topic AI

An amateur transmission reaches a victim receiver through only three doors: the radiofrequency stages, the intermediate frequency stages, and the audio stages via long speaker leads and other interconnections.

Television receivers use RF ranges of 50-225 MHz and 470-854 MHz, an IF of 33-40 MHz, and a video baseband of 0-5 MHz.

Broadcast radio receivers use RF ranges of 525-1606 kHz and 88-108 MHz, with typical intermediate frequencies of 455 kHz for AM and 10.7 MHz for FM.

Televisions and most broadcast radios are superheterodyne sets: tuning moves the local oscillator and never the IF, so IF breakthrough degrades every channel equally.

A signal inside the IF passband cannot be filtered out at the antenna socket, because the receiver's own IF gain amplifies it and the entry point normally lies downstream of the socket.

Last updated: July 2026

6.2 How Interference Enters Television and Broadcast Receivers

ACMA Exam Focus: Syllabus item 8.1 — the ways an amateur transmission enters a television or broadcast receiver: through the radiofrequency (RF) stages, through the intermediate frequency (IF) stages, and into the audio stages by way of long speaker leads and other interconnections. You must also know that televisions and most broadcast receivers are superheterodyne designs, and recall the examinable RF, IF and baseband frequency ranges for television and broadcast radio.

Three Doors Into a Victim Receiver

When a neighbour reports that your station is on their television, the useful question is not how much power are you running but which door is the signal using . The syllabus recognises three, each with a different remedy.

The radiofrequency (RF) stages. Your signal travels down the victim's antenna and feeder exactly as a wanted signal does. If it is strong enough it drives the first RF amplifier or the mixer out of its linear range.

The intermediate frequency (IF) stages. Your signal — more often one of your harmonics — falls on the frequency the receiver uses internally. It reaches the high-gain IF strip by leaking past the tuner or radiating into internal wiring, and is amplified as though the set had deliberately tuned it.

The audio stages, reached through interconnections. Long speaker leads , RCA interconnects, HDMI and USB cables and the mains lead all behave as antennas at HF and VHF, delivering radiofrequency energy straight into the audio section, downstream of every tuned circuit in the set.

Naming the door correctly is most of the job: a filter aimed at the wrong entry point achieves nothing.

Why Almost Every Victim Is a Superheterodyne

Television receivers and most broadcast radio receivers use the superheterodyne circuit. The incoming signal is mixed with a local oscillator (LO) to produce a fixed intermediate frequency , and virtually all of the receiver's gain and selectivity is applied there. Tuning the set moves the local oscillator; it never moves the IF. That creates two weaknesses the examiners expect you to know.

Image response. A mixer cannot tell which side of the local oscillator a signal came from. With high-side injection the image lies at

f(image) = f(signal) + (2 × f(IF))

so a set tuned to one channel also accepts a completely different frequency two IFs away, unless the front end has enough tuned selectivity to reject it.

IF breakthrough. A signal falling inside the IF passband never needs to be converted at all — it only has to reach the IF amplifier. Once there, the receiver's own selectivity is working for the interference instead of against it.