6.2 How Interference Enters Television and Broadcast Receivers
Key Takeaways
- 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.
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.
Which frequency range is the intermediate frequency (IF) of a television receiver?
The Examinable Frequency Ranges
Learn this table cold. Questions routinely quote one of these figures and ask which stage it belongs to.
| Receiver and stage | Frequency range | Significance for a VK amateur |
|---|---|---|
| Television RF, VHF | 50-225 MHz | The 6 m band (50-52 MHz on a Standard licence) sits inside this range — a direct in-band overload threat. |
| Television RF, UHF | 470-854 MHz | The 70 cm band at 430-450 MHz sits just below the lower edge, too close for a wideband tuner to reject. |
| Television IF | 33-40 MHz | The fifth harmonic of 7 MHz and the tenth harmonic of 3.5 MHz both arrive at 35 MHz. |
| Television video baseband | 0-5 MHz | The 160 m and 80 m bands (1.8 and 3.5 MHz) lie inside it, so pickup on internal wiring patterns the picture. |
| Broadcast radio RF, AM | 525-1606 kHz | The medium-wave band; a strong local station overloads your receiver as easily as you overload theirs. |
| Broadcast radio RF, FM | 88-108 MHz | The second harmonic of a 6 m transmission (2 × 50 MHz = 100 MHz) lands in this band. |
| Broadcast IF, AM | 455 kHz typical | Close to the 472-479 kHz allocation and reachable by mixing products from strong HF signals. |
| Broadcast IF, FM | 10.7 MHz typical | The third harmonic of a signal near 3.567 MHz on 80 m arrives at 10.7 MHz. |
The lesson: you do not have to transmit anywhere near a receiver's tuned frequency to interfere with it. A lawful 80 m or 40 m transmission has harmonics that land inside a television IF, a 6 m transmission is inside the television VHF range as a fundamental, and a 160 m transmission is inside the video baseband. Operating in band protects your licence, not your neighbour's picture.
Why an IF-Passband Signal Cannot Be Cured at the Antenna Socket
Candidates often assume any receiver problem can be cured by plugging a filter into the back of the set. For a signal inside the IF passband — 33-40 MHz for a television, 455 kHz or 10.7 MHz for a broadcast radio — that assumption fails for three reasons.
- Retuning is no defence. The IF is fixed by design, so changing channel moves only the local oscillator. The interference appears on every channel with equal strength and the viewer cannot tune away from it.
- The receiver amplifies it deliberately. The IF strip provides most of the set's gain through filters built to pass exactly that band, so the receiver cannot distinguish your signal from a properly converted wanted one and hands it the full IF gain.
- The entry point is downstream of the socket. Breakthrough usually occurs by radiation into chassis wiring, ribbon cables and the mains lead, or by leakage around the tuner can — all past the antenna socket, so a filter fitted there is bypassed. Unplug the antenna as proof: if the interference survives, nothing at the socket can fix it.
The genuine remedies lie elsewhere: screening and lead decoupling at the receiver, and on your side of the fence a low-pass or harmonic filter on the transmitter output. Harmonics are your emissions and your legal responsibility, and stopping a 35 MHz harmonic at its source is the fastest fix available to you.
Which pair correctly gives the radiofrequency ranges used by broadcast radio receivers in the ACMA syllabus?
Diagnosing the Door
| Observation while a friend transmits | Entry point | First step |
|---|---|---|
| Interference stops when the victim's antenna is unplugged | RF stages, via antenna and feeder | Filter and choke the feeder ahead of the tuner |
| Interference continues with the antenna unplugged | Audio stages, via mains lead, speaker leads or interconnects | Treat the leads, not the antenna system |
| Every channel affected equally and retuning changes nothing | IF breakthrough | Filter the harmonic at the transmitter; screen the receiver |
| One channel only, two IFs from another strong signal | Image response of the front end | Improve front-end selectivity or change the channel allocation |
Running that test costs nothing, and it prevents the classic mistake of fitting an expensive filter to the wrong end of the wrong cable.
A 35 MHz harmonic from an amateur transmitter is being reproduced on every channel of a neighbour's television. Why will a filter plugged into the television's antenna socket generally fail to cure it?