6.3 Masthead Amplifier Overload and Audio-Stage Demodulation

ACMA Amateur Radio Standard Recognition Certificate - Theory Exam

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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

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6.3 Masthead Amplifier Overload and Audio-Stage Demodulation

Key Takeaways Explain This Topic AI

Masthead and distribution amplifiers are wide-band, high-gain devices with no input selectivity, so a strong nearby amateur signal drives them into non-linearity.

Overload produces gain compression, cross-modulation and intermodulation, giving the classic symptom of every television channel breaking up whenever you transmit.

A filter must be fitted ahead of the masthead amplifier's input; anything plugged in at the television set is downstream of the overload and cannot undo it.

The non-linearity of an overloaded audio amplifier demodulates RF because a non-linear junction acts as a detector, so SSB speech emerges from a hi-fi, PA system or telephone that contains no receiver.

Cures for audio-stage demodulation are ferrite chokes on every lead, shorter screened leads, bypass capacitors at the amplifier input, and star earthing.

Last updated: July 2026

6.3 Masthead Amplifier Overload and Audio-Stage Demodulation

ACMA Exam Focus: Syllabus items 8.2 and 8.3 — masthead amplifiers and distribution amplifiers used for television reception are wide-band devices that a strong nearby amateur signal overloads easily, and the non-linearity of an overloaded audio amplifier can demodulate a radiofrequency signal. For each mechanism you need the cause, the tell-tale symptom and the correct cure.

Part 1: Masthead and Distribution Amplifier Overload

A masthead amplifier is a small low-noise preamplifier bolted to the antenna mast, powered by direct current sent up the coaxial cable from an indoor injector. A distribution amplifier does the same job indoors, feeding several outlets from one antenna. Both are common on Australian rooftops, and both are the most likely victim in a television interference complaint. Four design choices make them fragile:

They are wide-band. A typical unit passes the whole VHF and UHF television spectrum in one stroke with no input selectivity whatsoever , so it cannot reject your HF, 6 m or 2 m signal.

They have high gain. Fifteen to thirty decibels is normal, applied to everything arriving at the input, wanted or not.

They are badly located. The masthead unit is outdoors at roof height, often the closest active electronics to your antenna and at a similar elevation.

They have almost no headroom. These are small-signal stages built to lift microvolts; a local transmission is orders of magnitude larger than anything they were designed to handle.

What Overload Actually Does

Push an amplifier past its linear range and its transfer characteristic bends. Three consequences follow, and all three appear in exam questions.

Gain compression. The stage runs out of headroom, so wanted television signals are squashed while your signal is present — on digital television, pixelation, freezing or complete loss of lock for as long as you hold the key down.

Cross-modulation. The non-linearity transfers the modulation of the strong unwanted signal onto the wanted carriers, so your speech pattern rides on the television signals.

Intermodulation. Pairs of signals mix in the bent characteristic, producing sum, difference and higher-order products that land on channels nowhere near your transmit frequency.

The signature is unmistakable: every channel is affected at once, only while you transmit. Trouble confined to a single channel points instead at IF breakthrough or an image response.

Where the Cure Must Go

Overload happens at the amplifier input, ahead of the coaxial run, the splitter, the wall plate and the television, so anything plugged into the back of the set is downstream of the damage and cannot undo it. Remedies in the order worth trying:

Filter ahead of the amplifier input — a high-pass filter that passes the television bands while rejecting HF, or a band-stop (notch) filter tuned to your band where your frequency lies close to the television range.

Reduce transmitter power to the lowest that maintains the contact; overload is a threshold effect, so a few decibels often clears it outright.

Increase the separation — move, raise or re-orient the amateur antenna, change its polarisation, or turn a null towards the affected house.

Replace the amplifier with a properly screened unit that has input filtering and only the gain the installation needs. Many are fitted where the signal was already adequate, so removing one altogether is a legitimate free cure.

Check screening and the DC feed — poor braid coverage, cheap cable and unscreened splitters admit signal downstream of any filter you fit.