6.3 Masthead Amplifier Overload and Audio-Stage Demodulation

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.

1. 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.
2. 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.
3. 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:

1. 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.
2. Reduce transmitter power to the lowest that maintains the contact; overload is a threshold effect, so a few decibels often clears it outright.
3. Increase the separation — move, raise or re-orient the amateur antenna, change its polarisation, or turn a null towards the affected house.
4. 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.
5. Check screening and the DC feed — poor braid coverage, cheap cable and unscreened splitters admit signal downstream of any filter you fit.

Part 2: Unwanted Demodulation in Audio Amplifiers

This mechanism catches people out because the victim contains no radio receiver at all. A hi-fi amplifier, a public-address (PA) system, powered computer speakers, an intercom or a telephone suddenly reproduces your single sideband (SSB) speech — intelligible, and often with the volume at zero and the source switched off.

The Mechanism

No tuned circuit is required anywhere in the chain:

1. A lead acts as an antenna. A 3 m speaker lead is close to a quarter wavelength at 25 MHz; mains leads, interconnects and telephone lines are efficient pickup elements across the HF bands.
2. The RF reaches an input stage that cannot handle it. An overloaded audio stage is non-linear — its transfer characteristic bends exactly as the masthead amplifier's did.
3. A non-linear junction is a detector. The base-emitter junction of the input transistor is a diode; driven hard by RF it rectifies, recovering the modulation envelope of your transmission.
4. The recovered audio is then just audio. The remaining stages amplify it and the loudspeaker reproduces it.

Two consequences follow, and both are examinable. Rectification happens before the volume control, so turning the volume down does not help. And because an envelope detector needs an amplitude-varying signal, SSB and AM come through as intelligible speech, whereas FM — with its constant envelope — more often gives a buzz or a click on keying. Corroded joints and loose metalwork form the same kind of non-linear junction, which is why the effect can appear in places containing no electronics at all.

Curing Audio-Stage Demodulation

Every cure does one of two things: keep RF off the leads, or shunt it to chassis before it reaches a junction.

• Ferrite chokes on every lead — clamp-on ferrites, with several turns threaded through where the cable is thin enough, on speaker leads, interconnects, the mains lead and the telephone line. They act as common-mode chokes, raising the impedance to RF current on the outside of the cable without touching the audio.
• Shorter, screened leads — a shorter lead is a poorer antenna. Screened interconnects, twisted speaker pairs and routing well away from your feeder all cut the pickup.
• Bypass capacitors at the amplifier input — of the order of 1 to 10 nF from each input pin to chassis at the socket, presenting a low impedance at RF and a high impedance at audio, so the interference is shunted away before it reaches the first junction.
• Star earthing — bond the equipment to a single common earth point rather than daisy-chaining it, so RF currents do not circulate through the signal ground and no earth loops are formed.

Nothing on that list involves an antenna socket or your transmitter's harmonics. This is an immunity problem in equipment that was never designed to reject RF, and the fix lives on the leads.

Symptom, Mechanism, Cure