Television interference

Television interference (TVI) is a particular case of electromagnetic interference which affects television reception. Many natural and man-made phenomena can disrupt the reception of television signals. These include naturally occurring and artificial spark discharges, and effects due to the operation of radio transmitters.

In this article, only conventional UHF (or VHF) AM TV will be considered. Satellite TV tends to be FM TV and operates around 6 or 10 GHz (microwaves). While this page is mainly concentrated on UHF AM TV, many of the principles can be applied in cases where other devices are being troubled by poor reception. Also the advice on radio transmitter interference can be helpful in cases of non-radio equipment such as Hi-Fi units and stereos.

Static electricity and sparks

The sparks generated by static electricity can generate interference.

Many systems where radio frequency interface is caused by sparking can be modeled as the following circuit. The source of energy charges C1 via a resistance, and when the spark gap breaks down, the electricity passes through L and excites the resonant LC circuit. The energy in the LC circuit is then radiated through the aerial.

As an example, when a person walks over a nylon carpet, the rubbing of shoes on carpet performs the role of a battery and resistor, while the person acts as a capacitor (C1 and C2), and the air between a hand and a door knob is a spark gap. Stray inductance acts as L.

Sparks and allied phenomena

Horizontal lines randomly arranged on a television screen may be caused by sparking in a malfunctioning electrical device. Electric railways can also be a strong source of this type of interference.

Other possible sources of such interference include:

• Thermostats, fridges, freezers, fish tank heaters, central heating systems
  • These can create sparks as they turn on or off; as they age they can become worse. In some rare cases they can create non-stop interference through sparking.
• Electric motors
  • Motors which have a commutator can suffer from sparking at the brushes.
• Ignition systems on cars and motorbikes.

Devices which switch at powerline frequency

• Power line hardware, this can generate sparks at either a 100 or 120 Hz rate

• Light dimmers and other solid state power control devices.

Thyristor and Triac regulators without proper chokes are a common source of EMI as well. It is likely that a thyristor (SCR) power controller using the variable phase angle method will generate harmonics of the mains supply, while the spark at a contact will be a very wide band source whose frequency is not related to the power supply frequency. In Thyristor control systems the potential for EMI problems can be minimised by using zero crossing switching where the thyristor is switched on at the moment of time when the AC voltage changes from one direction to the other.

A typical SCR based light dimmer which dims the light through phase angle control. This unit is wired in series with the load. Diodes (D₂, D₃, D₄ and D₅) for a bridge which generates DC with lots of ripple. R and C form a circuit with a time constant, as the voltage increases from zero (at the start of every halfwave) C will charge up, when C is able to make ZD conduct and inject current into the SCR the SCR will fire. When the SCR conducts then D₁ will discharge C via the SCR. The SCR will shut off when the current falls to zero when the supply voltage drops at the end of the half cycle, ready for the circuit to start work on the next half cycle.

Devices which switch faster than 200 Hz

• Computers and other digital electronic equipment. These devices create and use signals which are switched on/off at great speed. It is the case that any repetitive signal can be reduced down to a Fourier series of sine waves. It so happens that a perfect square wave is

E = E^o sin (ωt) - {E^o sin (2ωt)}/2 + {E^o sin (3ωt)}/3 ......

As the term goes on for ever to higher and higher frequencies the square wave contains harmonics of the fundamental which go on upwards in frequency for ever, these harmonics are responsible for much of the interference created by computers. Always remember that a modern PC is a device which is operating in the VHF/UHF using square waves. As the cases on many computers are not perfect shields, some of this radio frequency energy can leak out and cause interference to radio (and sometimes TV) reception.

• Switch mode power packs can be a source of interference. These are used in consumer electronic/electrical products and in some lighting systems.

Strong TV signals

It is possible to also get a bad picture if the signal strength of the TV transmitter is too high, for instance in the Bromley area of south east London the signal from the TV mast is so strong that it may cause the TV's front end to be overloaded. Check for this by inserting an attenuator inside with the TV aerial connection. If you start by trying with 10 dB, and then move to 20 dB then this might provide a cure.

In that part of London the TV transmitter (Crystal Palace Transmitter) and the Croydon Transmitter tower which has VHF pager transmitters are both such strong sources of radiowaves that FM only UHF (432 MHz) and VHF (144 MHz) radiosets can become overwhelmed when they are attached to a beam aerial which is pointed at the Crystal Palace and Croydon towers respectively. The receiver of the FT-290R2 which is a 144 MHz multimode (FM/CW/SSB) radio is more able to cope with such strong out of band signals.

So this problem of overloading is not confined to TV sets only. One of the reasons these FM radio sets have this shortcoming is the fact that they often use two diodes which are wired across the front end of the RF front end. These two diodes act in the same way as the rusty bolt, the receiver then experiences a great array of mixing (intermodulation) products some of which fall upon the frequency which the radio is tuned to.

It is vital that before you attempt to start to locate an unwanted source of radio signals that you first check that the unwanted signal is not being generated in the front end of your radio set. By adding either an attenuator or a band pass filter to the radio its front end can be protected from these out of band signals.

See also

• Television interference (ghosting)
• Television interference (Co-channel reception)
• EMC problem (excessive field strength)