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Solving TV Interference Problems

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TV interference problems (often referred to as "TVI"), can be difficult to diagnose. This self help guide provides advice and troubleshooting tips on how to solve TVI from various sources including Tetra masts which can interfere with analogue and terrestrial digital TV channels, or overload a Sky Digital distribution amplifier, Freeview box or VCR.

Television Interference can take many forms but on Freeview reception, TVI problems often result in picture freezing or breaking up. On analogue TV, interference problems often result in picture patterning (picture below).

TVI can often be solved by fitting an appropriate TVI filter, but TVI filters are available in many different types so it is important that the right filter is used to completely cure the TV interference problem. This article will help you pinpoint the interference source.

Common types of TVI filter include:

TV Bandpass Filters - typically used to pass UHF TV Ch21 - Ch68 with low signal loss, but rejects other interfering signals both above or below the passband.
Notch Filters - often used to reject one of two strong interference sources. The frequency of the interfering signal must be known in order to design or select the right notch filter. Typically used to solve Tetra, 2m or 70cm amateur radio TVI.
Bandstop Filters - a band-stop filter is a filter that passes most frequencies with little attenuation, but attenuates those in a specific frequency range.
Low Pass Filters - Occasionally used to reduce interference from strong interfering signals above the UHF TV band (Ch21 - 68).
High Pass Filters - Frequently used to solve TV interference where the interference source is a lower frequency than the UHF TV band. These types of TVI filter are particularly good at rejecting VHF interference to UHF TV.

Common Causes of TVI

Television interference (TVI) can be the result of co-channel UHF television interference due to abnormal weather, high pressure or atmospheric conditions > read more.

Patterning interference on analogue television or can also result from the poor installation of a VCR, Sky Digibox or Freeview receiver. Each of these set top boxes has a built in UHF modulator, which must be carefully adjusted to avoid TV interference. So, it's important to determine accurately whether patterning interference is coming from the prevailing weather conditions, an external source or whether it emanates from your Sky Digibox, Freeview box or VCR modulator.

Analogue TV Interference Help

Read our useful tips for solving analogue television interference problems:-

1. Permanent Patterning Interference

Where permanent patterning interference is seen on one or more TV channels, try connecting your television directly to your aerial, so that the aerial isn't feeding the TV via all your peripherals (video recorder, Sky Digibox or Freeview receiver). Then check whether the TVI is cured. If the interference is eliminated, it could be due to either:-

Interference pickup by poor quality coaxial cable interconnects like this one, between your video recorder or set top boxes --> read more
The UHF modulator of your peripherals --> read more

If interference is still present after running this test, then it is likely to be caused by a nearby radio transmitter, phone or Tetra mast which is overloading your TV distribution amplifier or signal booster if you have one.

Where interference is from a nearby PMR or Tetra radio transmitter, a suitable TVI filter may help. If your aerial has a signal booster, curing the problem will be more difficult. Read more

2. Intermittent Patterning Interference

Where patterning interference of analogue TV signals is intermittent in nature, it is likely to be from a nearby PMR radio transmitter mast, taxi base or amateur radio station. Note the times of the interference and contact Ofcom for help, if the problem persists. It may also help to find out more about Ofcom's TV and Radio Interference Investigation Service.

Interference from PMR radio masts can usually be cured by fitting a high pass or notch TVI filter or by improving the aerial installation and replacing any unscreened high gain aerial amplifiers. Screening the aerial from the source of the problem (if identified) can also help.

3. Aerial Amplifiers and Poor Reception Areas

In areas which suffer poor Freeview reception, aerial amplifiers (also known as signal boosters) are frequently used to improve terrestrial digital TV reception (DVB-T). Unfortunately, high gain aerial amplifiers (signal boosters) overload easily when strong radio signals are present nearby, often leading to serious TVI problems. In many cases, the nearby radio transmitter that is blamed for the interference is actually not at fault. The real culprit is the aerial amplifier which is being overloaded by the very strong signal of the nearby radio transmitter or Tetra mast. A single stage signal booster (with a 10dB to 15dB signal gain) is a much less likely to suffer TVI than a extra high gain type (25db - 50dB) - avoid these at all costs!

Many older TV signal boosters comprise twin stage wideband amplifiers, with a high gain of 25dB or more making them particularly prone to overload. To make matters even worse, many aerial amplifiers are totally unscreened like the one pictured here from Fringe Electronics which, when tested, had very poor interference immunity!

When aerial amplifiers suffer strong signal overload from a nearby radio transmitter mast, you'll need to fit a suitable TVI filter between the aerial and the aerial amplifier input, not at your TV to reduce the level of the interfering signal. The TVI filter will need installing close to the aerial if a masthead amplifier (signal booster) is used. In these cases always waterproof the TVI filter connections with self amalgamating tape if outdoors. If the signal booster is an older unscreened type, it should be upgraded to a newer fully screened masthead amplifier which offer much better interference immunity. In addition, always inspect the TV aerial and down lead for deterioration and water ingression before fitting TVI filters. Sometimes, corrosion of aerials and feeders can exaggerate, or even be the sole cause of TV interference problems.

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To get the best signal in poor reception areas, always fit a high gain digital compatible TV aerial (such as a Triax Unix 52, Unix 100 or similar aerial) fed with double screened CT100 coaxial cable mounted above roof height.

This may increase the TV signal sufficiently for any masthead amplifier to be removed altogether. Where an aerial amplifier is still needed, always fit a screened masthead type for best results, with a gain not exceeding 16dB.

4. Interference to Sky Digital Distribution Amplifiers

It is not uncommon for a Sky TV distribution amplifier to suffer TVI and picture patterning. In some cases interfering lines may move across the screen giving a poor picture. A Sky distribution amplifier is usually powered from the Sky Digibox (which provides 12 volts to the coaxial cable). UHF distribution amplifiers are often mounted in the loft of a house, feeding all TV points around the home.

A Sky TV interference problem can occur on one or more distributed UHF TV channels, including the Sky TV UHF channel itself. To diagnose the exact cause of the problem, it is wise to read all of this page to get a better overall understanding. Interference to a Sky distribution system can be caused by a number of different issues. Common causes are Sky TV distribution amplifier overload due to being too close to a TV transmitter, or by interference from a nearby radio transmitter. However, in some cases re-tuning the Sky Digibox UHF modulator to a more appropriate (totally clear) UHF channel resolves the issue.

If interference is caused by a nearby radio transmitter, the solution is to fit a suitable TVI filter to the "Aerial In" of the Sky Digibox (assuming your TV aerial isn't fitted with a signal booster). If your TV aerial has a signal booster, then the filter must be fitted at the aerial input of the signal booster. The signal booster may be in the loft or on the roof for masthead amplifier types.

5. Bursts of Electrical Interference

Electrical interference appears as bright bursts of white lines or dots on an analogue TV picture and usually lasts less than one minute. It can cause temporary Freeview picture break up.

Common causes of are a faulty central heating thermostat, boiler thermostat, intermittent plugs, switches or sockets around your home which are arcing. Check for this by temporarily turning off your central heating or hot water at the time the electrical interference occurs to see if the problem is solved.

It is also possible that a neighbour's property is the source of the electrical interference. Commercial garage premises and MOT Centres can have arc welders for example, which can create high levels of electrical interference. Ofcom can help diagnose this kind of interference too.

Electrical interference to Freeview reception will always be worse with a loft TV aerial and even more of a problem where the coaxial cable down lead used is not double screened and run close to house electrical wiring. To reduce electrical impulse interference to Freeview reception, always use FT100 double screened satellite grade coaxial cable as your aerial down lead and mount your TV aerial well away from 240V domestic wiring and loft pipe work. We offer a dedicated page with tips for installing loft aerials.

6. Ignition Interference from Vehicles

Impulse interference from car ignition systems should have been largely eradicated as all modern automobiles, mopeds and motorcycles incorporate resistive ignition leads which provide suppression. However, there are always exceptions and poorly maintained old vehicles can still cause significant impulse type interference which can cause problems with Freeview reception - freezing the picture and bright white dots on analogue pictures. The problem can also affect FM and DAB reception.

When vehicle impulse TV interference is experienced, always ensure that the coaxial cable used to feed your aerial is the double screened FT100 type for the entire run. It may also help to raise the height of your TV aerial and to reposition it as far away as possible from the source of interference (I.E. nearby roads). Also try earthing the braid of the coaxial cable at the TV.

7. Reducing Mains Borne Interference Pickup

When tidying up the cables to your TV installation, it's fine to use cable ties to bunch the cables but just make sure that you don't cable tie power cables to your TV aerial cable as mains borne interference can then be passed into your TV aerial feeder much more easily.

Causes of Freeview TV Interference

Freeview TV interference is likely to cause total picture loss or the picture freezing. Such interference can be the result of electrical noise from a defective central heating thermostat (particularly in areas of the UK where there is a poor Freeview signal) and where a loft aerial is used.

It is also possible that interference from nearby radio transmitters or a Tetra mast could result in Freeview signal loss, a frozen picture or break up. In these cases, it is often a good idea to check your analogue TV picture at the precise time of Freeview picture break up, to see how that is being affected. If patterning interference is noticeable, then it is likely that a nearby radio transmitter may be the cause. Where bright bursts of white dots or lines are on the screen, suspect electrical interference (see points 5 and 6 above).

Types of TVI Filters

There are several different types of TVI filters as described earlier in this article. The two most common types are as follows:-

High pass TVI filters - These pass UHF TV frequencies with low loss, but attenuate frequencies below the UHF TV band. High pass filters and braid breakers are particularly useful for solving interference from HF and VHF radio transmitters. Tetra High Pass Filters with a very sharp cut off frequency are also suitable for reducing UHF interference from Tetra masts for example.
Notch TVI filters - Notch filters are effective at reducing TVI on one or two different frequencies. Different types are available depending on the frequency of the interfering signal.

Important Note:

TVI filters will not help reduce to the effects of electrical interference on your TV. They will only help when interference is due to a strong signal from a nearby radio transmitter.

Police Tetra Masts

The vast majority of Tetra TV interference problems are caused by the use of 2 stage aerial amplifiers (high gain 25dB signal boosters) and to a lesser extent by Sky TV distribution amplifiers which inherently have less gain and are less susceptible to Tetra interference.

Some aerial amplifiers (particularly 2 stage high gain signal boosters used in extreme fringe reception areas) can get overloaded by a nearby Tetra mast which operate in the frequency range 380MHz to 420MHz. If you are experiencing Tetra interference problems, buy the 6 section HPF6 high pass filter (below) or a Tetra UHF notch filter from Garex. These types of TVI filter have noticeable insertion loss, so if your TV reception is poor, you may notice the degradation in analogue TV picture quality and Freeview reception problems such as picture drop out may result if the DVB-T signal was already marginal.

Where TVI is experienced from Tetra masts, Ofcom state that most Tetra TV interference problems are caused by aerial signal boosters. There are a variety of filters available to solve Tetra interference from police base stations and similar services including a TV bandpass filter which covers the entire TV band (470MHz - 869MHz) and a few different notch filter designs including a double notch filter where two interferening signals are present on different frequencies. Good advice on choosing the best filter and curing Tetra TVI can be found on http://www.wrightsaerials.tv/Tetra.htm.

Important

If you use a TV signal booster, any TVI filter must be fitted between the aerial and the booster.

Garex HPFS - High Pass TVI Filter

		The HPFS is a high pass TVI filter for reducing interference from both HF (short wave) and VHF PMR radio transmitters. These TVI filters are available from Maplin.co.uk (stock code QT25C - unsuitable for Tetra TVI)
Garex 6 Section High Pass TVI Filter
		The HPF6 filter is highly effective at reducing HF, VHF and UHF aerial borne interference below 450MHz with a particularly sharp cut off making it ideal for solving Tetra interference. It is available from Garex. Garex also supply a range of notch filters to help eliminate interference from amateur radio frequencies.
Build a TV Notch Filter for 2m
		Notch filters are effective at reducing TV interference from a particular frequency band. For example, where TVI problems are experienced from the 2 metre amateur radio band, a 2m VHF notch filter design can prove effective. It is possible to build a tuneable notch filter by using a simple series tuned circuit or by connecting an open circuit quarter wave coaxial cable stub in parallel with your TV feeder and cutting it to length to minimise the interfering signal.
I made the tuneable 2m amateur radio notch filter pictured above using an old 35mm film container. The internal design is shown here and the circuit diagram here. It is highly effective at solving TV interference from 2 metre amateur radio transmissions. In addition, you can read another useful article on building low loss band pass TVI filters by clicking here.

Sky TV Reception Problems

Read our new Sky TV troubleshooting guide for comprehensive help and tips to solve Sky Digital picture break up and pixilation issues. For more advice on improving terrestrial digital TV reception, read our Freeview troubleshooting guide.

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