TV interference from TETRA

TETRA (TErrestrial Trunked RAdio) is an international standard for digital mobile communications. TETRA systems are gradually being adopted in the UK by police forces and other emergency services. TETRA bears some resemblance to the digital mobile phone networks, but generally the cells are larger and the transmission powers higher.

The introduction of any new transmission system is likely to cause some degree of interference to existing services. The problems caused to TV reception by TETRA, however, seem to be much more widespread than anyone anticipated. There are deep political issues involved with the implementation of TETRA, and in many districts where a TETRA mast has appeared there is local feeling that the Home Office, having committed billions to the scheme, is riding roughshod over TV interference problems. I don't want to go into that here, so I will simply outline the technical problem and suggest remedies.

The frequencies allocated in the UK for police and fire service TETRA use are 380 - 385 MHz (mobile) and 390 - 395 MHz (fixed). The company operating TETRA, O2 Airwave, is building a network using these frequencies, and the process should be more or less complete by next year.

TETRA and TVI

There is nothing particularly pernicious about these transmissions. TETRA interference has become newsworthy simply because the transmitters are new, quite high powered, and are often in residential areas. When dealing with TETRA interference most of the normal TVI (television interference) remedies will apply. Remember though, that because the interfering signals are digital, some familiar TVI symptoms such as audio breakthrough will not be present. Since the base stations transmit continuously the interference does not stop and start, like for instance TVI from a taxi base station. Apart from these points, my remarks here about 'TETRA' interference apply equally to any strong interfering transmissions, especially to those between 300 and 450MHz. TETRA interference problems are most likely to arise when one or more of the following apply:

• the TETRA mast is within a few km
• the TV aerial is vertically polarised
• the TV aerial is Group A (chs 21- 35) or wideband
• a masthead amplifier is in use, especially a two-stage one
• TV field strength is poor.

Since TETRA transmit aerials are flat panels designed to radiate most power towards the target area, very close areas in the shadow of the mast might have rather less field strength than might be expected. At the same time, areas on high ground two or three km from the mast (on the other side of the valley for instance) can be pretty well swamped with signal.Where the TETRA field strength is relatively low, interference problems are uncommon unless a masthead or distribution amplifier is in use. However, at locations with very high field strength, interference is quite common even where TV field strengths are good and the aerial is connected directly to the TV set with no signal amplification. If the interfering field strength is very high the unwanted signal can enter either via the aerial or directly into the TV set or VCR. This is really an EMC (electromagnetic compatibility) shortcoming on the part of the TV set or VCR. The symptoms can be very peculiar, and might not look like TVI. More or less anything is possible, although a common symptom is faint patterning, even when the TV set is working from a scart input. I'm generalising from a small sample here, but the very cheap 'supermarket specials' seem to be most susceptible.

If you have a set that works perfectly on your bench but persistently misbehaves in the customer's house, have a look round the immediate neighbourhood for a transmission mast. Since TETRA often shares a site with cellphone and other transmissions the mast will probably not be new. Look for new panel arrays. These will often be mounted below the cellphone panels on arms that position them away from the mast. Some TETRA sites are on flats, so TV sets on the higher floors of adjacent high rise blocks may be in a very strong interfering field (as are the occupiers - and there are contentious health issues about this). TETRA transmissions are, of course, properly engineered, and the out-of-band radiation should be minute. Nevertheless, the second harmonic does coincide with UHF TV channel 60, so if you encounter strange problems on that channel TETRA could be the culprit.

Masthead amplifiers

Normally it's the fundamental frequency that causes the trouble, because 400MHz is well within the passband of most aerial amplifiers, and the vast majority of problems arise when some sort of aerial amplifier is in use. If you think about it, the use of a masthead amplifier means that the TV signals are likely to be on the low side, so a fairly close TETRA transmitter might produce field strength 30 or 40 dB higher than the TV signals. Even allowing for the fact that the aerial is pointing at the TV transmitter rather than the TETRA mast and isn't designed for 400MHz, it's quite likely that the masthead amplifier will be presented with enough input signal to cause overload. In its resulting non-linear state the amplifier will add interference to TV channels that have no obvious relationship to the interfering frequency.

The effects will vary. DTT (Digital Terrestrial Television) will often withstand this interference surprisingly well, and will remain unmoved when analogue channels are quite badly affected. But beyond the point where the carrier to noise ratio falls off the edge of the 'digital cliff', reception will be impossible. Because the TETRA signal is digital, the familiar telltale signs of analogue 'breakthrough' are not present on analogue TV reception. The word 'breakthrough' implies that the modulation of the interfering carrier is visible or audible in recognisable form at the TV set, and this doesn't happen with TETRA. There are no disembodied voices whispering 'You go round the back while I watch the front'-or suchlike police chat-coming from the customer's TV set. Interference to analogue reception can be mistaken for that caused by an oscillating masthead amplifier, but a closer examination of the screen can often suggest the real cause of the trouble. The appearance of the patterning varies a great deal depending on the relative strength of the interference, but fig 1 is fairly typical.

The distinctive white horizontal bars, each made up of diagonal stripes, have given rise to the description 'tyre marks'. This effect is presumably the result of the pulsed nature of the TETRA signal. There may well be cross-modulation, with one TV channel appearing faintly behind another. In a really severe case TETRA can completely wipe out the TV signal. The first time I encountered TETRA interference the symptoms were much less dramatic though. There was slight cross-modulation and some rather vague patterning on the screen. I wondered at first if the masthead amplifier had a weird fault. I connected the aerial lead to the spectrum analyser and saw, just as I expected, a huge spike some way below the UHF TV band. Thinking the amplifier was oscillating I unplugged its power supply unit as a double check. Of course, this should kill the oscillation completely. To my surprise the spike, although greatly reduced, did not disappear! Using a small screwdriver as an aerial the spike was very tall. I climbed onto the roof and soon found that the signal, on 392MHz, came from the direction of an innocuous looking cellphone mast about 3km away.

Two-stage masthead amplifiers are much more likely to be affected than single stage ones. Two-stage amps have a gain figure of about 25dB, and single stage ones about 14dB. Quite often the extra gain of a two-stage amplifier is unnecessary, and the only thing you need do to get rid of the interference is replace it with a single stage one. The rule of thumb that I use for assessing whether a two-stage masthead amplifier is necessary is simple. Assuming that downlead losses are less than 3dB, I would only consider the use of a two-stage amp if the weakest analogue channels were below -8dBmV (52dBmV) at the aerial terminals. If TETRA or other signals below the UHF TV band are a problem, it's better to use a UHF-only masthead amplifier. These incorporate a highpass filter that can give a good degree of protection against 400MHz interference.It's also important to use a fully screened masthead amplifier. Most if not all of the major manufacturers now have fully screened products available. Some of the more 'vintage' masthead amps were poor performers by modern standards, and were very susceptible to strong out-of-band or in-band interference. Maximum output levels were often rather low, meaning that the cross-modulation threshold was easily exceeded by unwanted transmissions.

Filters

Having sorted out the masthead amplifier, if there was one, the next thought when faced with strong out-of-band interference will always be 'filters!' Fortunately there are lots of products available. Four basic types of filter are relevant.

A bandpass filter will pass only the required frequency range. A high pass filter will reject everything below its cut off frequency. A notch filter will reject a spot frequency. A channelpass filter will pass only the exact channels required. There are, of course, pros and cons to each alternative, but whatever type of filter is used it must normally be inserted before the first amplifier in the system. All filters lose a little bit of signal, and with the filter in front of the masthead amplifier the carrier to noise ratio will inevitably be degraded, if only slightly. This is unavoidable, but in marginal reception areas it's a good reason for not fitting a filter if it can be avoided. In a difficult case where it is felt that the slight loss in the filter is unacceptable before amplification, it might be possible to use a low gain masthead amplifier before the filter. Further amplification to overcome cable or distribution losses can follow the filter.In many cases a bandpass filter will do the trick, with no further complications. These items are also called 'group pass filters'. Since the filter will be before the masthead amplifier it will usually be outdoors, so it must be built into a weatherproof housing. Fringe and Maxview amongst others have these filters. The Maxview MHF range is fully screened and has 'f' connectors. It is available for channels 21-68, 21-37, 35-53, 48-68, 35-68, and 21-42.

To reject TETRA on 400MHz use the filter designed for the highest group of channels possible. Through loss on the wanted channels is less than 1dB, and the rejection at 400MHz is better than 25dB.Highpass filters are most familiar to us as in-line 'CB' filters, examples being the Antiference TVI and TVI-U. Generally the cut off frequency of these filters is not high enough to reject TETRA signals.Notch filters come into their own when a distribution system carries VHF-FM and DAB radio as well as TV. A notch filter tuned to the exact TETRA frequency should be fitted in line on each aerial feed before the amplifiers. Aerials for DAB can receive a lot of TETRA signal, but fortunately a double notch filter can have very high rejection - as much as 40dB is possible. Accurate tuning is difficult without a spectrum analyser. Taylor Bros (Oldham) can supply notch filters for any frequency. Alternatively, bandpass filters for VHF-FM are available from Taylor Bros, (type TBP2) and for DAB (type TBP3). These are indoor units and are intended to be fitted just in front of the distribution amplifier.

In the unlikely event that a masthead amplifier is used for VHF FM or DAB, the filter can be fitted into a standard ABS electrical junction box. A 120 X 180 X 50mm box with a rubber seal on the lid is ideal.It is unlikely that you would have to resort to channelpass filters on a simple domestic installation. On the other hand, the majority of distribution systems would benefit from having such a filter on the aerial input, even where out-of-band interference isn't a problem. But if the aerial receives TETRA or other unwanted signal at any significant strength then a channel pass filter/leveller is a must. By 'significant strength' I mean about 6dB lower than the digital TV multiplexes. Since TETRA will often appear at the aerial terminals at higher strength than even the analogue TV channels it is very likely in TETRA coverage areas that a distribution system will need a channel pass filter/leveller. We now take this for granted, the only exceptions being small systems of 20 outlets or less in places where the TV signals are very strong at around 20dBmV or more. Filters for five analogue channels and six digital multiplexes can cost as much as £150, and believe me we wouldn't use them if there were a cheaper alternative that worked as well.

Aerial work

Don't start adding filters and changing masthead amplifiers without giving the aerial a cursory glance at the very least. A bit of aerial work can pay dividends here both technically and financially, the latter because customers seem to see more value in shiny things on the chimney than in boxes in the loft or your time.

The fundamental principle is to improve the ratio between the wanted and unwanted transmissions as much as possible. Firstly, I'll state the obvious. The aerial should be pointing accurately at the TV transmitter, and it should be sited as advantageously as possible. If raising the aerial or moving it to the other chimney gives it better line-of-sight to the TV transmitter without improving the view towards the TETRA mast, then it's worth doing the work. Apart from alleviating the TETRA problem, there will be a general improvement in reception. Don't waste your time putting the aerial off beam from the transmitter in the hope of 'nulling out' the interference. It doesn't work.TV aerials designed to work right down to the bottom of the UHF TV band (470MHz) seem to work surprisingly well on 400MHz, even exhibiting some directional properties and gain! If possible avoid the use of wideband aerials. These are often installed unnecessarily these days, thanks largely to the widespread misinformation that DTT always needs one. If all the analogue and digital TV signals are in Group B or C/D, the low frequency response of a wideband aerial is merely contributing troublesome interference such as TETRA. The performance deficit of wideband-compared to grouped-aerials can also make a difference.

As a desperate last resort, it might be possible to improve the signal to interference ratio by re-locating the TV aerial to a spot screened from the TETRA mast. Unfortunately 400MHz signals aren't attenuated by buildings and trees quite as much as UHF TV signals, so this might not be all that effective.
Aerials for VHF FM should be fixed with horizontal polarisation, assuming that the transmissions include a horizontal component, as most do. Unfortunately DAB aerials have to be fixed vertically.

In conclusion

Over the years the electromagnetic spectrum has become ever more crowded. During my time as an aerial installer I have seen all sorts of innovations that that can cause problems for UHF TV reception. In the late 70s and early 80s there was the CB craze. Then we had the illegal high-powered cordless phones. Meanwhile, many of the CB fanatics migrated to amateur radio and started chucking out lots of watts on 144MHz and 432MHz, usually in the middle of crowded housing estates. Then illegal video senders became popular (I could write a book about the trouble they caused). Low powered RSL radio stations have sprung up all over in recent years, cellphone masts adorn every bit of high ground, and we now have five analogue channels and six digital multiplexes competing for space in the UHF TV band, with the old channel spacing rules thrown to the wind. No wonder we have interference problems. We must remember though, that the majority of these problems are not caused at the interfering transmitter, they are the result of poor EMC (electromagnetic compatibility) at the receiving end. Poor amplifier design, the wrong type of amplifier, cheap co-ax cable, poor TV set design, and an inadequate aerial installation can all open the door to interference.

Over the years, ensuring good EMC performance has become an important part of the installer's work. TETRA is no more than the latest challenge. I'm sure it won't be the last

Bill Wright

Please mail your comments about this article to wrightsaerials@f2s.com