earn valley aerials digital tv & radio reception solutions
Earn Valley Aerials, Perth, firstname.lastname@example.org (Please use contact page) Copyright © Earn Valley Aerials 2016
site last updated 18th Aug. 2017
(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 -
TETRA mast at Court Farm, Dyke Road, Brighton and Hove, combined with a 3G mast (right). Courtesy of www.tetrawatch.net
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-
· a masthead amplifier is in use, especially a two-
· 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 -
TETRA transmissions are, of course, properly engineered, and the out-
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-
If TETRA or other signals below the UHF TV band are a problem, it's better to use a UHF-
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-
Having sorted out the masthead amplifier, if there was one, the next thought when faced with strong out-
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-
Click here to read part two
This excellent article was researched & written by Bill Wright of wrightsaerials.co.uk
Earn Valley Aerials, Glenturret Place, Perth, PH1 3FP. email@example.com (please use contact page) Copyright © Earn Valley Aerials 2016
site last updated 19th October 2016