earn valley aerials   digital tv & radio reception solutions


             Earn Valley Aerials,  Perth,  sales@earnvalleyaerials.com  (Please use contact page)   Copyright © Earn Valley Aerials 2016

                                                                                site last updated 18th Aug. 2017

Share on Facebook Share on Twitter Share via e-mail Share on LinkedIn Share on Google Bookmarks

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

                                                                                                                                                                                        Tetra mast, Worthing

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!

Tetra interference continued…