Digital Antennas

John Parncutt has been involved in Television and Radio Electronics for more than 45 years, in fact his first antenna installations were for short wave radio reception at the age of 14! Many of our customers have found themselves in a pickle since the introduction of digital TV transmission and he has helped sort out their problems. This article has been written by John to give simple advice on what to do if you want perfect pictures.

The introduction of digital television throughout Australia has solved a lot of reception issues for users but at the same time introduced some new challenges for antenna system installers. In the pre digital (analogue) era most installers used a simple signal level meter to adjust the orientation of the antenna and setup the gain distribution of various amplifiers and splitters in the more complex MATV (Master antenna TV) systems used in blocks of flats etc. In analogue TV systems the most important parameter is signal level which typically needs to be 60dB u/V to achieve a noise free picture. Another common issue affecting picture quality is that of ghosting which is the result of reflected signals reaching the antenna slightly later than the main signal. Typically viewers in more difficult reception areas often had to put up with snowy and ghosted images on some or all of the channels. Mast head amplifiers were often used to overcome these problems with varying results.

With the inception of Digital TV in Australia we now have a far more efficient service not only able to offer far more channels within the same band space but a system capable of offering extraordinary high quality images. Unlike the old analogue system with digital you generally get either a perfect picture or no picture at all! In reality this statement requires a little more explanation. You may have heard the expression “The digital cliff” what on earth does that mean? When a digital signal is transmitted it can be effected by atmospheric disturbances, electrical noise etc. This can result in some of the transmitted information becoming garbled or lost and this is where the magic of error correction comes into play.

Your set top box/inbuilt digital tuner has some extremely clever error correction software built in to it (in fact two levels, Viterbi and Reed-Solomon, named after their authors). Without this error correction Digital TV would not be practical. There is a point however where the loss of information becomes too great for the error correction to handle and the picture starts to break up (or reach the so called “cliff”.) It is the task of the antenna installer to ensure that the signal quality or so called “Bit Error Rate” (BER) stays well away from this cliff. Therefore the most important measurement we must make is the BER (prior to any error correction) and to do this requires specialised digital signal analysis equipment. Sometimes in more marginal reception area’s occasional picture breakup will occur due to spikes of interference caused by the switching of electrical equipment (Fridge motors, Fluorescent lighting, etc.) Ie. The interference momentarily forces the signal over the cliff!

So How Do We Increase Signal Quality? 

A great deal of the old analogue installations utilised components which are not suitable for digital reception. These components such as plastic cased splitters and old style TV coax are not properly screened against interference and should be replaced with modern high quality parts. In the major Australian cities the analogue antennas were designed to receive channels 2,7,9,10,28 & 31. The digital band plan replaces these channels with 6, 8, 11, 12 and 29 (the future of 31 is yet to be decided). The old antennas were not designed to receive on channels 6, 11 & 12 and therefore suffer reduced performance. You may get away with this in a strong reception area, but it is wise to ensure that the analogue antenna is replaced with a digital antenna.

The requirements for a professional and effective installation are:

  • A professional appraisal with the appropriate diagnostic equipment at the location with regard to television reception.
  • An antenna mounted outside the roof line with a clear view to the respective transmitter (particularly for UHF).
  • The appropriate antenna for the area.
  • Low noise and well shielded cable matched with the appropriate F connectors, splitters and filters.
  • Matched equipment.
  • Balancing of signal strength inputs.
  • A guarantee.

It’s not surprising that when the home owner attempts to install an antenna themselves, the results are generally less than optimal. When selecting your antenna installation professional, remember to ask about his training and background. Good antenna installers have been properly taught and trained as professional domestic television antenna installers. They don’t need to advertise as enough work comes from stores like Audio Trends and referrals.

Conclusion: Often problems can be attributed to either price or simply lack of experience. With regard to price, never choose the lowest price and always ask why a price is higher than another. Good materials cost more! Your guarantee before you contract any work should be one that guarantees, not OK pictures but rather perfect pictures and in multiple rooms if required.

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