- Improving mobile television reception

While television reception at sea is generally free of the picture rolling, fading and ghosting associated with signal weakness due to path obstructions and multi-path reflections on land, it is not totally free of picture deterioration. The most significant factor that affects reception is the reversal of the polarisation, which occurs consistently at UHF frequencies.

Receiving only part of the signal, the picture quality suffers from fading and fading in and out of colour and sound distortions where the received signal is weak. Breaks in transmission can also occur where there is not enough information received to display an image. Moreover, as the frequency of the channel increases, these problems increase in magnitude and picture quality deteriorates correspondingly.

Having the ability to receive both vertically and horizontally polarised signals at UHF frequencies is therefore a very significant advantage - and not just to overcome signal deterioration from polarisation reversal.

Added to the problems of polarisation reversal, are those associated with the move from analogue to digital transmission, which more than compound the issue.

As has been evidenced from mobile phone usage, lack of signal in analogue systems leads to partial reception or fading. In digital systems, however, lack of signal strength usually results in the call dropping in and out, or out completely.

The same is true of digital television, where the affect is gaps in picture or picture and audio reception, or the inability to display a picture at all. All too often the promised increases in quality of reception do not eventuate. In reality, quite the reverse often occurs.

These changes in spectrum allocation and technological developments in television broadcasting, have made it necessary to design antenna systems specifically to overcome these problem.

Where it was once adequate to have a single element to cover the whole band from 40 to 860 MHz, this is no longer adequate to give reliable performance on the UHF bands. The solution is to provide separate VHF and multiple UHF arrays, enabling the antenna to receive both vertically and horizontally polarised signals. Arrays are coupled via a built-in diplexer. In this way it becomes possible to receive the whole of the signal and translate it into a quality picture.

When the VHF and UHF arrays are of a closed loop design, this helps to minimise reception of lower frequency (mainly HF) radio transmissions from other shipboard sources. Obviously amplification of the signal is necessary. However, it is not just amplification that is important but also where the amplifier is located. By placing the amplifier right at the antenna, only the signal is amplified. This eliminates cable losses and amplification of noise maximising the quality of reception to give excellent results across the bands from 40 to 860 MHz - all TV channels. Amplifiers placed at the end of the cable cannot do this.

Mobile antennas are usually subject to the effects of vibration, shock and corrosion. This can interfere with performance and over time cause damage, shortening the life of the unit. Protection needs to be provided for the elements by housing them within a radome which is then filled in an inert foam.

System losses that may have been acceptable previously, no longer can be tolerated. This includes strict attention to quality in choice of coaxial cable and connectors - especially in the marine environment where cable with both tinned inner and outer conductors and F type connectors are highly desirable.

Distribution systems that are currently available on the market, unless top of the line, are often quite limited in their capacity and rarely offer consistent performance throughout. Of those offered for use on ships at sea, many are merely land based systems trying to survive in a marine environment. At sea an isolated DC ground is essential to provide adequate protection against the possibility of electrolysis. The provision of isolators in the line protects the system and, more importantly, personnel from faulty TV receivers feeding 240 volts AC onto the cable.

In the ideal distribution system, there is uniformity of signal output with output balanced from station to station, no matter how far from where the signal is first received, providing the same quality for all from the closest to the most distant outlet.

If going digital and going up the bands have a significant impact on normal television reception, the affects on multiple distribution of the signal are even more important. The repercussions of any deterioration in signal reception or losses in the system will mean significant decreases in output across the board. O ring sealed F type connectors together with corrosion inhibited low loss coaxial cable will help minimise system losses.

Distribution systems are comprised of amplifier modules which receive, mix and distribute television and FM radio signals to the outlets and modulator modules which make it possible to distribute audio and video signals from VCRs, cameras, satellite receivers and FM tuners. A distribution network of cabling, directional couplers, splitter/combiners and line isolators carries the signal to the outlets. A main RF trunk line is used with taps and efficient directional couplers to supply the individual outlets.

With fixed systems, it is usually not possible to control the level of signal at the outlets so that they are uniform, as, obviously, cable lengths will often vary considerably between outlets. With custom design systems, however, this can be taken into account at the design stage, so that signal output levels can be predetermined and planned at the outlets and additional in line amplification provided wherever necessary.

Network calculations based on actual installation plans, are carried out to allow for progressive cable loss for outlets located further along the line. Appropriate adjustments are made at the outlet line taps to make sure the signal is evenly distributed.

A custom designed system can also accommodate individual requirements where a complex upfront distribution of media signals serves a small number of outlets, or where just the television signal needs to be distributed to a complex chain of outlets on multiple levels.

Mark II active omnidirectional TV Dome
TV/Multi Media Disribution System