UNDERSTANDING YOUR BROADCASTING ENVIRONMENT

 

 Background:  Those interested in Low Power TV (LPTV) may need some guides on how to understand the broadcasting environment and what information to collect in order to make useful assessments about the required equipment.

 

 Basics of signal transmission:  This is a complex subject which is covered in more detail when we train people to evaluate sites and install equipment.  What follows are a few basic concepts in layman’s language.

• A TV signal / transmission decays (reduces in power) in a uniform way as it travels away from the transmitter antenna.  Such a loss is unavoidable and can be calculated, but will also depend on the terrain it covers.

• LPTV signals can usually reach any home you can see from the antenna.  Line-of-sight is a good rule of thumb to work with.  Anything you get behind the horizon is a bonus!

• We need to know what broadcasters are active in the reception area and what power they use.   Many national and commercial broadcasters use 1 kilowatt and more.   If the difference between our LPTV signal and that of other broadcasters is very great, our signal may be ineffective.   For example, where other broadcasters are using 1 kilowatt (1000 watts),  a 10 watt LPTV signal would not work well, but 100w would.

• What can a viewer do to improve the signal in the reception area?

2)     A mast head amplifier.  Adding modest amplification on a home antenna, will make significant difference to reception. 

The economics of signal transmission

It is commonly assumed that more power (watts) is always better!   This is not necessarily so.

Some African countries have been persuaded (probably by the salesmen) that they need at least 1 Kw to reach anywhere.   So when we come with 100w, we are not taken seriously.  Everybody talks in Kilowatts! 

But Low Power TV covers a special niche.   It is interesting to know that LPTV installers that cover small towns in rural areas very successfully use relay transmitters from 2 – 10 watts in power.

Let us consider a case study, comparing a 100w and a 400w transmitter, costing 53% more.

While the 400w unit gives 4 times the power of the 100w unit, what is really important (and significant) is that the real gain in power (usually measured in decibels - dB ) is only 6dB  (50dB and 56dB respectively) – this is a 12% increase in dB output.

The effect in the reception area needs to be considered.   Most of the viewers will be in the town, surrounding the antenna.   Some will be on the outer perimeter – say 10%.

The extra 6db has little impact in the town and will not matter, as it is not marginal.

However on the outer perimeter of the signal, the extra 6db can make the difference between reception and non-reception, where it is marginal.  As the population of viewers is usually more sparse on the perimeter, you may gain 10% more viewers.

Looking at the economics, you are paying 53% more to gain 10% more viewers, which is not a good investment.   It makes more sense to increase the number and distribution of LPTV transmitters.  Instead of installing 400w transmitters (in 2 towns), gaining 20% more viewers, you could install 100w transmitters in 3 towns, at the same cost, gaining 50% more viewers!  (In reality, a whole new town!)

The 100w units therefore are very cost-effective for their relatively low price and we should avoid installing 400w units, unless for very compelling reasons.

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