What was it?

The answer to yesterdays question!

I didn't expect anyone to get this this - completely unfair of me I know…but, it was good fun.

Yesterday's blip is the far end of your tv before its transmitted. It is the Radio Frequency output of a digital tv transmitter. Its where all your terrestrial digital tv services originate.
The pipes are actually coaxial cables just like the aerial input of your tv - only much much bigger. These are a couple of inches in diameter.
At this stage, the signal is high power (on this kind of transmitter, potentially 1-2kW. On others, as much as 18kW (our highest isn't quite that high though). With that kind of power from anything electrical comes a massive amount of heat hence the substantial fans and ventilation system at the top.

That is an awful lot of RF - By the time it is radiated from one of the antennas (the white cylinders or square panels you see on masts and towers) the effective power can be increased dramatically because of the focussing effect of the antenna itself (There's no point in radiating the signal up into the sky - there's nobody there to receive it).

So, how do you get your tv channels?
Our tv transmission is managed as bundles of individual tv & radio channels - Digital tv has allowed individual analogue channels to be replaced by digital services each carrying far more channels.

There are two types (from a operating license perspective). There licenses are called Public Service Broadcasting or PSB. This licence obligates the operator to cover the most of the UK. The BBC operates two systems one of which provides HD channels and the third is done for ITV, Ch4 etc. There are over a thousand tv transmitters but many are much smaller than this.

The other kind of licence are the commercial licences. These COM licences have lower service requirements and cover only about 80% of the UK. Currently 4 of these bundles operate with a 5th coming shortly - do you need more tv?
There are just over 80 large stations transmitting these services - you may not be able to get these.

Finally, local tv started earlier this year. These systems cover specific local geography - you may have one, you may not (even if you are close to a large transmitting station) but are also to contain national channels.

Transmitters like yesterdays blip transmit some of the more recent new COM services which contains HD channels from the BBC and other operators, but, itself, isn't operated by the BBC themselves.

When you scan your tv for channels, it is searching for these groups of services - that's why when it stops, the number of channels jumps by a few or even lots.

This particular system blipped yesterday is actually a training system just 20m from my desk. It doesn't do any real work, but is a fully functioning system as if it were.
Before the signal gets to this high power stage, there is an awful lots of clever electronics and distribution. After it, the outputs of the individual transmitters are combined - This bit is a wonder of engineering in its own right. Then it goes up the mast to the antenna. It contains up to 7 separate transmitters at this stage (yup - upwards of 100kW). Now, if that were a microwave oven….tea in two seconds ash in three :-)

The big systems on the really tall towers have feeder cables which are 8-10" diameter. By the time the signal gets from the transmitters at the bottom to the antennas at the top, a great chuck of power has already been lost. These feeders can hum due to the power being carried.

So, that's how you get your tv. All very complicated stuff put together by lots of engineers with brains the size of planets - just so you can watch Eastenders, Corrie and MOTD!
Fascinating….