The Final Countdown

Written on 19/10/2016
Toby Mills

Popoiti Broadcast Tower in Gladstone

This is the last of our behind the scenes posts on what it takes to setup a new local TV channel.

This week we cover the complex process of the actual broadcast itself so that you can pickup the signal with your television set.

Our signal is broadcast with a very narrow beam from Masterton, 23km up to Popoiti Hill where our dish picks up the signal and delivers it into the Freeview network.

The view from the tower  is spectacular. You can see all the way to the ocean to Masterton.

The challenge with Popoiti Hill is that the wind there is insanely strong, winds of up to 200km are common and equipment on the tower is constantly being damaged by strong winds.

The signal we are transmitting consists of an UDP broadcast over a standard network link. When it reaches the tower, the signal is combined with several other channels into a MUX which is basically a bundle of channels on one frequency (this is why we can have way more channels than we used to when the signal was analog).

The MUX is then amplified by a broadcast amplifier to around 200 watts.
There are multiple amplifiers amplifying multiple MUX's in the broadcast tower. The amplified signal is then connected to a special combiner which combines the amplified signals so that it travels down only two extremely heavy duty antenna cables. These cables connect to the antenna arrays on the tower so that many channels can be broadcast from a single tower / antenna array.

Each one of these units on the right is a MUX amplifier which amplifies 6 to 8 TV channels each.
The signal from TVNZ is coming in on the green cables at the bottom and ours is in there somewhere.

The antennas on the tower are aligned in a special way so that the signal from the two feeds combines and constructively amplifies the signal far greater than it would if it was just 200watts.
This clever trick also concentrates the signal so that it goes in a very vertically narrow but wide beam circle that looks a bit like a dougnut. This means that power is not wasted transmitting the signal up into the air and down into the ground and that it goes out evenly across the land where people actually live. If you look closely at the pictures on the tower you can see how the alignment of the antennas at the very top of the tower achieves this.

You can see the special curve of the antennas at the top that makes sure the signal goes out in a narrow doughnut shape.

In your lounge, your Freeview decoder tunes into a MUX on a specific frequency, then you can select and view a channel in that MUX and your decoder ignores all the data from the other channels in the MUX that you are not watching. This is why sometimes when you change channel it takes a little longer, because the channel may be in a different MUX and your decoder needs to tune into the new MUX and pull out the channel information within it.