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How To Get Into Digital TV, Pt.2

Last month, we introduced the topic of Digital TV and strongly advocated the purchase of a High-Definition Set-Top Box or Personal Video Recorder. In this second part, we discuss the antenna installation and what you need to do to ensure the most reliable and interference-free reception.

By Alan Hughes & Leo Simpson

Many people thinkthat there is no need to change an existing TV antenna in order to pick up digital TV broadcasts. In some cases, your old antenna may do the job.

In most cases though, you will be better off with a completely new antenna installation, with an antenna selected to suit the digital broadcasts for your area.

AUSTRALIA

AnalogDigital
Real ChannelsFrequency Range (MHz)Real ChannelsFrequency Range (MHz)
0 - 5A45 - 144Not used due to interferance
6 - 11174 - 2226 - 9A, 10 - 12174 - 230
28 - 69526 - 82027 - 69519 - 820

NEW ZEALAND

AnalogDigital
Real ChannelsFrequency Range (MHz)Real ChannelsFrequency Range (MHz)
1 - 344 - 68Not used due to interferance
4 - 12174 - 23027 - 34518 - 590
28 -34, 38 - 62526 - 83838 - 62606 - 838

Table 1: comparison between the existing analog channels and their digital counterparts for both Australia and New Zealand. As you can see, in the digital bandplan the bottom end of the band is used in neither country.

First, let’s look at the situation where your existing antenna is a VHF Yagi, log periodic or other antenna type such as a phased array. Most likely, this will be sized to cover all the channels in the VHF band, including channel 0. In other words, it will be a large and ugly structure of aluminium tubing that the local bird life has enjoyed for years.

And while this may have done sterling service for the analog channels, the fact that it is large enough to cover channels 0-5 will be a distinct drawback when analog TV broadcasts cease at the end of 2009.

Why? Because the antenna will continue to pick up all manner of extraneous signals which can interfere with digital TV reception, including high-power AM & FM radio transmissions and interference from power lines. The simple fact is that digital TV broadcasts in the VHF band are only on channels 6-12.

So your large VHF array is still an antenna but now it also picks up signals that you don’t want!

VHF digital antennas
are smaller

A new VHF antenna for the digital channels will be much smaller (because it doesn’t have to get down to channels 0 and 2) and less obtrusive. Not only that, because it does not have to cover such a wide frequency range, it will more than likely have slightly more gain than your previous analog antenna.

Click for larger image
Fig.1: the digital cliff, represented by the blue line, shows how too little or too much signal results in a "no signal" message on your TV screen. For analog (the red line) there will always be something on the screen, even if it is only snow.

Perhaps even more importantly, there are some areas of Australia where digital television will only be transmitted on UHF. No VHF antenna will receive UHF properly, despite the anecdotes from "a mate who knows someone" who receives a marvellous UHF picture from the antenna they erected in 1956 . . .

Table 1 shows the wanted channels on the VHF and UHF bands for digital TV in both Australia and New Zealand.

We will discuss the UHF TV bands later in this article. For the moment, let us compare analog and digital signals and how they perform with varying signal strength.

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