TV Aerials

 

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Digital TV Aerial Options
As digital switchover gathers pace, and demands on aerials and installers increase, Digitech is extending its range of aerials with innovative and exciting solutions to reception problems. Switchover means more transmitters carrying more digital TV and radio at higher levels. This increases the importance of aerials that receive only the signals you want, whilst rejecting the signals you don’t.
A major focus for Digitech is log periodic technology, recognised for flat gain across the desired band, high directivity and superb impulse noise rejection. In other words, just what is needed for digital reception.  Digitech offers a number of models favoured by broadcasters and test houses alike that are sure to become favourites for aerial installers as well.
For those who prefer to use more conventional technology, we have yagi designs. In addition to market leading models, new additions push the boundaries of yagi aerials with more compact sizes delivered at more budget-friendly prices.


Yagi aerials
The most conventional aerial design.
The yagi has a single boom, a series of director elements and an isolated dipole to collect the signal.


High gain aerials
Essentially one yagi stacked beside another on a single boom holding chains of directors. Signals from each chain are added together by the full wave dipole and a PCB balun ensures matching to the cable. These designs are popular because they have more gain than a similar length yagi.


Log periodic aerials
A chain of dipoles on two parallel booms. Each element pair acts as a dipole at one frequency and a director at another. Each screens the next from the rear so no reflector is required. As open 75ohms dipoles, a balun is not needed. Log periodic aerials work better than yagi's at rejecting impulse noise and are more directive- making them ideal for digital TV reception.


Phased arrays
The phased array aerial design consists of two identical aerials working together. They are carefully positioned so the incoming signals are added together and matched in a mast mounted balun or combiner. This will easily out perform a yagi, high gain or single log periodic aerial of a similar boom length.


Radio aerials
Aerials designed to receive VHF/FM in band II or VHF/DAB in band III. Frequency modulation has been around for many years and remains popular, but super-clean and interferiance free digital audio broadcast radio represents the new digital era of national and local radio.

Signal wave lengths
A receiving aerial must intercept the electromagnetic wave radiated from a transmitter. The wave front creates a field around the aerial, which “induces” a current in the conductors forming the aerial. If the aerial can be made to resonate at the frequency being received, the capacitative and inducted components will cancel, leaving only the resistive component. The aerial is most efficient under these conditions, which will occur when the aerial is half the wavelength long with the cable connection at its centre.
Another way of looking at the situation is to consider what would happen if the aerial where to be one whole wavelength long. At any point in time, if one end was receiving a positive peak of waveform, the other end would be receiving the next positive as well and there would be zero current at the centre.
If the aerial is only half a wavelength long, one end would be at a positive peak whilst the other end would be at a negative peak and the current at the centre would be at a maximum – this implies that the resistance is at its lowest.


Aerial lengths
Since we have already calculated the wavelength of 300MHz is 1 metre, then the theoretical length of an aerial half a wavelength long to receive 300MHz will be 0.5 metres. In practice, it will only be about 95 percent of its theoretical value, because radio waves travel through a vacuum.
This type of aerial is called a half wave dipole and the coaxial cable linking it to the receiver is connected to the middle, one side being connected to the inner core and the other side to the outer screen of the cable.


Vertical and horizontal dipoles
A vertically mounted dipole will pick up signals in the horizontal plane equally from all directions, but it will not pick up anything from above and below it. Its pick-up characteristics are equivalent to the shape of a ring doughnut.


Ominidirectional and directional dipoles
As more elements are added, these effects become more pronounced. Most aerial designers utilise one parasitic element behind the dipole, called the reflector, and one or more director elements in front of the dipole. Reflections are typically 5% longer than the dipole and director elements are 5% shorter.

For best TV reception your TV aerial should be mounted outside and as high as possible, though you may also get good reception if it is inside, for instance in the loft, or even with a set-top aerial.

To get the best reception you need to make sure that your aerial is in good condition and pointing towards the best local transmitter. An outdoor aerial can deteriorate or be knocked or blown out of its correct position, and cables and connections from the aerial to the TV set can become corroded. If you live near the sea, corrosion can happen quite quickly.

Poor reception

If your aerial is broken or out of alignment or the connections are corroded, the signal reaching your TV set will not be strong enough. This will result in problems with the picture or sound, such as 'snow' (a faint or grainy picture) and jumbled words on Ceefax (teletext).

If you are experiencing such problems, first try to find out if there is another cause:

Check today's transmitter work to see if your local transmitter is affected by a fault or maintenance work.

If possible, check another TV set connected to a different aerial - for instance, at a neighbour's - to see if that has the same problem.

Make sure that your aerial lead is securely plugged into your set-top box or TV and check any other connections.

If there is still no improvement, you should have your aerial and its connections checked for faults.

Important: checking, repairing or installing an aerial

We suggest that any work on your aerial is carried out by a professional aerial installer.

Ghosting

Ghosting is a double image on the TV screen. Signals bounce off solid objects such as hills, tall buildings or cranes and reflect off shiny surfaces such as the sea. This can create a second, delayed signal and produce a ghost-like repetition of the main picture on the TV screen. Slightly changing the angle of your aerial, so that it is not pointing directly at the transmitter, may improve the picture. Alternatively, it may help to replace the aerial with a more directional one that focuses on the main signal and ignores other ones.

Other reception problems

Other problems, such as horizontal bars or white dots or flashes, are more likely to be the result of interference caused by electrical equipment, radio communications or the weather.

Communal aerials

If you share a communal aerial (for example, in a block of flats) and you are having reception problems, the aerial may be faulty. See if other residents using the same aerial have the same problem. If there appears to be a fault with the communal aerial, you will need to report it to whoever is responsible for the block - the council, landlord or property manager.

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