The status of DTT in the UK has been in the spotlight for many months. Following the launch of digital terrestrial free to air receivers, DTT installations are still very much a part of every rigger’s workload. Quality installations are in demand now more than ever, so what aerial should you be using to ensure you deliver the digital signal reception your customers expect. Jeremy Kennedy, Technical Director at Solutions Group, explains what you should be installing and why.

In September 1999 the Government announced the criteria for digital television before analogue terrestrial television broadcasts could be switched off:

Availability
Everyone who currently receives free-to-view analogue TV channels (BBC 1 and 2, ITV, Channel 4/S4C and Channel 5) must be able to receive those channels digitally (target: The current 99.4% coverage for analogue terrestrial channels other than Channel 5)

Affordability
Switching to digital must be an affordable option for the vast majority of people (target: 95% of consumers have access to digital equipment). It remains the government's case that this could start to happen as early as 2006 and be completed by 2010.

Each analogue channel of 8MHz can provide at least 6 (and possibly 7) DTT channels of a quality similar to analogue. This means the present UHF TV frequency range has the capacity to provide many more TV Channels, or other TV received services or non-TV services.

The government is currently reviewing the alternative uses for this frequency band and is expected to conclude on its preferred position during this year. If at that time, the preferred position is to allocate some of the range to non-TV services (such as mobile telephone), then international agreement will have to be reached before re-allocation can be made which is unlikely to be reached before 2005 at the earliest.

So what should we be installing today to meet our customers' requirements throughout this changeover period?

Before considering the characteristics that are more critical to digital signal reception, here is a reminder of the important specifications of any terrestrial TV aerial and why they are important.

Forward Gain
This is a measurement of how much the off-air signal is amplified by the aerial when the aerial is pointed directly towards the transmitter. If the received C/N (Carrier to Noise) and /or BER (Bit Error Ratio) is insufficient, no amplifier will correct this situation. However it should be noted that masthead amplifiers do have a use but only when the received signal from the antenna is adequate, otherwise only a re-sited aerial or one with a higher gain will fix the problem.

Two units (types) of measurement are used; one uses a half wave dipole known as dBd as the reference and the other, dBi is relative to an isotropic emitter.

A manufacturer that quotes its aerial gain relative to dBi will show a higher gain figure than dBd, and this is why some manufacturers use this measurement as a marketing tool to present 'better' numbers!

Flatness
There are two measurements for this; the most simple is to measure the gain across the claimed operating range of the antenna, the flatness can then be specified to be for example 10dB ±2dB and this is what is shown on most manufacturer's performance graphs.

The other kind of flatness is measured within a single channel and is often referred to as ripple. The antenna should have no more than 3dB variation across its operating frequency band as anything much greater than this is likely to cause problems.

Trying to maintain all of these channels within the operating window can become a challenge, particularly when you have the combination of large differences in transmission power and TV channel frequencies.

For in channel ripple - the lower the better. Poor RLR (Return Loss Ratio) has a significant influence upon this (See Impedance Matching).

Frequency Range
The UK UHF terrestrial television coverage has historically been broken up geographically into different limited frequency bands for both economic and interference reasons. By having adjacent areas operating in different frequency bands, interference from adjacent transmitters is minimised and the costs at both the transmitter and receiver are reduced.

For example, if we compare the gain specifications of Yagi aerials of the same type ofconstruction and the same number of elements (and therefore similar cost), we find a Group 'A' aerial to be 2-4dBs higher than a 'WB' (Wideband) aerial. The difference reduces to around 1-2dBs for Group 'B' and only 0.5dB for Group 'CD'. (This is why some aerial manufacturers do not make both 'CD' and 'WB' aerials, as the difference in gain is small).

Front to Back Ratio
This is the difference (quoted as a ratio) in gain for signals reaching the aerial from the front compared to the rear and is quoted in dB. It is important in reducing the interference that could be caused by any signal reaching the aerial from the rear and is particularly relevant where buildings or other structures behind the aerial can reflect the wanted signal. It is affected by the design and size of the reflector.

Beamwidth
This is a measure of the directivity of an aerial and is used to define the reception arc into the front of it. If an aerial is peaked on to a signal and is then swung to the left until the received signal drops by half (3dB) and repeated to the right, this total swing in degrees is known as the Beamwidth.Generally the more forward elements, and thus higher gain, the narrower the Beamwidth.

Side Lobes
For TV reception, the ideal aerial would have good forward gain, with no gain in any other direction. Unfortunately this is not achieved with the aerials that we use! So we would say the lower the gain, other than in the forward direction, the better to minimise interference from unwanted signals.

Remember that 'Side Lobes' exist 360 degrees in both horizontal and vertical planes around the aerial and can therefore potentially pick up interference from almost any direction.

Impedance Matching
The TV industry has standardised upon 75ohm impedance so televisions have 75ohm input impedance and we use 75ohm coax cable. However, most antenna dipoles are not 75ohm.
Whenever there is a difference in impedance in a transmission line interface, a mismatch (reflection) occurs. This is referred to as either VSWR (Voltage Standing Wave Ratio) or RLR (return loss ratio); the greater the mismatch the more signal that is reflected at the dipole/cable interface and the greater the chance of interference.

There are several methods of reducing this mismatch; the most commonly used for Yagi antennae is an impedance matching transformer (BALUN meaning BALanced/UNbalanced) while another method is to use a matched dipole such as is used on the Antiference TC and RX antenna ranges.

Some antennae, such as Log Periodic do not require a BALUN for impedance matching as they are inherently near the required 75ohm impedance. In addition to reducing signal reflection, a BALUN can reduce an aerial's susceptibility to Noise interference.

A BALUN reduces the inherent mismatch (resulting in signal loss) between the dipole and the 75ohm feed cable.

Traditionally, the connection to the aerial has been with a 'saddle and clamp' but aerials are now available with 'F' connectors. The advantage of these aerials is that the connection
to the aerial is consistent and repeatable, whereas the 'saddle and clamp' arrangement is dependent on how far the clamp has been tightened.

Mechanical Considerations
Any quality antenna should provide years of quality viewing for the customer, which is achieved by good design and installation techniques.

Note that if a cradle support is used the performance of the antenna will be degraded. Some antennae are supplied with 'tilting mechanisms' on the basis that in some circumstances tilting the antenna up may reduce ground noise to a greater extent than it reduces forward gain, thus improving C/N Ratio. The wider the Beamwidth of an antenna is, then the more likely that tilting will be required.

All the above are important to any aerial, whether for analogue or digital signal reception; so what makes an aerial suitable for digital signal reception?

At present there is no agreed standard for a 'digital aerial' although a committee is working on this and we will advise when such a standard is available. In the meantime we need to consider the above specifications and the probable future of DTT. Particularly as we will see a significant number of new STB's released some with dual tuners others with diversity tuners requiring two aerials connections!

Analogue
If at the time of an analogue installation the reception is acceptable, and over time the received signal varies due to atmospheric conditions or obstructions for example and the sources of interference also vary, then in the vast majority of cases the picture quality may deteriorate but the picture will not disappear.

Digital
In the case of DTT (Digital Terrestrial Television), the picture quality does not deteriorate, as is the case with analogue, but will in adverse conditions 'freeze' or disappear altogether.

Up until 2006, it is probable that current DTT services will continue to be transmitted in the present 'bands', but new services (such as pay TV) may use different bands and after 2006 there may well be a re-allocation that would make the present 'band' structure obsolete. Therefore, to provide the best probability for a 'future proof' DTT installation, you should install a WB aerial with sufficient gain and a good match (low RLR) to the feed cable.

In addition you should use a high quality approved cable such as H109F, CT100 and PH100 to minimise interference entering the distribution system after the aerial and to ensure any electronics (such as mast head or distribution amplifiers) are well matched to the cable. Units with 'F' connectors 'in' and 'out' are our preferred solution.