Difference between revisions of "Installing a TV aerial"
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Revision as of 02:41, 2 March 2009
There are many choices to make in selecting and installing a TV aerial. These choices can make the difference between lousy reception with no digital channels and near perfect reception.
Choose the first location in this list you can use. Each step down the list results in further loss of quality that can not be made up for in any way.
- Use a rooftop aerial whenever possible
- Loft aerial is 2nd best
- A medium gain indoor aerial mounted as high up as possible
- Set-top aerials are a last resort, and give a junk quality picture in most locations. For use when portability is necessary.
As an example of how much difference aerial height can make, here is an example of someone finding a 15-21dB difference between loft and roof mount.
Size & gain
- CAI 2 aerials are suitable for most locations
- Very high gain (CAI 1) aerials are good for fringe reception areas, but are only suitable for rooftop use. They don't work well in other locations.
- CAI 3 lower gain aerials are suitable for houses close to the transmitter
- Some aerials have less than honest manufacturer's specs.
Many aerials and related equipment lack features which, while unnecessary for analogue recption, are important for good quality digital reception. CAI approved equipment meets the necessary requirements.
Some aerials are available in group versions as well as wideband. Typically only the wideband version is CAI approved, due to costs, and the group versions are normally built to the same standards. These can thus be regarded as if CAI approved, even though they're not.
Non-approved aerials are ok for analogue reception, but are prone to causing problems with digital TV, which is more demanding in some respects.
CAI approved aerials are divided into categories 1,2 and 3.
- 1 is for very high gain aerials for difficult reception areas (eg Televes DAT75)
- 2 aerials are suitable for most properties
- 3 are lower gain for locations close to the transmitter
Many aerials currently being sold are wideband. Wideband means it will receive the full range of TV transmission frequencies used in the UK, making them a safe (but not necessarily optimal) choice. Traditionally all the TV transmissions from single transmitter were grouped together in a smaller subset of the available set of frequencies. This enabled the use of more efficient "grouped" aerials for reception. However with the changes being introduced to accommodate the switchover to digital transmission, it has not always been possible to stick rigidly to this plan. As a result there are some areas where a wideband aerial will either be required now (during the transition), or after switchover is complete, to receive all the channels.
There are two different types of wideband aerial that you will encounter. The commonly available Yagi design (this is the traditional TV aerial design) is available in a wide ranges of sizes (and hence gains) and qualities, and the log periodic. The log offers slightly lower gain that the higher gain yagi aerials but does perform very consistently across the full range of frequencies used for TV transmission in the UK. Hence if you need wideband and are not in a fringe reception area, it is often a good choice.
Make sure when choosing a wideband aerial to go for a CAI approved one.
Grouped aerials receive less than the full range of TV transmission frequencies, but perform significantly better within their more limited frequency range (particularly noticeable on the lower frequencies). Hence where wideband is not needed, performance can be improved, or a smaller and cheaper aerial used, by selecting a suitable aerial for the group.
Choose an aerial that is:
- the suitable band
- the same model type as the CAI approved wideband version
All the channels you need to receive can be determined by looking up the channels on which you:
A grouped aerial will need to receive all those frequencies.
Depending on the power of your selected TV transmitter, and how far you are from it (plus the effects of any obstructions etc) the strength of the signal available to you may vary considerably. This means that you need to select an aerial appropriate for the location. A strong signal will require a modest aerial with a relatively low "gain" figure. Poorer reception areas however may require an aerial with more gain. Fringe reception areas may require a high gain aerial and some active amplification to get a useable signal.
In its simplest sense, the gain of an aerial is a measure of how much signal it can deliver for a particular strength of transmission where you are. However gain is a complex subject when dealing with TV aerials, since although many catalogues will quote the gain as a single figure (say 12dB), this does not really tell you much, since the gain of any aerial will vary with the actual frequency you want to receive - and the figure quoted will usually be just the peak figure. In reality you need to see a detailed graph of the response of the aerial plotted against frequency to make a meaningful comparison of any two aerials. Its not uncommon to find that a big high gain wideband aerial will have a larger gain quoted than a smaller far less elaborate grouped aerial, and yet the grouped aerial will outperform it at the frequencies that are of interest to you.
Have a look at the excellent ATV site to see how real aerials compare.
Lower gain aerials tend to:
- Be cheaper
- Physically smaller and lighter - and hence produce less wind loading on their mounting masts and fixings
- Last longer
Higher gain aerials tend to:
- Be more expensive
- Are often more fragile
- Will require more substantial masts, brackets, and fixings etc
- Will have a narrower angle of acceptance - and hence require better alignment
Very high gain aerials (CAI 1) are used for problem locations, but very high gain aerials can be very large and have all the issues described above as well as being ugly! So ensure all other installation issues are solved before resorting to one of these.
TV signals are broadcast with either horizontal or vertical polarization. The usual yagi TV aerials should be positioned horizontally for horizontally polarized signals, and vertically for vertical signals. Check (online) the polarisation of the transmitter you're receiving, or just turn the aerial to see which gives better reception.
Nearly all TV aerials are described by the manufacturer as 75 ohm aerials, yet nearly all are really designed as 300 ohm aerials, with construction details reducing this to some degree, but not to 75 ohm. Connecting a 300 ohm aerial to 75? cable causes loss of gain, tendency to reflections, and some degree of degradation of signal quality.
Better aerials have a small balun inside that transforms the 300 ohm signal to a genuine 75 ohm feed. All CAI approved aerials have this.
Loft, Rooftop & Set-top
Small set-top aerials vary in performance from poor to useless in most areas. In strong signal locations one may be ok for portable TVs. The advantage is minimal cost, no installation & portability.
A rooftop gives the best vantage point for TV aerials for several reasons.
- Being higher up there's less to get in the path of the signal from the transmitter.
- There is no interaction with roofing materials.
- The aerial is further from sources of electrical interference in the house, improving S/N ratio
- TV aerials are affected to some degree by metalwork in the loft. Very high gain aerials are especially prone to this.
In practice roof mounting gives a much stronger and better quality signal than a loft aerial.
A loft aerial will be quite satisfactory in many cases, but not always. Rooftop is of course less convenient and more expense. Outdoor aerials, poles and cables have limited life, so the expense is repeated over time.
Manufacturer's specs aren't always honest, making comparison of manufacturer specs misleading. Also real life aerial performance varies widely with frequency, so a single dB figure is of limited use. Real aerial performance can be compared using a graph of frequency vs gain for the major brands of aerial. (Link needed to graphs.)
Loft aerial choice
Very high gain aerials (CAI 1) are designed specifically for rooftop mounting. If used in a loft they generally give poorer performance than a standard medium gain aerial.
TV aerials don't make good VHF FM aerials, but they're often much better than internal VHF antennas, telescopic rods, or the popular rabbit's ears. If you've decided not to spend the money on a separate VHF aerial, in some cases you can get a real improvement in FM reception by using a diplexer at the radio to split off any VHF received by the TV aerial and pass it to the radio.
A metal diplexer only costs 2 or 3 pounds. Avoid £1 unscreened plastic diplexers, these let interference into the aerial system, affecting digital reception.
Avoid using a splitter instead of a diplexer, as these cause an extra 3-4dB loss, often degrading reception.
Some TV aerials are designed to give VHF recption as well as UHF. These give significantly better VHF reception than the ones that aren't, though they're still far from perfect. A dedicated VHF aerial will always outperform a UHF aerial with VHF reception, but adds costs of course.
For more info see VHF aerial
To obtain long distance (DX) TV reception , the following methods can be used to boost reception range. They're not required for standard domestic setups.
Tower mounted aerials can achieve long distance line of sight reception, avoiding all obstacles. (Don't attach huge rods to chimney stacks, they will only cause a collapse.)
Phased aerial arrays combine the outputs of multiple aerials for greater output without the addition of noise that an amplifier causes.
Narrow band aerial arrays consist of a set of narrow frequency aerials, with outputs combined. The result is full frequency reception with the better performance of narrow band aerials.