Difference between revisions of "Installing a TV aerial"

Latest revision as of 12:37, 27 June 2020

Practical advice on installing a TV aerial.

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.

Installing your own aerial

In theory this does not sound like a difficult task, but it may involve working safely at height. Also note that while some installations are straightforward and can be attempted by any competent DIYer using common sense and some educated guesswork, the more tricky installs present a not insignificant technical challenge, and may require access to special equipment. If you have doubts about your ability to complete the job safely, then its better to call for professional help than risk falling off a roof. Producing a long lasting installation also requires some careful attention to detail to ensure that aerial installations remain secure, and are not affected by the ingress of water.

Aerial position, Loft, Rooftop & Set-top

Small set top aerial

The location you choose for your aerial can have a huge impact on its performance. In a strong signal area, this may be of little concern, however in poor reception locations, it can make the difference between a good picture and something unwatchable.

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.

Rooftop
Loft
Indoor aerial mounted as high up as possible
Set-top aerial

Small set-top aerials vary in performance from poor to useless in most areas. In strong signal locations one may be ok. The advantages are minimal cost, no installation, and portability.

A loft aerial may be quite satisfactory in some cases, but not always. Its often easier to install a loft aerial than a roof top one, and they will usually last longer than one exposed to the elements.

In practice roof mounting gives a much stronger and better quality signal. However, roof mounting is of course less convenient and more expensive. Outdoor aerials, poles and cables cables also have limited life, so the expense is repeated later.

A rooftop gives the best vantage point for TV aerials for several reasons:

Being higher up there's usually less to get in the path of the signal from the transmitter. Even trees will have an effect on signal levels - more so when in full leaf.
There is no interaction with roofing materials.
The aerial is further from sources of electrical interference in the house, improving the signal to noise (S/N) ratio
TV aerials in a loft are affected to some degree by metalwork in the loft. Very high gain aerials are especially prone to this.

As an example of how much difference aerial location can make, here is an example of someone finding over 100 x the difference in signal level between loft and roof mount.

Practical advice on installing an aerial

First choose your aerial, and suitable cable.

While not optimal, in reasonable signal areas a loft aerial may be fine. If you are unsure it may be worth trying anyway since installing in a loft is safer and easier, and the aerial will probably last indefinitely without being used as a rest stop by every overweight pigeon in the area!

Loft aerials

Aerials are often bigger than they look from the ground

Tips:

Keep away from wiring and metalwork where possible.
Results looking "through" a gable wall may be dramatically different from looking through the tiles or slates.
Don't choose a position where it will have your eye out every time you climb through the loft hatch!
Try alternate positions if you're not getting the result you'd like
If you have a metal roof, a loft aerial won't work

Be prepared to accept that a loft mount may just not be good enough.

Roof Mounting

Roof mounted aerials usually have the best chances of giving satisfactory performance. Aerials mounted on a gable wall or a chimney stack adjacent to a wall are the easiest to install. Mounting on a chimney that is in the middle of a hipped roof can present more of a challenge.

General Guidance:

Choose appropriate mounting hardware - you generally need a lashing kit for fitting to a chimney, and you will normally need a bracket to hold the mast and provide some standoff to move the mast away from the building or chimney. Buildings with sizeable corbelled sections of brickwork or deep overhangs at the eaves may require longer brackets or a "cranked" mast (one with a double bend in it).

Lashing kit and light weight galvanised bracket - suitable for small aerials
Galvanised bracketing lasts much longer than painted or powder coated steel.
Ali masts are light and easy to handle, but choose an appropriate size. 40 - 50mm diameter ones with 1.6mm walls are far stronger than a thin walled inch thick mast.
Spread the load - with a gable wall fixing, use two brackets (often called T and K brackets) for the larger aerials and space them well out.
Drill into the centre of the bricks, not the weaker mortar joints.
Only use screw fixings where lashing isn't possible, and avoid screwing into the top 3 courses of brick
Take care near the top of a gable wall - there is little weight of brickwork resting on it, so it is not that strong and you can easily dislodge a brick.

Bricks near the tops of walls have little weight on them, and can be easily pulled out
Pay careful attention to the wind loading of an aerial - big aerials can exert a significant force in a strong wind that can pull out fixings, or snap masts that are too long or thin. In extreme cases you can easily bring down a slender chimney stack with an oversized aerial!

Too much wind loading from a pair of aerials, resulting in chimney damage
Use appropriate fixings - ordinary wallplugs and screws may only be acceptable for small aerials in more sheltered locations. Bigger aerials may need coach screws and matching plugs or even expanding sleeve anchors.
Take care when fixing near to the edge or top of brick and block walls to not dislodge bricks when drilling, or to fracture them with expanding fixtures. Remember that the top courses of a gable wall for example have little weight of bricks over them, and hence little strength.
Cranked masts impose a strong twisting force on the U bolts in the chimney/wall bracket.

Adjusting position for difficult reception conditions

If more than one location is available, do a survey at each position to see which is best. The easiest chimney to get to might not offer the best results!
Height is important - raising an aerial as little as a metre can make a big difference to the signal level (and may be preferable to using a larger aerial). The counterpoint to this advice may be where the problem is cause by tall trees. Using a lower location that can see under the branches may actually help here.
If line of site is obstructed by a hill, try angling the aerial up slightly, or aiming slightly to the side of the hill even if that is not as accurate as straight at it.
Multipath interference (i.e. what was known as "ghosting" on analogue transmissions), tends to be less of a problem with digital receivers which are better able to cope with it. However it may explain poor signal quality in some cases. To reduce it, try a more directional aerial, and try different bearings. It may be preferable to switch to a different transmitter if one is available.

Polarization

Aerial with vertical polarisation

TV signals are usually broadcast from the main transmitters with horizontal polarization. Hence the TV aerial should be positioned horizontally. Some smaller relay transmitters however use vertical polarisation, and with these, the aerial will need to be rotated through 90 degrees (i.e. with its elements running up and down rather than across) when using these transmitters.

Check the polarisation of the transmitter you're receiving, or just turn the aerial to see which gives better reception.

Splitters, combiners, diplexers

Splitters split the signal from one wire into 2 or more feeds. Simply connecting multiple wires together causes greater reduction of signal, impedance mismatch, and signal distortion. (you can't simply join and split TV coax like an electrical supply cable)

Diplexers separate UHF TV signals from VHF or DAB signals sent down the same lead. Avoid using a splitter in place of a diplexer, as these cause an extra 3-4dB loss, often degrading reception.

Combiners combine the output from UHF and VHF aerials, feeding them both down the same lead.

Its best to avoid cheap unscreened plastic diplexers & splitters. These let interference into the aerial system, known for causing picture freezes on digital TV. Prefer metal cased devices that offer better screening from external interference.

Extreme installations

To obtain long distance (what is sometimes referred to as "DX") TV reception, the following methods can be used to boost reception range. They're not required for standard domestic setups.

Tall mast and tower mounted aerials

Raising the height of an aerial can help with long distance line of sight reception, avoiding all obstacles. Generally speaking, increasing aerial height does wonders for signal reception. However long masts will need serious fixing to match since they give the wind loading of an aerial extra leverage

Bayed and Stacked aerials

These arrangements combine the outputs of multiple aerials for greater output without the addition of noise that an amplifier causes. Designing and implementing them effectively normally requires specialist knowledge and test equipment, without which they tend to fail. Discussion of issues.

However the simplest version with two aerials bayed side by side, is not to difficult to implement if one ensures the cable feed from each aerial to the combiner, are exactly equal in length. Baying of aerials like this increases signal strength without needing to use large fragile expensive extra high gain aerials. It also tends to make the aerials more directional since signals hitting the pair off the main axis tend to cancel somewhat due to the different signal paths.

Pair of bayed 18 element aerials on a mast (shared with an FM radio dipole aerial)

Stacked aerials (note the aerial installation industry tends to use the phrase "stacked" for both bayed and stacked arrangements) can also be useful for rejecting unwanted signals introduced from above or below, such as caused by overflying aeroplanes.

Narrow band aerial array

Consists of an array of grouped frequency aerials - each covering a different group, with outputs combined as above. The result is full frequency reception with the better performance of narrow band aerials. This is often seen where it is necessary to combine reception from more than one transmitter in order to receive all the available channels. Special aerial combiners called diplexers can be used here - these include an element of filtering to accept only a specified range of frequencies into each input.

Good quality aerial mast mounted amplifier, shown with its weather protecting cover raised

Low noise Aerial amplifier

A good quality aerial amplifier is often necessary in poor reception areas. One should select one with the lowest possible noise figure so as to amplify the signal with the minimum of noise introduction. System performance with an amplifier typically is at its best when the amplifier is placed close to the aerial on the mast, rather than close to the TV. Amplifiers of this type typically need a separate power supply. This can either be a dedicated box that feeds power up the co-ax to the amp, or some TVs and set top boxes also have the option of providing power up the co-ax.

Screening

For the lowest noise operation, all components should be fully screened to avoid picking up interference, which would degrade quality of a weak signal. This not only means a good quality foil screened co-as like CT-100 or similar), but also using screened accessories. These can usually be recognised by the "fully enclosed" nature of any terminations. The modern screw-in F Connectors also offer good screening - so these are also often seen one the backs of wall plates etc.

Cable joints

Cable joint made with F plugs and a barrel connector. Once encapsulated in self amalgamating tape this yields a very low loss effective cable joint not affected by water ingress

For any but the simplest systems, cables need to be jointed at various places. Any joints that are exposed to the weather must also be protected by being fully enclosed in a waterproof box, or with self amalgamating tape. Self amalgamating tape needs a little practice to apply well (the tape must be stretched prior to being used), but when mastered makes a fully water proof connection. Connections made in normal electrical junction boxes or bits of terminal strip, or worse, from taped up twisted together wire joints will dramatically lower the signal quality in any TV aerial system.

Ensure that when making connections to terminals, that only the minimum required length of inner wire from the co-ax is exposed. All connections should be crimped, soldered, or fixed in screw terminals.

Even low cost traditional Belling Lee style aerial plugs can be crimped with a pair of side cutters

Testing

The simplest test is to "suck it and see". If the picture quality is fine, then there is a fair chance you have an adequate solution. You can add a little finesse to the testing process by trying the new aerial with a 6dB attenuator inline with the connection to the TV. Ensuring it still works with this should mean there is a little "headroom" to cope with days where signal reception is not as good generally, and with the slight drop in performance as the system ages. Try all the channels as well.

Test equipment

The simplest test equipment for an aerial installation is a TV. However taking this up onto a roof is another matter.

A small TV (eg 5") can be very useful here.
A large TV on the ground can be used. For decent results, if possible also have a camera with telephoto lens, binoculars or similar. Its not a good solution, but better than not seeing the picture at all.
A remote helper watching the TV while in voice contact (via mobile phone if necessary) can work, but its harder than it sounds.
Signal strength & quality measuring devices work well, and need no tv, but most DIYers don't have access to them.
Some digital TV set top boxes also have diagnostic capabilities that allow them to report signal strength and signal quality. Search the menus to see what yours has.

More tricky installs can be very frustrating to DIY, because much of the time you are working without adequate information. Alas a multimeter is not going to help here, and a set of professional TV reception test gear could set you back several thousand pounds. Rental may be an option, however there are some intermediate level test gear products that can provide massive assistance without breaking the bank such as Terry from Swires Research [note of 2020, possibly no longer trading.]