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Aerial system components - 
the good, the bad and the ugly

There are only three things you really need to know about siting aerials -

  1. Height matters. The higher, the better. Rooftop is better than loft, loft is better than set-top. Use terrain and you will fairly easily see why. What that doesn't show you is most of the clutter shielding your aerial from line of site is close to you - neighbouring houses, trees and stuff. More height gets you above it all.
  2. playing a long second fiddle  to height, generally the more metal you have in the sky the better. 18 elements is usually better than 14 - but another three feet in height is much better in most urban situations!
  3. Use decent cable for digital. This has little to do with loss and everything to do with keeping unwanted interference causing occasional picture freezes. If you have to use 15 year old brown coax then put your aerial 20 feet higher :-) Seriously, just don't do it. Here's why.

So, here's the cast of characters you can use for DTT. 

Start here if you want to see how to do it right, or carry on for the order that those with more enthusiasm than understanding often try...

the good

Tape and Braid screened cable (CT00, WF100 and the like)

Use no other cable anywhere in the RF path of your system between the aerial, other components and the TV. Here's why.

Loft aerial £20

If you're in a good service area, generally within 10-15  miles of the transmitter, this is a good solution for analogue TV. If you're in a cul-de-sac and don't mind the odd freeze when some kid goes past on a cheap motorbike, you can get away with this for DTT. If your aerial is close to a main road you may have problems on digital. For a £20 investment you can suck it and see - it is your greatest likelihood for a DIY win, and you can take a dry run by taking the downlead through the loft hatch and down the stairs on a temporary basis.

The problem is that the aerial is close to a lot of sources of impulse interference - light switches, mains wiring in the loft carrying mains-borne hash, central heating pumps going off. None of these sources is particularly strong, but their influence falls away with the square of the distance to your aerial. In a typical loft the mains wiring is two feet from the aerial. Put that on your roof and it's ten feet from the aerial - so the interference will be 25 times weaker plus your signal will be more than doubled (you lose about 6dB measured at the TV input through a typical tile roof ). That's a 50:1 improvement to impulse noise resistance!

When buying an aerial for the loft, the obvious temptation is to get the biggest and best you can, right? Don't. A 14-element is plenty. Quality still matters - get a CAI benchmarked aerial as DTT aerials really ought to be balanced to reduce impulse interference. The grading is

 You're in the 2 or 3 mark. Don't even think of using a Standard 1 in a loft- it will be big and won't work right there. You don't need to tangle with aerial grouping as you're not doing an analogue install - the CAI benchmarking will look after performance issues for you. If you have a clear line of sight and are within 8 miles of the transmitter a 3 will probably see you ok, else use a 2 - provided you have a clear  line of sight.

If it isn't good enough in the loft put the same aerial outside - a bigger aerial is unlikely to work right in the loft. You will, of course, use decent cable and wire your aerial plugs properly.

The reason you shouldn't try to use a big aerial indoors is that an aerial is designed to work by the interaction between the conductive elements, and obviously, for a big aerial there are a lot of these. The aerial is designed assuming it is mounted on the roof with air space all around. That is not the case in your loft - there's all sorts of bits of metal and often a great big water tank through  which you definitely do not want the aerial pointing.

Typical loft aerial in 3 bed house-  red is the mains lighting circuit (source of impulse interference) and blue is the water system.

Typical loft aerial in 3 bed house-  red is the mains lighting circuit (source of impulse interference) and blue is the water system.

The metalwork of the water system and mains wiring will also act as conductive elements, and will generally act to detune your aerial and degrade its performance. Avoid metalwork in front of the aerial. If possible avoid any metal or wiring nearer to it than its length other than behind the aerial where it shouldn't be a problem, and a vertical plane through the boom for a horizontally polarised aerial. where it is also not particularly sensitive to detuning (this is normally where the aerial pole is mounted in a rooftop situation).

This all gets harder to do as the aerial gets bigger, and the effect of spurious surrounding metalwork is worse on a bigger aerial which relies on the interaction of more elements. I found an Antiference Xtragain XG16 WB lost 7dB gain on channels  44-52 when mounted halfway in the loft, compared with the same aerial on the gable wall outside at the same height. This is an aerial more than 2m long and its performance was reduced to less than that of the  much cheaper XG5W at a third the size! This is not a reflection on Antiference's product - it is a comment on my misuse of it!

pros:

Easy DIY. 

effective in many situations particularly analogue installations. 

Weather doesn't get into the aerial. 

Loft is a good location if you're going to use a distribution system.

cons:

DTT is particularly prone to impulse interference here. 

Doesn't work on some kinds of roof insulation with metallic foils.

Low aerial height means less signal strength than outdoors. 

Roof easily halves your signal strength, more when wet. 

Aerial takes up a lot of loft and performance can vary if objects are added/moved in loft.

Outside aerial £140, can be less DIY if things work right first time

This is what the broadcasters designed the system for. If you get a CAI contractor to do this job then you don't have to worry about the technology - just specify which channels you want and he will select the parts to deliver that to you. If you are going to do this yourself then you might as well avoid the cheapest aerials which don't last in the British weather - get a CAI benchmarked  one so you know it at least performs ok. Use decent cable. And think about how to stop water getting into the aerial connections and cable.

pros:

Your best shot. If this doesn't work, nothing else will.

cons:

difficult/higher up front cost, though when you've added together everything you need for a DIY job the contractor approach doesn't look so expensive for a guaranteed win...

weather degradation - an outdoor aerial should only be expected to have a 10 year service life in the UK.

the bad

Amplifiers, boosters £20

Boosters are not inherently bad, and in some cases can be good. In the hands of the unaware, they're usually bad. If you have a reception problem, very rarely will a booster get you out of it, in DTT particularly.

Let us start of with the misuse of these first. Amplifiers (boosters) do not solve the problem of insufficient signal at the aerial - 

never use a booster in the same room as the TV unless to split the feed.

Yep, if you have a snow/noise problem, or picture freezes etc on digital, a booster sounds like a great idea. Unless you are in a rural area don't go there. At least try and borrow one first. You need a better aerial site or to fix the fault with your old one (often degradation of aerial, cable or connectors but could include not being sited right).

First law of signal processing - what has once been lost can never be regained.

So what do most people use boosters for? To make up for inadequate aerial installation. To compound the error, there's the 'mo betta' school of thought that says if one booster can make something out of nothing, howsabout adding another one? In fact, the more the better - why not have three in line! Mo' betta is wrong.

Never, ever, directly cascade UHF amplifiers/boosters

Really, don't do it. There are no circumstances I have ever come across or can think of under which this would be of any earthly use in a normal domestic situation. To view this more graphically, take a look at this experiment where boosters were added to get weaker and weaker signals up to nominal signal level. 

Obviously if you have a ranch trying to get your aerial feed to a  a distant summerhouse 1km away then you have to break the cable every 100m (20dB of loss) and whack in an amplifier - as I did in my cable TV days, albeit with fatter cable. You, my friend, do not need to do this, and you particularly do not need to do this at the the back of your TV set. 

It's a common proposal on message boards. I can guarantee you that if cascading boosters does anything other than seriously impair your picture you have something very, very, seriously wrong. Like no aerial on the end of the cable or the power to your distribution amp in the loft has failed.

After the many ways of abusing booster amplifiers, here is what you should use them for

The correct use of a booster amplifier in an urban area of good reception is to correct a known loss after the amplifier

  1. correcting the loss in long runs of cable following the amplifier (typically more than 20m, which excludes most domestic systems)
  2. correcting for the loss of splitting one aerial feed to a number of TVs. Often the split is built into the booster and it then gets called a distribution amplifier.
  3. in rural areas of low signal strength (far from the transmitter) an amplifier mounted at the aerial (masthead amp) can be used to lift the signal a little to compensate for the attenuation of the downlead. Power is injected into the cable from a power supply at the TV end, but the amplifier is at the aerial.  A 10m CT100 downlead will have a worst-case attenuation of 2dB @ 860MHz. So don't put in a 20dB amplifier, 6dB would be plenty. If you have too much gain, you won't get any more performance, but large out of band signals will push the amplifier over the top earlier. Got a mobile phone? Your TV aerial probably still has gain at 900MHz which some networks use. Too much booster gain and that will cause picture freezes by overloading your set-top box and/or the booster.

There is one specialised exception to the rule that amplifiers only compensate for losses after the amplifier. In DTT a booster close to the aerial can lift the signal in the downlead enough to reduce the impact of local interference picked up on the downlead. That works if, and only if, the amplifier is not being hit with high level analogue TV channels which drive it into distortion. Which tends to rule out the bargain basement type sold in B&Q which do not have the output power capability to do this. If you really, really, must use 15 year old brown TV 'low-loss coax' because it's behind the plaster and you are plagued by impulse noise then you could use an amplifier of maximum gain about 10dB to run the cable at a high level and possibly use a 6dB attenuator/splitter at the end of the cable before the TV/set-top box. You have to try with and without, because the attenuator is only needed if the interference or analogue signal levels overloads the TV/set top box.

If the impulse interference is getting into the aerial rather than the downlead, like a lot of vehicle/lawnmower/power tools interference, a booster will do nothing for you, and may make thinks slightly worse. Siting a rooftop aerial at the back of the house away from the road can help, as the roof tends to shield the aerial from the interference.

pros:

Effective if used to compensate for signal loss after the amplifier. 

Best way to break out one aerial feed to multiple TVs if you do not have enough signal for a passive splitter.

cons:

Often destroy signal quality in the hands of the untrained who view this as a panacea for bad installations. Let's keep this simple - a booster in the same room as the TV is not a correct response to insufficient signal strength. This is experimentally verified here.

Non double screened 'Low-loss' TV coax, usually brown.

Served us fabulously for 40 years of analogue TV. We will hate it and curse it for the next 20 years of DTT. Do not even think of reusing this cable. Here's why. Even in the flylead between the aerial socket and the TV this will give you grief from impulse interference. Just say no.

The ugly

Set-top aerial, unamplified, a.k.a indoor aerial, £5

There's an attractive honesty about the unamplified set-top aerial - it doesn't promise much, doesn't deliver much, and isn't actually the worst option. The price of about £5 is right, even better if it comes free with your set. Often used by tenants and students whose landlords take a dim view of holes drilled through walls.

I don't know under what circumstances one of these will work right for DTT since I've never experienced them. My dad's house is 5 miles from Crystal Palace in London. You can actually see the transmitter aerial from the window!   On a set-top aerial placed on the windowsill the DTT picture freezes about once every three minutes. Admittedly, this is a first-generation OnDigital set-top box, but the same box works flawlessly from the loft aerial!

Experimental verification that settop aerials suck and how much they suck compared to a basic outside aerial is available here. Basically the improvement from a set-top aerial to a basic roof aerial is greater than the improvement from the basic roof aerial to the biggest rooftop aerial you can commonly get in the UK. That first step from a set-top aerial to anything else is the greatest. Using a set-top aerial on my main downstairs TV reduces my signal level by 100 times compared to the roof aerial, and about 10 times if used in an upstairs room.

pros:

cheap

cons:

Doesn't work, basically

Set-top aerial, amplified, £20

They should make it a crime to make set-top aerials with internal amplifiers. This gets the award for worst value for money of all the options. It seems so obvious - a weak signal. Amplify the damn thing! And yet it's so wrong. Below are two signals received at 14dBm the lowest limit this Sony receiver could cope with reliably. This is 46dB below minimum specified signal level. The signal is then amplified with a booster to nominal signal level. It's still a rotten picture.

Signal received at 14dBm Same signal, 
amplified 44dB

 

First law of signal processing - what has once been lost can never be regained.

You have a tiny aerial, perched on top of your TV. Your aerial is in the wrong place, which is why it is not picking up enough signal. Noise however, is all around. If you took your set-top aerial and mounted it on your roof [2] or in your loft it would pick up about the same amount of noise, but one heck of a lot more signal.

Experimental verification of the worthlessness of amplifying a set-top aerial  is available here. The addition of an amplifier gave me zip all increase on the 0% of DTT channels available on the set-top aerial. The magic is in the aerial, not the amplifier.

Nobody would try to borrow a hearing aid if you're in a crowded pub and can't understand what your friends are saying above the noise. You have to get close to your friend (raising the aerial to get more signal), get him to shout louder (call up the BBC and get them to turn the transmitter up) or move outside to talk  (move away from the noise). Amplifying it will just give you earache and you still won't understand what he says.

Don't believe the hype. You will never get perfect reception from a set-top aerial. Your signal is probably not large enough compared to the electrical noise the aerial also picks up. Amplifying the signal will make the signal bigger, but it will also amplify the noise, so your signal to noise ratio stays the same. Don't do it. And if you have to, get the version which is an unamplified aerial bundled with a booster amplifier. Better still, just get a booster amplifier to use with the unamplified set-top aerial you already have. It won't work any better, but you've have saved some money, and the booster amp may let you run two TVs off one feed later on.  Booster amplifiers are a topic in their own right and easy to misuse, but they have some uses.

pros:

Can look space-agey, if you like that sort of thing. [3]

cons:

More expensive, no better perfomance.

Doesn't work, basically this is an unamplified set-top aerial with go-faster stripes. 

Takes up another mains plug. 

Wastes power to no particular useful purpose. 

 

More information

http://www.wolfbane.com/articles/tvr.htm   answers the question which transmitter should I use, and how hard will I have to work to get it. Also a source of the values you use to plug into
Terrain shows you the line of sight to the transmitter.
Digital Spy DTT tech forum A broad mix here - some excellent advice, but also some not so excellent rumour. Beware the expertise of anybody using a set top aerial further than 2 miles from the transmitter or with a booster amplifier next to their TV!

 

More links - 

http://www.bbc.co.uk/reception/ ( the BBC have a really bad habit of swapping their site about so if this link is dud enter reception in their search box)

http://tx.mb21.co.uk/info/index.asp

 

Please don't write me and say I'm talking bull and you get perfect reception on your particular choice of amplified set-top aerial, cascaded boosters and 50m of brown 20-year old cable. If you're happy with it I'm happy for you and good luck to you.

 To me good DTT reception means NO picture freezes and sound disturbances  even if my central heating starts, the neighbour is using their lawnmower, the other side is using an electric drill and the kid across the road is trying to impress his girlfriend doing wheelies on his cheap motorbike. Most people seem to have a tolerance for picture and sound interruptions which I don't have - working in TV a few years back made me intolerant of impairments.

Too much rubbish is being sold by companies pandering to the natural human desire for an easy win. Knowledge is your friend here. There are cheap things that do work for many a DIYer, like a loft aerial. There are cheap things that don't work for most people - like set-top aerials, particularly anything of an unusually fancy design.  And then there are expensive things that don't work for most people, like huge outdoor aerials in the loft and strings of boosters after a poor aerial. This page is a different take on the "just whack in this super ultragain amplified set-top aerial for perfect reception" 

Selecting and siting your TV aerial

Why DTT is different from an analogue install

 

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  1. Source: CAI website, July 2005, typical range £80-£190
  2. Set-top aerials are not usually balanced, so you will pick up more interference on the cable than you would with one designed for the job. It'll still be a lot better on your roof or in the loft than on top of your TV!
  3. Run, do not walk, from anybody trying to sell you a UHF TV aerial with a parabolic dish on it. To be effective the diameter of the dish needs to be at least one wavelength, which is 0.5m at 600Mhz for the middle of the UHF TV band. That is one BIG set-top aerial. Any dish at UHF TV frequencies is marketing frippery, not engineering.
  4. 60dBuV minimum, 80dBuV maximum into 75 ohms if you want to be pedantic.

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Text and photographs RM 2005 unless otherwise credited