This site is optimised for a screen resolution of 96 DPI.

Main Page List

Splitters, Amplifiers and Diplexers

Subjects on this page are listed in the following order :   (also see Sales / Mail Order Sales)

Amplifier Basics (including Attenuators and the importance of Variable Gain)

Aerial Splitters   (including “Head End Amplifiers”, used in conjunction with splitters)

Mains Aerial Amplifiers

Mast Head Aerial Amplifiers

More than one aerial on your house ?

Diplexers (Combining the signals from more than one aerial, plus use of diplexer as a filter)

We are more than willing to give advice to those actually purchasing from us.  

Could those only seeking information please just ring an aerial installer local to them or try calling reception advice on 08700 100 123 (BBC) or 0844 881 4150 (ITV).

Amplifier Basics (Including Attenuators)

Sometimes known as “boosters”, there are more misconceptions about amplifiers than

any other area in this trade. The crucial point to understand is the difference between

signal strength and signal quality. Assuming the downlead is OK, the only way to

improve signal quality is at the Aerial, either by changing its location/direction or by using

a higher gain antenna. If one has a good quality signal but it is of small amplitude (hence

the term amplifier) then an appropriate amp can work wonders for picture quality. However,

if the signal is of poor quality and one adds an amplifier, it will give no improvement. All you will get is a large poor quality signal as opposed to a small poor quality signal ! The best example of a poor quality signal is ghosting, but a low "signal to noise ratio" can also

give symptoms of low gain, i.e. a grainy picture. Furthermore too much signal can be just

as bad as too little. A television tuner circuit has an inbuilt "AGC" (Automatic Gain Control) which will generally enable the receiver to operate satisfactorily with signals from around

one millivolt up to about ten millivolts (= 60 to 80 dBμV). If the signal is outside this range there is a risk of a grainy picture (if the signal is too weak) or cross-modulation interference if the signal is too powerful. If one lives in a particularly strong signal area,

even an unamplified signal may still be too great for the tuner to cope with.

In these circumstances it is necessary to use an attenuator to reduce the input level.

Just like amplification, attenuation is also measured in dB, it`s just minus dB !

Those on Belmont or Rowridge whose aerials face toward the continent can often benefit from using an attenuator if (in certain weather conditions) they get co-channel interference from transmitters abroad. Attenuators can also be useful if you are picking up more than

one transmitter with the resultant co-channel, a classic example being Chesterfield and Emley Moor. In this situation one would vary the attenuation in order to tip the unwanted signal down the Digital Cliff Edge, leaving the required signal at the top.

It must be stressed that any amp in the system is just as likely to be “overdriven” as the

TV tuner. Thus if the input signal to the amp is excessive the attenuator must be utilised

at this point, not just at the “setback”. A variable attenuator is obviously the most useful !     

The amount of gain provided by an amp (or aerial) is measured in decibels ( = dB), plus 6dB is double and minus 6dB (i.e. attenuation) is half, see article on the Decibels.

Although doubling the signal might sound like a lot, in most instances this amplification level (as opposed to gain at the aerial) would not make that much difference to the picture, unless it is the critical amount needed to attain the level of one millivolt. Even then most tuners will not improve from a poor picture to a perfect one with just a 6dB increase in

signal. The possible exception to this are Digital signals, where a small improvement can sometimes make all the difference (see "Digital cliff edge").

One should also bear in mind that all amplifiers introduce additional "noise" to the signal and this should obviously be as little as possible, around 3dB or less, generally mains type amps are inferior in this respect (by 1 to 2dB) to a masthead amp, and that is in addition to the extra noise that is picked up by the cable running from the aerial to the “set back”.

Wherever possible an amp should have variable gain and generally speaking the gain control should be turned down as far as possible to achieve the desired results,

doing this will help to minimise cross modulation and/or co-channel interference.

A variable gain facility could well be vital when the transmitting power is increased at DSO, for this our “4 way” M/H amp is particularly well specified as it turns down to only 1dB.

Finally all amplifiers (or splitters) should be "screened" to help eliminate interference etc.

It cannot be over emphasised that gain at the aerial is much more significant

than any gain added by subsequent amplifiers. It is only at the aerial that the critical signal quality can be achieved, see Aerials.

Aerial Splitters

As the name suggests these are used to "split" the signal from the aerial to feed more than one point. If your signal strength allows it I would generally advise the use of a splitter over an amplifier, it is simpler, more reliable, cheaper, introduces less noise and is less likely to give cross modulation problems. The latter could will become even more significant at DSO.

If you purchase a splitter from us, but discover an unacceptable fall off in signal, provided it is returned in the same condition we are happy to swap it for an amp.

A splitter should always be used because just "splicing" the aerial downleads together is very bad practice from both the signal point of view and (if it`s mounted outside) weather proofing considerations. Aerial splitters are designed to maintain a 75 ohm "impedance matched system". All TV tuners are designed to work with a 75 ohm load, all TV cable should be 75 ohm and all decent aerials are designed to have (as close as possible) a 75 ohm impedance at the dipole or balun. That is one of the reasons why one should always use a splitter rather than (for instance) just connecting two cables to the aerial, as the bodger has in our photograph. Doing so destroys what should be an impedance matched system, and it can also introduce other problems such as reflections of the signal within

the cable. Any installer who uses an "insulation tape splitter" should get back to

“The OK Corral” and do some cowboying there, you can quote me on it.

Having said all of that (above), as I have repeatedly stated on this site, if you live in a strong signal area (or are just plain lucky), bodges may well work, up to a point anyway. We can only advise you of best practice and then it`s up to you. Also see “Digital cliff edge".

Four way screened outdoor / mast head splitter

The picture above is a close up showing it with,

and without, the cover in place. It is equally suitable

for mounting on the aerial pole or on the wall

according to which is more convenient.

Note the screening can.

We also stock a two way version.

Loss on the 2 way = 3.5dB and on the 4 way = 7.5dB

Obviously one will only get a proportion of the signal out of each leg, but so long as an adequate signal is present at the input, it is quite acceptable to split the signal 2, 3  or 4 times (or even more). The crucial point is that there is still about 1 millivolt of signal at each TV tuner input. In strong signal areas we have split unamplified signals up to eight ways

with no problem at all. It should always be remembered that it is the splitter (and the

cables from it) which represent the load on the signal, it is irrelevant whether or not there

is a TV on the end. If it is more convenient, a powerful one way masthead amp can be used as a “head end amp” and the signal then split where it is required. If using the latter technique remember that Power pass (through the splitter to the M/H amp) considerations must be satisfied, all the splitters we sell enable power pass.

The loss through a 2 way splitter is about 4dB and most locations can accommodate

this with no problems and if you have a reasonable signals a four way should be fine.

Note  dB = decibel

If using a “head end amplifier” and then a splitter at a more convenient location one can work out the nett amplification quite simply by taking the dB loss of the splitter from the dB gain of the amplifier. For example if one uses a 24dB gain Mast Head amp feeding a 6 way splitter (with a loss per output of 10dB) one still has a nett amplification of 14dB.

Commercial systems use the same principle to feed up to 100 points (or more) by using head end amps with very high gain (e.g. 50dB) feeding splitters and/or taps.

Even using a relatively small head end amp of say 35dB one could feed an 8 way splitter (loss 12dB) which could then feed 8 more 8 way splitters (at another loss of 12db) which

still gives a nett gain of 11dB at each output, for 64 points !

Loss (approx) on splitters, 2 way = 4db, 4 way = 8dB, 6 way = 10dB, 8 way = 12dB   

ATV sell 2 way, 4 way, 6 Way and 8 Way indoor splitters plus 2 way and 4 way

"Mast Head" (outdoor weatherproof) types. The latter can be fitted outside and are simpler/neater to use than "indoor"splitters because only one hole (i.e. just in to all the rooms) is required. In addition these splitters can also be fitted on the aerial if the cables

are required to feed different sides of the building.          All our splitters are screened.  

Any F connectors required with our splitters (or amps) are included in the price,

and they`re decent ones !                

Diplexers

A Diplexer is a form of "combiner/splitter" which only passes signals of certain frequencies through each leg. A Diplexer generally has a through “loss” of between 0.5 and 1.0 dB as opposed to that of a splitter/combiner which loses 3 to 4dB.

The most common use is to combine the signals from a UHF/TV aerial and a VHF/FM/DAB (radio) aerial onto one downlead. Note that not all diplexers accept FM and DAB signals, some have a filter and only accept FM or DAB.

If required, this diplexed signal can then be fed through a distribution amplifier (or splitter if sufficient signal is present) to feed more than one point.

Remember most mast head amps are only UHF compatible, if both VHF (FM/DAB) and UHF (TV) are required to be amplified we supply a combined M/H amp to specifically achieve this. If your mains distribution amp has separate FM/TV inputs (like our 4, 6 and 8 way amps) it already has a diplexer on the input. This is an important point because if

you feed a combined FM/TV signal into the TV input the amp will only “accept” the TV component ! Furthermore not all models of amplifier which have TV/UHF + FM/VHF inputs will accept DAB on the latter input, the ones we sell do accept DAB ! Incidentally when

we checked the output of a Sky receiver with our spectrum analyser, it did pass FM/DAB, and they both emerged from RF1 and RF2. This information could be significant when planning your system.

At the other end of each lead, another diplexer should (ideally) be used to split the signal back into it`s constituent parts, e.g. setback or surface plate types. One could just use

a splitter though the through loss is significantly higher.

If a feed from a satellite dish was involved the use of just a splitter would not be recommended, for this a diplexer should definitely be utilised.

Diplexers can also be built into the surface plates so that it has two outputs. For example one socket could be marked "TV" and the other "FM". On the subject of surface plates,

we do not recommend them, see surface plates.

It is also possible to diplex a signal from your satellite dish with one from your TV aerial (and one from an FM antenna as well, if required), but this combined signal cannot feed multiple points simply by splitting it. If multiple feeds from a satellite dish were required, it would be necessary to use a DiSEqC switch which is outside the scope of this article.

The fact that a diplexer can be utilised for satellite frequency signals is one of the most significant reasons why all cabling should use decent quality satellite grade cable. You never know whether you, or someone who buys your house, will need to utilise existing wiring (possibly installed behind the wall....) to carry signals from a satellite dish.

More rarely a diplexer can be used to combine the signals from two TV aerials,

(also see “phased array”). Although it is possible to just use a splitter "in reverse", there is the possibility of signal interaction or ghosting and in addition the loss on a diplexer is

lower. For example you may be in a situation where you can get good analogue off one

transmitter (with poor digital, or none at all) but you are able to receive a reasonable digital signal off another. Alternatively you may be in such a poor spot that at different times of the year (or indeed of the day ! ) the best signal is off different transmitters, trees are the most common cause of the latter. If the two transmitters outputs do not overlap in the band, and

a diplexer with a suitable crossover is available, both aerials outputs can be combined onto one downlead.

Sometimes a wideband is required for a particular transmitter, but because the response

of these aerials is poor at the bottom of the band two grouped aerials are diplexed together off the same transmitter.

Remember that if you are only using analogue off one aerial and only the digital off

another you can always run two cables, e.g. one to your Freeview box and the other

straight to your TV for the analogue signals.  

We sell four different diplexers which are listed below together with examples of their use.

Many other combinations are possible, check your transmitters channels/frequencies.

The best way to check possibilities is using the Channel Allocation Guides

(e.g. Bilsdale`s) for the transmitters for which we have a dedicated page, the latter also include the “post DSO” channels.

Channels 21 to 33 combined with 35 to 68 (i.e split at CH34)

Belmont (analogue plus MUX1) plus Sutton Coldfield

Channels 21 to 35 combined with 37 to 68 (i.e split at CH36)

Bilsdale (excluding MUX6) plus Emley Moor

Rowridge (excl MUX2) plus Hannington

Rowridge plus Mendip

The Wrekin (excl MUXES 5/6 + subject to radiation pattern coverage) plus Sutton Coldfield

Channels 21 to 37 combined with 39 to 68 (i.e split at CH38)

Crystal Palace plus Sudbury

Crystal Palace plus Hannington

Bilsdale (excl MUX6) plus Pontop Pike

Channels 21 to 51 combined with 53 to 68   (i.e split at CH52)

Wenvoe plus Mendip (exc C5)

Hannington (excl C4) plus Mendip (exc C5)

Guildford (excl MUX6) plus Midhurst

Since wideband aerials have such poor gain for the A group frequencies, if you are

in a poor area and have to use a wideband but you require more gain at the bottom of the band, you`re best option is to diplex an A group with an E group, or even an A group with another (high gain) wideband if you already have one of these.

In the example below an XB16E has been diplexed with a Yagi18A using

a CH21 - 37 + CH39 - 68 diplexer. The graph, which illustrates the above example of diplexing, also shows that a diplexer can effectively be used as a filter if only one leg is

used. If using a diplexer as a filter, best practice is to “load” the unused leg with a 75 Ohm resistor and this can easily be added the the diplexers we sell as they use “saddle and clamp” connections. This use of a diplexer/filter can be useful to eliminate co-channel interference on your system (e.g. with the output of your Sky box) or other unwanted

signals from just above (or below) the required band.    

Diplexers Those in the above picture are TV/FM but we also stock TV/TV and TV/Satellite plus TV/FM/DAB though technically the latter is a triplexers.

Back to the top of aerialsandtv.com  Amplifiers, Splitters & Diplexers

More than one aerial on your house ?

Unless two antennas of different groups are required to obtain good signals (and this is pretty rare) no one need have multiple TV aerials on their roof, or wall. Not only does it look a bit of a mess but there will be greatly increased wind loading as well. Only one antenna is needed to feed multiple points through either a splitter or amp as required. If the latter is used you may even end up with better pictures than you had before !

Mains Amps

Generally speaking (in domestic installations) amplifiers fall into two basic categories,

Mains and Mast Head. The most commonly fitted are mains powered amps with relatively low gain, which are sometimes (misleadingly) called "setback amps". These tend to be

used when the TV signal needs splitting to feed more than one set, i.e. distribution amps.

A typical gain figure would be around 6dB but if this is used in (say) a 4 way amp one must add in the loss which would occur if a passive 4 way splitter were to be used in its place (see - Splitters). That loss would probably be around 8dB (according to the quality of the splitter), so the effective "gain" would actually be 8dB + 6dB, i.e. 12dB.

Lastly bear in mind that most mains amps have inferior noise figures to masthead types

and furthermore most do not have the benefit of variable gain.

Note. Not all mains distribution amps with TV + FM/VHF inputs will accept

DAB on the latter input. The ones we sell DO accept DAB !

ATV supply 2 way mains amps with a variable gain of up to 17dB per output.

We also stock 4, 6 and 8 way amps.  

All our amps are fully screened.                (Also see Sales / Mail Order Sales)

Mast Head Amps

The other type of amplifier is the so called Mast Head amp, see picture. This can be something of a misnomer as they are also be mounted on the wall or even used in lofts, which can be convenient if no mains power socket is available. These are powered by 12V sent back up the aerial cable by a "power supply unit" (PSU) which is plugged into one of the aerial points. These PSUs are often mistaken for amplifiers but in fact provide no amplification at all. Furthermore if one fails (or is omitted) the amplifier on the "mast head" (or in the loft) will be inoperative and no signal will reach the aerial point. It should be noted that if the PSU is required to supply its voltage back through a surface plate, the latter cannot be of the isolated variety as the capacitors built into it will not pass DC. Whilst on the subject of surface plates we do not recommend them, particularly where a PSU is in use. As with amps and splitters the PSU should be of the screened type to minimise interference. The major advantage of the mast head amp is the improvement in "signal to noise ratio" as it can be situated close to the aerial (the ideal distance is about 1 metre)

and therefore does not amplify any noise introduced to the signal by the cable. Most M/H amps seem to have superior noise figures to mains types though I`m unsure why this is.

Maybe it`s just because noise is more significant with a small signal (and this is what is expected with a M/H amp) so more priority is given to this parameter during the design stage(s). Note that the vast majority of M/H amps only amplify UHF, not VHF and UHF, as most of the mains ones do.If both VHF (FM/DAB) and UHF (TV) are required to be amplified we supply a combined M/H amp to specifically achieve this.  

Mast head amps are generally used in poor signal areas and can (provided the signal is of good quality) give spectacular results. They can also be used to overcome cable loss

(particularly important in long runs) and as distribution amps. By making the cable runs simpler / neater / quicker they can compensate for their slightly higher price over a mains type amp, see use of outside splitters.

The usual considerations regarding screening and appropriate gain should be observed.

ATV stock one way and four way mast head amps, all of which have variable gain.

The 1W = 7 to 22dB, the 4W = 1 to 16dB and the 4W “kit” (c/w PSU) = 12 - 24dB

I am of the view that amps with variable gain capable of being decreased to a low level are

more appropriate, and are more flexible in their application, than those with higher gain.

If you really feel you need more than 16dB of gain I think you want to be looking at your aerial, for example can you use a grouped one ?

We also stock the PSUs. F connectors are included with our amps !

All our amps & PSUs are screened.              (Also see Sales / Mail Order Sales)

Whatever combination of aerials you install on your mast remember that the single most important factor is that nothing is placed in line between the reflector, the dipole and the elements of the aerial, or indeed the path to the transmitter Transmitter !

See article on this subject.

Installations using Mast Head amplifier(s).

The picture above shows typical installations involving Mast Head / external amplifiers.

On the left we have a simple one way amp feeding one TV point, note how the distance

that the weak unamplified signal has to travel is minimised, ideally about one metre.

On the right is an installation utilising a for way Mast Head / external amp, though in this case only three outputs are in use, thus a spare is still available if ever required.

I have added an external splitter on one of the outputs simply to show that this is possible, but an additional benefit is a neater install. In fact, if it results in more elegant cable runs,

and less use of cable, it is perfectly acceptable to use a one way amp near the aerial

and then run into one or more splitters (external or internal) to feed the various points.

Note that many people fit the Mast Head amps in the loft instead, if this is a more

convenient location for the cable runs, e.g. through the ceiling(s).

The above graphic is showing the use of amplifiers, but if you are in a decent signal area

the four way amp could just be a four way splitter instead, in fact it should be a four way splitter !  If you live in a strong enough signal area, then the use of a second splitter off

one of the splits (as in the above right example) would be fine, despite the fact you`d only

be getting half of a quarter (i.e. an eighth) of the original signal