Main Page List

Splitters, Amplifiers and Diplexers

Subjects on this page are listed in the following order :   

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

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

Mains Aerial Amplifiers   (including Sky Digi Eye compatible amps)

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)

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.  

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 below.

Doing so destroys what should be an impedance matched system, and it can also

introduce other problems such as possible reflections of the signal within the cable.

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

Could those only seeking information please just find the answer somewhere on this site,

or ring an aerial installer local to them, or call the reception advice phone numbers.

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 (particularly a mast head amp) can work wonders for picture quality. On the other

hand 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) or digital picture break up. The noise referred to is background RF (Radio Frequency) noise which is around us all the time.

Any installer who uses an "insulation

tape splitter" (or connects two cables

to an aerial without a splitter) should

get back to “The OK Corral” and do

some cowboying there, and you can

quote me on it. The graphic below shows how it should be done.

Having said all of the 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".

Using a splitter will obviously only supply 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 (except the Co Ax splitter) 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 decent signals even a four way with a loss of 8dB

should be fine. Remember you are only losing signal level the quality should be exactly the

same on the split signal. If you use a splitter and then find unacceptable signal degradation

it`s fine to just add an amplifier before the input to the splitter.      Note that 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 22dB gain Mast Head amp feeding a 6 way

splitter (with a loss per output of 10dB) one still has a nett amplification of 12dB.

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 (less cable loss) 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 !                

A typical gain figure for a mains powered distribution amp 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,

so the effective "gain" would actually be 8dB plus 6dB, i.e. 12dB.

Many 4, 6 or 8 way mains powered distribution amps give 12V “line power” out of the

aerial input socket to power a mast head amp without the need for a separate PSU.

Unfortunately it is fairly common for this to be at an insufficient current to actually run

many mast head amps ! Line power should be at least 50mA, and preferably 100mA, all of

the amplfiers we sell supply 100mA.

Some mains distribution amps have two inputs (one for UHF/TV and one for VHF/FM/DAB)

which are combined together in an internal diplexer to give TV and/or FM/DAB out of all the

outputs. However, not all amps accept DAB on the VHF/FM input.

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.

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

We also stock 4, 6 and 8 way distribution amps.      All our amps are fully screened.

Some amps have an “IR (infra red) return path”. Sky boxes can be remote controlled from

different rooms utilising “Digi Eyes” (next to the TVs) which pick up the remote commands

and send them back down the Co Ax cable to the RF2 output of the Sky box. An IR return

path amp (or “Digi Link amp”) allows the passage of these remote control signals.

Typical uses of external and internal splitters.

For the external splitting job we`d recommend the mast head type splitter.

For the internal splitting job we`d recommend the F connector type splitter.

2 way and 4 way outdoor / mast head splitters

The picture above is a close up showing the splitters with, and without, the covers in place. They are equally suitable for mounting on the aerial pole or on the wall according to which

is more convenient. Power pass on one leg.   Note the screening cans.

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

Typical displays from an RF spectrum analyser. Each blue waveform represents an RF

transmission and this is the type of display you would get for a transmitter whose broadcast

channels are “grouped”, e.g. Crystal Palace,  Emley Moor,  Winter Hill or Sutton Coldfield.

The amount of amplification given to the signal by an amp is measured in decibels ( = dB),

plus 6dB is double, and minus 6dB (i.e. attenuation) is half.

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 the

graph on the right illustrating the effect of the

"Digital Cliff Edge".

See this customer Aerial Report.

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 and a variable attenuator is obviously the most useful. Incidentally a variable attenuator is probably the most under used

piece of diagnostic equipment considering they`re only about £6 ! Just like amplification, attenuation is also measured in dB, but it`s minus dB. 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”.

Unless you`re absolutely certain of the signal strength in your area I`d make sure that any amp you`re thinking of using is capable of accepting a reasonably large input signal, even if this is not important now it may be after DSO when the power will be significantly increased.   

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 interference, 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.    

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, although mains type amps generally are inferior in this respect (by 1 to 2dB) to masthead types. And remember 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 this Aerial Report.

Aerial Splitters

As the name suggests splitters 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 use a splitter but

discover an unacceptable fall off in signal, you can

always add an amplifier in front of it, and it can then

be removed after switchover if required.

Mains Amplifiers

Generally, in domestic installations, amps 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.

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. That said, there will be a slightly

greater loss “on the shoulders” of the input frequency range on each leg, e.g. on a TV/TV diplexer

( = UHF/UHF) the loss at plus + / - one channel would be about 3dB.

The most common use of a diplexer 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. But 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 a

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 its constituent parts, e.g. setback or surface plate types. For TV/FM/DAB signals one could

just use a splitter (though the through loss is significantly higher) but if a feed from a satellite

dish was involved 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 don`t really recommend them, see surface plates.

More than one aerial on your house ?

Unless two antennas of different groups are required to obtain good signals (and this is very

rare * ) no one need have multiple TV aerials. It`s obvious if you think about it, if you couldn`t run more than one TV off each aerial then the size of tower blocks would be limited by the number of aerials they could fit on the roof !  

Not only do multiple aerials 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 (in some circumstances) even end up with better pictures than you had before, particularly if a mast head amp is used.

* It`s very rare to require two aerials on the same transmitter, though it`s slightly less rare to have two aerials on two different transmitters, often diplexed together. This may be to receive different local news or because the different transmitters give better signals on different channels.

Installations using Mast Head amplifier(s).

The picture above shows typical installations involving Mast Head (i.e. 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 four 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 in a strong signal area 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.

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 !

See article on this subject.

In the graphic on the right an A group aerial is shown diplexed with an E group. The diplexer would normally

fit on the pole.

Below is a graph of the gain curves resulting from the diplexing of an XB16E with a Yagi18A using a

CH21 to 37 + CH39  68 diplexer.

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. Only those in the very worst signal areas need to do this however.

The above graph also hints at the possibilities of using a diplexer as a filter by only

utilising one leg. 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.

The graphic on the right shows a TV

aerial diplexed with an FM half wave

dipole. The latter antenna can give good

results on both FM and DAB so the

resultant combined feed can supply TVs,

and/or FM tuners, and/or DAB tuners.

For more DAB signal (but less FM), a

DAB antenna could be substituted for

the FM aerial. For maximum FM and DAB a triplexer could be used with separate inputs for a TV aerial, an FM aerial and a

DAB aerial.

In poor signal areas the VHF/UHF diplexer can be replaced by a VHF/UHF mast head amp.

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 to a number of “set top boxes” were

required it would be necessary to use a multiswitch (and a four way LNB) 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.

Any UHF frequency (e.g.the RF output from a CCTV camera or a Sky box) can be diplexed with a

TV signal, though only if your transmitter is grouped and therefore has unused channels above

or below its output, e.g. Crystal Palace where a CH38 diplexer could be used, the “added” RF

output being set to any channel above CH39. Note that it is not possible to use a diplexer like

this if your transmitter is a wideband (e.g. Sandy Heath) because using any of the splitting

frequencies available would still lose some of the broadcast signals. Under these

circumstance you would have to take your chances using a splitter in reverse (as a combiner),

although this loses half the input signals and can result in signal interaction.

It should be pointed out that in the case of Sky it is far more usual to add the TV signal to the Sky

RF output at the actual Sky box itself, the latter effectively has a built in combiner for this purpose.

More rarely a diplexer can be used to combine the signals from two TV aerials, though do not

confuse this with a “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 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, though this is rare.

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.  

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 it 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 they can give spectacular results provided the signal is of good quality. They can also be used to overcome cable loss (which is particularly important in long runs) and as distribution amps in conjunction with splitters. 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 gain of our 1 way = 7 to 22dB, the 4 way = 1 to 16dB and the 4 way “kit” (only available with with the PSU) = 12 to 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)

Mast Head Amplifiers

The other type of amplifier is the so called Mast Head amp although 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.

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

Many other combinations are possible, check your transmitter`s 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.

Channel 34 diplexer (i.e CH21 to 33 combined with 35 to 68)

Belmont (analogue plus MUX1) plus Sutton Coldfield

Channel 36 diplexer (i.e. CH21 to 35 combined with 37 to 68)

Bilsdale (excluding MUX6) plus Emley Moor

Crystal Palace (excl analogue C5) plus Hannington (excl analogue C5)

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

Channel 38 diplexer (i.e. CH21 to 37 combined with 39 to 68)

Bilsdale (excl MUX6) plus Pontop Pike

Crystal Palace plus Sudbury   

Rowridge plus Hannington (excl analogue C5)

Channel 52 diplexer (i.e. CH21 to 51 combined with 53 to 68)

The CH52 diplexer is now no longer in production,

we are currently sourcing a replacement, but this will be split at CH51.

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.

Also see Separate DAB & FM Inputs On Your Tuner ?

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

Each aerial point in the above illustrated example can receive any transmission from the aerial(s)

plus the channel tuned in on the Sky box. The latter can be remotely controlled via Digi Eyes if

required. Only three of the four outputs are in use in the system shown above.

However it is important not to confuse the term Sky compatible amplifier.

1 No amplifier can split/distribute a signal from a single LNB (on the dish)   

   into a number of Sky boxes, this requires a Quattro LNB and a

   multiswitch system.

2 Any aerial amplifier can distribute the RF signal from a Sky box

   (i.e. the output from the RF2) to a number of TVs.

3 Only an IR compatible (Digi Link) amp will allow remote operation of

   the Sky box from a number of rooms. But if the use of remote eyes is

   not required then the IR return path capability of the amplifier is

   superfluous, see 2 above.

In addition to conventional mains powered Digi Link amps (we stock an 8 way version) a

remote powered model is available which utilises the 9V output from the Sky box. This

compact type of amp is particularly suitable for mounting in a loft or other location with no

mains power present. We stock a 4 way version of this type of amp, although not all of the

outputs on either of the amps have to be used, obviously !