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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

Terminators

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.  

On the other hand, if your aerial worked fine on the lower power transmissions before switchover,

the higher power transmissions after DSO should enable you to split the signal 4 or even 6 times

and still end up with the same signal level at each TVs input that you had before on one TV.

This is because the power is being increased at switchover by about 10dB.

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

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.

just connecting two cables to the aerial, as the bodger has in our photo on the right.

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.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 pictures and graphic below show 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.

And that`s 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 reliably run some mast head amps and I`ve

seen quoted figures as low as 25mA.  Line power should be at least 50mA, and preferably

100mA. All of the 4, 6 and 8 way amplifiers we sell supply 100mA, though not the 2 way.

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 CoAx cable [at 5 to 30MHz] 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.

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. Emley Moor (see Wright`s Aerials spectrum display of Emley Moor).

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

In the RF reception game 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, which equals 60dBμV.

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 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 well

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 the

amp can then be removed after switchover if required.

Mains Amplifiers

Generally, in domestic installations, amplifiers

fall into two basic categories, Mains amps

and Mast Head amps.

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 signal

needs splitting to feed more than one set.

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.

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 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.

Installing the amp (or splitter) outside can often make the cable runs simpler / neater / quicker,

but they can also be used in a loft where a lack of mains power precludes a mains amp.

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 mast head amp = 7 to 22dB,  the 4 way = 1 to 16dB,  and the 4 way “kit” = 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 ?

Note that grouped mast head amplifiers do exist, though their use is rare now. If you aren`t getting all the channels for some unknown reason, check the M/H amp isn`t a grouped one !

All our amps & PSUs are screened and the F connectors are included !

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. These

PSUs are often mistaken for amplifiers but in fact

provide no amplification at all. Furthermore if one

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.

If only one additional point (the solid green leads in the above graphic) is required the

connections to/from the Sky box would be the same but no amplifier would normally be required.

Unable to configure your system as in the graphic ?  Possibly because the cable from your

aerial only goes into the loft so you only have one cable running between your Sky box and the loft ? See using a diplexer to add a second RF signal to the input to your distribution amp.

Also see Sky Digiboxes general information on the RF output/loop through.

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.

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. However Digi Eyes will not

   work back through an amp (or, usually, any splitter) if it has no IR return path.

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.

Diplexers

A Diplexer is a form of "combiner/splitter" which only passes signals of certain frequencies

through each leg. They are passive devices and as such don`t require any external power. Diplexers generally have 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, particularly "budget" unscreened ones, see article on 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.

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.

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.

We sell three 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. Note some of these combinations will change after the switchover.

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)

Crystal Palace (excl analogue C5) plus Sudbury (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

Rowridge plus Hannington (excl analogue C5)

Channel 51 diplexer (i.e. CH21 to 50 combined with 52 to 68)

Hannington (excl analogue C4) plus Mendip (excl MUX4 post 2011)

Rowridge plus Mendip (excl MUX4 post 2011)

Rowridge plus Midhurst

Wenvoe plus Mendip (excl MUX4 post 2011)

If the two transmitters you`re interested in have overlapping frequency bands, and therefore it

isn`t possible to use a diplexer, the other option is to run a cable for each aerial down to a 2 way CoAx surface plate (a decent quality screened one obviously, like this one). Use of twin cable would help make this particular job neater and quicker. Then, when you want to swap the signal from one transmitter to the other, just pull out the lead from one socket and put it in the other !

These splitters are equally

suitable for mounting on the

wall or the aerial mast

according to which is more

convenient. All Mast Head

products come with a strap

(to attach to the mast) but I`d

always tape them on as well,

in case the tie wrap breaks.

Our splitters have power

pass on one leg.   

Loss on the 2 way = 3.5 dB

and on the 4 way = 7.5 dB

There are three schools of thought about whether terminators are necessary. Some say

terminators are hardly ever required in domestic jobs. Some say they are advisable when half

(or more) of an amplifier or splitter`s outputs are unused. Some say all unused outputs should

be terminated. The one thing that everyone agrees on is that a terminator is required for the “Full

Output” of a mains distribution amp, if it`s unused obviously ! When I did some simple tests this

was certainly confirmed because omitting the terminator from the Full Output meant the amp was

actually giving out less (on average) than the input, i.e. not terminating  the full output effectively

“lost” 6dB of gain !  A splitter on the Full Output had the same effect as a terminator, even if the

splitter`s legs were unterminated. Plugging in an unterminated 30m length of cable to the

Full Output resulted in the amp “only” losing 4 to 5dBdB, i.e. it had a slight loading effect.

Terminators

Let`s get one thing clear straight away.

When we`re talking terminators  we`re talking what to do with any

unused outputs on an amp or splitter, it`s got nothing to do with an

unfeasibly large Austro American actor.

Any aerial system should be impedance matched and therefore

amps/splitters are intended to work into a 75 Ohm load. On the other

hand most amps/splitters are designed to accommodate a

proportion of the outputs being unused without significant effects.

Terminators are designed to put a 75 Ohm load on an unused

output of an amplifier, splitter or tap. The latter are used in system

work and this article is not concerned with their use.

However the situation is rather different with the unused standard outputs of a mains amp, or

indeed the outputs of a Mast Head amp, or a splitter. In my tests the average difference (across

all 11 test frequencies) between the worst case scenario (i.e. only one output of a device being

used) and all being used or terminated was generally less than 1 dB. The standard outputs of mains distribution amps were least affected by lack of termination.  That said, some of the

frequencies had bigger differences than others. In these, admittedly simple tests, use of terminators had an inconsistent effect on the losses of different splitters, but it was always slight.

On the other hand they marginally increased the gain of all the amplifiers.

Terminators should not normally be fitted to unused outputs of IR return path (Digi Link) amps.

Conclusion If a distribution amp has a Full Output (not all do) and it is unused it should definitely

be terminated. In other cases, for domestic systems, terminators are not that important , though they do have an effect on the margin.

In the graphic on the right an A group

aerial is shown diplexed with an E group.

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.

The two aerials could be receiving from

the same transmitter, or they could be on

two different transmitters, e.g. the A group

aerial on Rowridge and the E group

aerial on Hannington.

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 put a 75 Ohm “load” on

the unused leg and this can easily be added (in the form of an F Conn terminator) on the

diplexers that we sell.

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 are triplexers.

Also see Separate DAB & FM Inputs On Your Tuner ?

If you`ve found this site informative and, hopefully, interesting as well,

please help us increase the number of people reading it.

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

It is also possible to diplex a signal from your satellite dish with one from your TV and/or FM/DAB aerial, then, down by the TV/Sky box, this can then be split out again to feed each unit. However, it must be noted that this combined signal cannot  feed multiple satellite tuners 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 Quattro 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 together with a TV signal, but only if your transmitter is grouped and therefore has unused channels above or below its output, e.g. Winter Hill, or Crystal Palace. In the latter case a CH38 diplexer could be used, the “added” Sky RF output being set to any channel above CH39. In this case the TV aerial would use the “low” input of the diplexer and the added RF (e.g. your Sky)

would be connected to the “high” input”.

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 TV 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. That said, if your system has the cable from the aerial going (only) into the loft [or wherever your distribution amp is] using a diplexer to add the Sky output works quite well. In fact if the Sky input is on the low frequency leg of the diplexer (as it

would be with Winter Hill, Sutton Coldfield or Emley Moor transmitters) then IR return path/Digi Link signals will pass, but only if the Sky box is connected to the low frequency leg. Digi link signals will not pass if it`s connected to the high leg.

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 splitting frequency 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.  

In addition to conventional mains powered Digi Link amps (of which 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 !                                                                  (See graphic below)