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FM & DAB (Digital Audio Broadcasting) Aerials

Subjects are listed on this page in the following order :

Basic Aerial Facts

ATV`s Tests of FM/DAB Aerials   (also see ATV`s tests of TV aerials)

ATV`s Choice Of FM/DAB Aerials   (Including Wiring Up An FM Dipole)

Separate DAB & FM Inputs On Your Tuner ?

Basic Aerial Facts

Radio in the UK was always broadcast in three distinct bands, from Long Wave

through Medium Wave up to VHF/FM (Very High Frequency), but in 1995 a new digital

system was introduced just above the FM band. This was christened Digital Audio

Broadcasting, which is more commonly known as DAB. It is important not to get the latter

confused with the radio channels which are transmitted as part of the “Freeview” package

because these are a completely different system.

The frequencies of the bands (and their corresponding wavelengths) are given below :

Long Wave - 148 to 283 kHz = 2020m to 1058m

Medium Wave - 526 to 1606 kHz = 570m to 187m

VHF/FM - 88 to 108 MHz = 3.4m to 2.8m

DAB - 175 to 230 MHz = 1.7m to 1.3m

There is an additional band (Short Wave) between MW and VHF but this has never really

been used for public broadcasting in this country.

Notice how as the frequency rises the wavelength falls and the latter then requires shorter

element(s) on the aerial in order for it to become resonant. This is clearly illustrated in the

picture above and that of the amateur radio aerial array. Whilst it is not relevant for the

purposes of this article it should be pointed out that both Long Wave and Medium Wave

are amplitude modulated (AM) whereas the other two are frequency modulated hence the

term FM. Radio broadcasting is subject to the same polarity criteria as TV transmission,

although the Cross Polar Rejection is much less significant than it is for higher frequency

signals. Most FM (see BBC list * ) is transmitted in vertical (or mixed) polarity and all DAB

transmissions are vertically polarised. If the correct aerial is used then it will work pretty

well for both FM and DAB if installed vertically. ( See article on FM aerial polarity )

Don`t forget there is one other way to change polarity....

Due to the lower frequencies involved “line of sight” (to the transmitter) is less important,

than with TV transmission. Similarly installing an aerial in the loft for VHF is not usually so

deleterious to the signal as it is for UHF. That said, any aerial will work significantly better

without a roof between itself and the transmitter, it`s just that the difference becomes more

marked as the frequency rises. Higher TV frequencies (e.g. the top of the C/D group) will

tend to be attenuated a little more than those at the bottom of the UHF band, i.e. CH21.

As one gets up to satellite frequencies no significant signal will be received through the slates (about 10GHz with wavelength of down to 3cm for the signal into the LNB).

Although I am not an expert in the field of AM broadcasting I`m told that aerials to improve

MW/LW reception should consist of a length of wire as long as possible, but remember that

the wavelengths are quite large ! Try stringing the wire out in your loft but bear in mind that

these bands will suffer interference from the continent during hours of darkness and a more

efficient aerial may well just bring in more of this co-channel........  

Cable for FM / DAB should be 75 Ohm co-ax and although the lower frequencies make

cable quality less critical than for TV reception it`s still worth using decent stuff !

Finally remember that it is possible to Diplex FM/DAB signals onto TV downleads or into

a TV distribution system.

* See BBC radio transmitters page.

ATV`s Tests Of FM / DAB Aerials

After years of contradictory theory, inconsistent “advice” (from the manufacturers) and our

own variable experience we finally bit the bullet and in April 2007 we instigated a series of

comparative tests on DAB and FM antennas. Whilst we were at it we also tested some TV

aerials to see just how well they really picked up DAB / FM and then we deliberately

“mis-installed” a 3 element DAB to check if it really does disrupt the antennas tuning like

we said it would. It did.........

Note that we do not pretend that these are scientifically accurate tests (our TV aerials tests are more rigorous) but they do reflect the relative performance of the various antennas.

We rigged up a temporary test pole and the used our spectrum analyser to monitor the

received signal for both FM and DAB from :

A half wave open FM dipole (polarised vertically, horizontally, diagonally & “cut down”),

A folded FM dipole (polarised vertically and horizontally),

A 3 element FM aerial,

An FM “Omni”,

A DAB dipole,

A 3 element DAB aerial (including deliberately mis-installing it ! ),

Various TV aerials (a Log Periodic, a Yagi18B, a Yagi18WB and a Contract 10A),

And finally a shorted out test cable.

The latter was a control to check that the cable wasn`t picking up the signal as opposed to

the aerial(s). We used our decent quality cable and on this test absolutely no signal was

received whatsoever, thus proving its quality ! Similarly in the case of TV aerials it is

usually preferable for them not to pick up anything except TV broadcasts, so once again

the Log Periodic shows its genius.......

Incidentally these tests involved a large amount of work as the signal varied from day to day

and even from hour to hour. This helped to explain the inconsistent results we`ve had in the

past but it also meant that we had to keep repeating the tests in order to get meaningful

results. See below the results table for accompanying notes.

Pictures of our FM / DAB aerial test rig. On the left we have a diagonally polarised FM half wave dipole and on the right a deliberately mis-installed 3 element DAB antenna.

This bodge knocked about 3dB off the directional performance, which is what one would expect. Note that we have utilised an L section “loft kit” because the installation is only temporary (and not exposed to high winds ! ), we would not normally recommend the use

of this type of pole/bracket for an aerials outside. The same applies to the cabling !

ATV`s choice Of FM / Radio Aerials (plus musings on the above results.....)

It doesn`t take much study of the results table to discover that if you want FM and DAB

(from one aerial) the best all round antenna is the half wave FM open dipole, preferably installed vertically so it will work omni directionally and will also pick up DAB pretty well too.

The vast majority of sites in this country will get a perfectly acceptable FM signal with one

of these aerials, and the great majority a perfectly acceptable DAB signal as well.  

The model we sell will mount both horizontally and vertically, but it is also possible to use

it diagonally. If you are struggling with a particular frequency then utilising the latter polarity may help bring it in, it`s rare but at least it`s another option. On the other hand horizontally

it has a bit of a null off each end (and thus a bit of directivity) so this may be useful to

decrease a bit of interference if required. It is also quite acceptable (and easy) to shorten

the elements in order to improve the DAB (at the expense of FM) if you decide in the future

that DAB is more important to you. I`m not an expert in antenna design but we cut down

the elements from 75cm to 38cm and DAB performance improved to be just behind that of

a DAB dipole, though the FM dropped off markedly.........

Incidentally if you are planning to distribute DAB and FM (or just DAB) through your

system you must make sure that your amps and/or diplexers are compatible for it.

The amps (and diplexers/triplexers) that we sell are suitable for both, but some are not.....

Our half wave dipole is supplied with a 2” rotating clamp, also see wiring up an FM dipole.

Incidentally moving the aerial around the pole can make a difference to the received signal,

and not just where the pole is in front of, or behind, the aerial`s alignment onto the

transmitter. I was surprised to discover this and suspect it may to be reflected RF waves off

the roof. Results of the tests will appear in due course.

It should be noted that FM folded dipoles (also known as Full Wave dipoles) theoretically

have slightly more than a 1dB increase in gain over a half wave type, but that was the

average increase in our tests. It was apparent that the folded dipole antenna did not pick up

DAB as well as the half wave, though it was still better than the “Omni” and far better than a

TV aerial, i.e. it`d work OK for DAB in a moderate signal area.

The FM folded dipole we stock come with a 2” clamp to enable vertical, horizontal or

diagonal mounting,also see wiring up an FM dipole. The folded dipole is probably the

strongest made of the FM aerials as it has none of the relatively weak FM directors and

the dipole (being a loop) is obviously stronger than an open half wave type.

For maximum FM signal, and maximum chance of rejecting unwanted signals, go for a

3 element FM aerial though unfortunately they are significantly bigger than the dipoles

which could be important in exposed locations.   

The size / wind loading of multi element FM antennas in particular can be problematic and

they are not of the strongest construction either. Also bear in mind that it is difficult to

mount a multi element FM aerial vertically polarised because the mast would affect its

performance. The only practical way to mount it vertically would be on a side pole off the

aerial mast, alternatively you could use it horizontally polarised.

Having said all of the above, it must be admitted that whilst the 3dB in gain that a 3 element

aerial gives you is certainly worthwhile in weak signal areas, it is the aerials directivity (i.e.

its rejection of unwanted transmissions) that is possibly more worthwhile. These`s no

escaping the fact that a 3 element antenna (particularly if it`s horizontally polarised * ) will

give you a more reliable and consistent signal than any omni directional aerial. However

directivity will obviously be a problem if you want to pickup more than one transmitter, and

the vast majority of people don`t need a multi element FM (or DAB) aerial anyway.

If you`re unsure whether to to for an FM dipole or an FM 3 element, bear in mind that you can convert a 3 element to a Dipole by simply removing the director and the reflector !

* A vertically polarised aerial will be more susceptible to picking up off beam transmissions

because to those broadcasts from the sides the antenna is effectively a vertical dipole.

The round FM “omni” type antennas do not perform as well as the half wave dipole and

this reflects their design, which gives a theoretical minus 3 dB gain figure. That`s why we

think they`re crap and we don`t stock them. To be honest I`m at a loss as to why there

are so many around, a lack of knowledge perhaps ? Or the fact more suppliers sell them ?  

A half wave dipole is a much better bet for FM and many people find it far more effective at

reducing “hiss” than an Omni. Don`t forget the half wave dipole also works pretty well on

DAB, unlike the Omni, although if you`re in strong signal area you will still get DAB off one.

Incidentally, the BBC “do not recommend circular aerials for national FM broadcasts”.

Obviously if you want the most DAB signal, and are not that bothered about FM, then go for

the DAB dipole because it does give more DAB response than the half wave FM dipole.

The vast majority of sites in this country (which are within a DAB transmission area) will get

a perfectly acceptable DAB signal from one of these aerials on the roof.

It was apparent that the DAB dipole was not that good for FM, far worse than you`d expect.

The explanation for this became clear in Autumn 2008 when we changed the model of FM

folded dipole which we stocked. The previous model of the FM folded aerial had a balun and

its DAB response was poor, whereas the new model has no balun and its DAB response

was much improved. Our DAB dipole has a balun and it is this which cuts down the FM.

If really short of signal then consider a 3 element DAB aerial. Bear in mind that DAB is

supposed to be a “single frequency network” (for the main stations anyway) so a dipole can

pick up a signal from the strongest available transmitter. A directional aerial will make this

that much more problematic although the 3 element models do give a bit of gain, generally

about 3 to 4 dB. Three element aerials do have some advantages though !

If you`re unsure whether to to for a DAB dipole or a DAB 3 element, bear in mind that you can convert a 3 element to a Dipole by simply removing the director and the reflector !

Just out of interest we tested TV aerials for FM and DAB reception and to be frank we

were rather surprised that they gave any significant signal at all. To be honest it goes

against all theory that they seem to pick up as well as they do. Maybe the boom of the

antenna is acting as an aerial, but the bottom line is that if you get a decent FM (or DAB)

signal off a TV aerial it is probably because you are in a strong signal area anyway.  

One things for sure, it`s even more proof that RF is a black art, it`s not a science.....

Installing more than one aerial on a pole (e.g. TV plus FM) ?

see article on this subject.

On the subject of FM polarisations it is possible to change from vertical transmission to horizontal very quickly, and this occurred when a fire in 2004 caused the Peterborough

radio transmitter to fall down !

Picture copyrights, left Joe Landy/MB21 (before, when vertically polarised),

and right, Justin Smith/ATV (after, when horizontally polarised.......).

Joke copyright, Andy Burton !

For further reading on aerials / antennas see Wikipedia & Astrosurf articles.

The following is an article by “Mike from Choice FM” which seems to make sense.

Make your own mind up !

Polarisation Horizontal or Vertical ?
A topic which still causes lot of confusion when discussing aerials is polarisation -    whether aerials should be mounted horizontally or vertically.
In the early days (1950s) of FM radio it was found that when the signal was transmitted   with horizontal polarisation it seemed to be much less severely affected by vehicle ignition interference. Those were the days when there were very few portable, or in-car, receivers so nobody ever bothered dealing with the problem of ignition interference at source.
However by the 1970s, with transistors having almost completely replaced valves and radios appearing in cars, vehicle manufacturers started paying attention to the problem of engine interference (no point having a car radio if all you can hear is the noise off the spark plugs) and most radio listening was now being done on portable radios with small telescopic  aerials which 'pointed up the way' (i.e. vertically).
Suddenly horizontal polarisation didn’t make much sense anymore and broadcasters  started reverting to either mixed (horizontal and vertical), or pure vertically polarised signals. Nevertheless, old habits die hard and many aerial installers and even manufacturers still insist that FM aerials should be mounted horizontally, the fact that most (but not all....) TV transmitters [main transmitters that is, not repeaters] use horizontal polarisation probably adds to the confusion. In a very small number of cases (where a station one wants to hear  is using mixed polarisation and there is interference from one using vertical polarisation) it makes sense to use a horizontally mounted aerial but, 99% of the time, aerials should be mounted vertically.

(1) = No balun

(2) = With balun

(3) = Polarity made little difference

All performance measures are relative to the (directional) gain of a 3 element FM

or DAB aerial ( = EXCELLENT).

For a table of the approximate dB readings for these tests click here.

A disintegrating multi element FM aerial is a common sight to those

of us who spend far too long looking at aerial installations,

do you really need one ! ?

This page is primarily concerned with FM/DAB aerials and reception issues, those who

want to know more about the content should look on MDS975.

Some of the FM aficionados who buy aerials from us assure me that FM has a superior

sound quality to DAB. Personally speaking I only tend to listen to Radio 4 and Radio 5 so I

cannot really confirm this ! But if DAB follows the same pattern as the picture on Digital TV

then this rumoured superiority of FM wouldn`t surprise me at all. That said, just like with

Freeview TV, this supremacy of the analogue system would only apply if you have a decent

signal. Those who have an  imperfect signal, and/or a low quality system, would probably

find DAB superior.

FM & DAB aerial dimensions

The FM half wave and folded dipoles

are both 59” (1.5m) high, or 59” wide

if horizontally polarised !

The 3 element FM is 59” (1.5m) wide

by 34” (86cm) long.

The DAB dipole is 24” (61cm) high.

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.

It doesn`t matter which wire goes to which terminal, or it shouldn`t !

I would add here that the above applies more for omnidirectional aerials, and less so for multi element directional FM antennas.

Wiring Up An FM Dipole

Separate DAB and FM Inputs on Your Tuner ?

Many tuners come with separate inputs for DAB and FM signals which obviously causes a problem if you`ve only got one combined FM/DAB signal. I`ve been onto most of the manufacturers but as of the moment none make a “setback” FM + DAB diplexer to split

out the FM and DAB signals. This leaves you with a number of options.

1 Use a 2 way splitter. This will obviously only supply half the signal to each input but I`d have thought that most FM/DAB signals (esp from an amplified distribution system) would cope with this OK. This method doesn`t split the frequencies into the correct bands for

each input but in most cases that wouldn`t cause a problem. Splitters are available with either Co Ax or F connectors.

2 Use a 2 way amp which includes the FM and DAB frequency range, preferably a variable gain model. As with a splitter this method doesn`t split the frequencies into the correct bands for each input but in most cases that wouldn`t cause a problem. Our 2 way mains amp would be suitable for this. I`d only use an amp if you`ve already tried a splitter and the latter adversely affects the signal.

3 Use a TV + FM + DAB triplexer, ignoring the TV output if it isn`t required. The one we stock is normally used outdoors but can be removed from its casing to reduce its size.

I have to say that from a technical aspect using the FM + DAB outputs from a triplexer is preferable to using a 2 way amp or a splitter.

4 Install separate FM and DAB aerials and run two cables down to the tuner. This is obviously the most hassle (and cost) but theoretically will give the best results. That said utilising the same two aerials and one downlead, but using two* triplexers, wouldn`t be that

far behind !

* One on the aerial mast to combine the signals, and one at the set back to split them out.

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