Hillsborough
Sheffield
S6 2LL
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DAB and VHF/FM aerials amounted
on one pole. I cannot take credit for
this particular brainwave because it
was Pat (one of the aerial installers)
who came up the idea.
The exercise is particularly worthwhile
to appreciate the difference in size
between the various antennas and the
potential for mounting them using
different polarities. A picture of a
diagonally mounted FM dipole is
displayed below and this polarity
(along with vertical and horizontal) is possible with the model we sell.
FM & DAB (Digital Audio Broadcasting) Aerials
Subjects are listed on this page in the following order :
ATV`s Tests of FM/DAB Aerials (also see ATV`s tests of TV aerials)
ATV`s Choice Of FM/DAB Aerials
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 -
Medium Wave -
VHF/FM -
DAB -
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.
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-
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 !
(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.
Also see aerial report for FM Half Wave Dipole v Omni.
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 ! ?
On the subject of FM polarisations it is possible to change polarity fairly 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 !
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.
In June 2009 the Government published its "Digital Britain" report in which it said it`d like to switch off all (national) analogue radio in 2015. Remember they`re primarily talking national radio here and in any case this is an aspiration it`s not set in stone by any means, or it wasn`t when I last checked with Ofcom. Even the Government have said they won`t proceed until at least 90% of the population has DAB coverage, and that coverage must at least match FM`s.
I think we should bear in mind that as of the first quarter of 2009 digital radio (that`s DAB, Internet and Freeview) only accounted for 20% of all radio listening, so there`s some way
to go yet. Many people aren`t too happy about this proposed analogue radio switch off,
so we`ll see what happens....
Join the Campaign to save AM/FM.
However the fact that radio DSO may happen does mean that the FM half wave dipole aerial`s relatively good performance on DAB (and the fact it can be easily cut down to improve DAB still further) is a point in its favour !
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.
mounted with
the pole
here
It doesn`t matter which wire goes to which terminal, or it shouldn`t !
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.
For further reading on aerials / antennas see Wikipedia & Astrosurf articles.
If you`ve found this site informative and, hopefully, interesting as well,
Also see wiring up an aerial with a balun.
I would add here that the above applies more for omnidirectional aerials, and less so for multi element directional FM antennas.
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 -
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-
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
Above left we have Pontop Pike`s mixed polarity FM transmitting array, this transmits at 134kW.
Above right is Acklam Wold (just south of Malton in Yorkshire) which transmits vertically polarised at 2kW, the two vertical transmitting antennas are clearly discernible.
Acklam Wold, Transmitter, View and The Stoat (or why it`s better to walk than drive....).
How often do you see a Stoat ? How often do you get a chance to take pictures of it ?
Especially a photo when the Stoat is up close and looking straight at you !
Who says that visiting transmitters is boring !
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.
See Aerial Reports for the Half Wave Dipole.
It should be noted that FM folded dipoles (sometimes [incorrectly ?] 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 go 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, but, of course, just
because there`s a lot of something around
it doesn`t necessarily mean it`s any good.
Take chipboard for example. A building
material which has negligible physical
strength and isn`t even waterproof. It`s absolute crap, yet it`s used bleedin` everywhere.
A half wave dipole is a much better bet for FM than an Omni and many people find the
former is far more effective at minimising “hiss” than the latter. Don`t forget the half wave
dipole also works pretty well on DAB, although if you`re in strong signal area you will
still get DAB from an Omni, and reasonable FM as well come to that.
Incidentally, the BBC “do not recommend circular aerials for national FM broadcasts”.
See aerial report for FM Half Wave Dipole v Omni.
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.
One assumes that if a DAB dipole has no balun its FM performance would be significantly better, though it would obviously still be inferior to an FM dipole. On the other hand the lack of balun may well mean its DAB performance would suffer a little bit.
See Aerial Reports for the DAB Dipole
If you`re really short of signal then you could 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) and consequent directionality.
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) ?
FM Horizontal Or Vertical Polarisation ?
There is an article reprinted below which goes into the whys and wherefores of FM polarity and I agree with it generally speaking, consider this section a preamble to it.
As regards BBC National FM network all the main transmitters are mixed polarity like Pontop Pike in the picture below left. Many of the relay transmitters were vertical polarity
like Acklam Wold in the picture below right. This transmitter actually broadcasts BBC
Radio York (rather than the BBC National FM stations) but it`s a clear well lit photograph
which shows up the transmitting arrays well !
When I last checked, of the 114 BBC National FM transmitters 49 were mixed polarity,
this is 43% of the total, but this accounts for far more than 43% of the audience because this figure includes all the high power main transmitters.
Of the others, 63 were vertical polarity, which is 55%.
Only 2 were horizontal polarity, and both of the latter were small transmitters (20W & 30W).
dipole aerial
(vertically polarised)
DAB aerial
If the correct aerial is used then it will work pretty well for both FM and DAB if installed vertically polarised, also 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-
Cable for FM / DAB should be 75 Ohm Co-
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.
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-
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-
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.
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 television signals.
Most FM is transmitted in vertical (or mixed) polarity and all DAB transmissions are vertically polarised, see BBC radio transmitters.
