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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.
Before investing in an aerial for your radio I`d check it`s actually got an external aerial input connection ! Some of the cheaper radios (FM and/or DAB) can only be used with the supplied "set back" aerial because they lack an external aerial input socket. See this Forum thread on this subject. That said, in some cases (particularly for DAB radios) the supplied aerial unscrews and reveals a female F connector socket onto which a lead from a decent aerial can then be screwed. When it comes to aerial input connections, most FM tuners have a male CoAx connector socket, i.e. the aerial input lead requires a female
Co-
DAB tuners invariably have an F Connector socket for the aerial input.
Of the supplied "set back" radio aerials the worst are the single wire types, though as mentioned frequently on this site, signal strength is almost everything. If you live in a strong signal area even a crappy single wire aerial may well work fine ! "T type" wire aerials work better than the single wire types but a decent FM or DAB aerial will usually out perform a "T Type" even if it`s just propped up against the wall by the side of the radio ( ! ) though any aerial will work better in the loft and better still high up outside.
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-
Note the cabling is just "draped" and we have utilised an L section “loft kit” because the installation is only temporary. We would not normally recommend using a loft kit for an aerial outside. Having said that the windloading on an FM or DAB dipole is pretty low and, in fact, the mounting boom of a Half Wave FM is 1" so, if required, will fit straight into the mounting bracket of our loft kits.
(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, and 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 ! ?
mounted with
the pole
here
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 decent 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 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.....
Some crude tests at home indicated that a Half Wave dipole was giving about 5 to 10dB more (depending on frequency) than one of those single wire setback aerials that some small radios are supplied with. So it is entirely possible that putting a Half Wave Dipole in your room (as opposed to outside or in the loft where it will undoubtedly work much better) may be sufficient to improve your signal over the supplied setback aerial, make sure your radio has an external aerial socket though !
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, particularly this report.
For maximum FM signal, and maximum chance of rejecting unwanted signals, go for a three element FM aerial though unfortunately they are significantly bigger than the dipoles which could be important in exposed locations. Furthermore the size / wind loading of multi element FM antennas can be problematic and they are not of the strongest construction either, particularly the directors and reflector. Lastly it is not generally practical to mount them vertically, though this is a bit irrelevant anyway because because if fitting a multi element aerial then directivity is the whole point of the exercise *.
It should be remembered 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.
In May 2011 we started stocking a 6 element FM aerial (not pictured) for those who feel they really do need every last dB of signal, though most people don`t need a high gain FM aerial. The aerial is similar to the 3 element version but has 3 more directors. I would estimate that a 6 element FM aerial would give about 2 to 3 dB more gain than a 3 element.
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 dBd gain figure. That`s why we think they`re crap and we don`t stock them.
To be honest I`m at a loss to understand 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. Which begs the question, on what day did The Lord create chipboard, and would it not have been better if he`d rested on that day ?
(with apologise to Spinal Tap)
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. That said, the great majority of sites will get a perfectly acceptable DAB signal off an FM Half Wave dipole and fitting the latter would get good FM as well. On the other hand 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 primarily 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. However, 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 directivity.
Three element aerials does have one other advantage 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 !
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 frequencies (e.g. TV) being much more attenuated than lower frequencies (e.g. FM).
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-
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.
It should be noted that FM folded dipoles (sometimes -
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” rotating clamp to enable vertical, horizontal or indeed diagonal mounting. (Also see wiring up an FM dipole)
Basic Aerial Facts incl :
FM v DAB including : FM switch off, or not.......
Historically 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, or DAB, and, unlike FM, it is transmitted on a Single Frequency Network.
It is important not to get DAB confused with the radio channels which are transmitted as part of the Freeview package, that is a different system.
The frequencies of the bands (and their corresponding wavelengths) are given below :
Long Wave -
Medium Wave -
VHF/FM -
DAB -
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-
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
eceived 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.
Cable for FM / DAB should be 75 Ohm Co-
Finally remember that it is possible to Diplex FM/DAB signals onto TV downleads or into a TV distribution system.
Some sources say that for FM you should be aiming for a signal level between 60 and 75dBμV at the tuner input but most tuners will work fine down to 40dBμV (some even lower) and that includes the additional signal [15dB or so] that stereo decoders require.
Arguably a more realistic ideal about 50 to 55dBμV.
For DAB the quoted ideal is 40dBμV to 70dBμV but again many tuners will work perfectly well on less than this, say down to 35dBμV. Aim for 55dBμV ?
Whilst testing FM and DAB aerials on our back wall, not, it should be noted, up on the chimney, we were getting FM figures of 45 to 50dBμVs and DAB at 35 to 40dBμVs and the tuners were working fine. The FM half wave dipole on my roof gives mid/high 50s dBμV on both FM & DAB, interestingly it didn`t give much less in the loft. I also have a little FM/DAB radio in our kitchen which operates off its own poxy little aerial, the FM is probably in the mid 30s dBμV for FM and DAB yet it still works OK, though the FM may not be in stereo, I wouldn`t know because the speakers are only 2” apart. Incidentally, what is the point of having two speakers only 2” apart !
What all this means is that in the real world so long as you`re getting a decent quality signal from a dedicated FM aerial, you can split it quite a few times and, nearly always, it`ll still work fine.
Here are all the commonly available 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 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.
Radio Bands (MW, LW, FM & DAB)
External aerial input, have you got one ? ! ?
Loft mounting FM / DAB aerials (it often works well)
also see :
ATV`s Tests of FM/DAB Aerials including :
ATV`s Choice Of FM/DAB Aerials including :
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.
There is one seldom mentioned advantage of the DAB system. I`m told that down in London, and some other big cities, there are so many pirate stations around that getting interference free FM reception can be a little frustrating, but, at the moment, the pirates haven`t cracked DAB. Some cynics reckon that the powers that be are slow timing on prosecuting the pirates to try and force people to switch to FM !
In June 2009 the Government published its "Digital Britain" report in which it said it would like to switch off all national analogue radio in 2015, but they`re only talking national FM transmissions anyway (BBC Radio 1 to 4 and Classic FM), not local ones, and local includes the BBC local radio stations. So don`t go thinking FM broadcasting is going to end anytime soon ! (reconfirmed by Ofcom Aug 2012).
There are no plans at present to cease local radio FM transmissions.
Furthermore, this FM switch off is an aspiration it`s not set in stone by any means. 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) and at least 50% must be actually using digital radio. This position was repeated by the Government in July 2010. In fact at the 2010 Retra conference Leslie Burrage (of Roberts Radio) reckoned that the 2015 target was not achievable, he thought that 2020 was more realistic and even that could be ambitious. He went onto say that converting all car radios to DAB was the biggest challenge.
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. Some people doubt the FM signal will ever be turned off because low band VHF isn`t that suitable for mobile Internet (unlike UHF), so it`s not so worthwhile for the Government who are primarily concerned how much money it`d raise, particularly as the spectrum doesn`t seem to be worth as much these days !
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 !
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.
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 !
Lastly, one other advantage of a vertically polarised aerial is that perching birds are less likely to be a problem, which is more significant if :
1 Your aerial is a Half Wave dipole (they`re not as strong as the folded dipole).
2 Your car is a dark colour and parked underneath said horizontally polarised aerial.
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 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. Using an amp to split FM and DAB should be a very rare occurrence, I`d only try it if you`ve already tried a splitter (preferably a diplexer or triplexer) and the latter adversely affected 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, see diplexers v splitters.
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.
I would add here that the above applies more for omnidirectional aerials, and less sofor 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 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-
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 !
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).
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For people who want as much DAB signal as possible (and most people don`t need a high gain DAB aerial) in Sept 2012 we started stocking a 5 element DAB aerial.
I would estimate that the 5 element DAB aerial would give about 2dB more gain than a 3 element.
Note : In the case of this aerial we would recommend end mounting it because mid mounting would put the pole between the (vertically polarised) elements thus negatively affecting the gain.
