Hillsborough
Sheffield
S6 2LL
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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.
Note, the DAB dipole is of the folded variety but is seen side on so this is not immediately apparent.
We know the 3 element DAB is
kinked round towards the front,
but we`re not setting the whole
thing up again just for that !
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
We are more than willing to give advice to those actually purchasing from us.
Could those only seeking information please just ring an aerial installer local to them or try calling reception advice on 08700 100 123 (BBC) or 0844 881 4150 (ITV).
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.
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. 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 quite 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 (upwards of 1GHz with wavelengths of 2cm or less) no significant signal will make it through the slates.
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-
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-
we said it would. It did.........
Note that we do not pretend that these are scientifically accurate tests.
This is what happened in our location, and it supports the
experiences of our customers, in the real world.
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.
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 !
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 the best all round aerial,
particularly if you want FM and DAB, is a half wave FM open dipole, preferably installed vertically so it will work omni directionally and will also pick up DAB pretty well too.
The model we sell will mount both horizontally and vertically, but it is also possible to use
it diagonally. Utilising the latter polarity may help bring in a particular frequency that you
are struggling with so it`s always a useful 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. 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. The aerial is supplied with a 1.5 inch rotating clamp but a 2” is a no cost option, please ask for this if required.
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.....
It should be noted that FM folded dipoles should theoretically have slightly more gain
than the half wave type but the increase was too small to really show up in our tests.
It was apparent that the folded diploe antenna did not pick up DAB very well at all,
and similarly the DAB dipole was not that brilliant on FM. The dipole we stock can be
mounted vertically or horizontally.
Obviously if you want more DAB signal, and are not that bothered about FM, go for the
DAB dipole, or if really short of signal a 3 element antenna. 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 small amount of gain, about 3 dB. Unfortunately they are significantly bigger than the dipoles which may be significant 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 so we do not supply them via mail order. 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 ! ?
The round FM “omni” type antennas do not perform as well as the half wave dipole and this reflects their design, which gives a minus 3 dB gain figure.......
We tested TV aerials just out of interest 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 one things for sure, it`s even more proof that RF is a black art, it`s not a science.....
Perhaps the bottom line is if you get a decent FM or DAB signal off a TV aerial it is
usually because you are in a strong signal area anyway !
If 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 !
For further reading on aerials/antennas see Wikipedia & Astrosurf articles.
The following is an article I found on the internet, it 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 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 TV transmitters use horizontal polorisation
adds to the confusion).
In a very small number of cases (where a station one wants
to hear is using mixed polorisation and there is interference from one using vertical
polorisation) 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).
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 ! ?
