ATV's tests of FM and DAB aerials
After years of contradictory theory, inconsistent manufacturers “advice” and our own variable experience we finally bit the bullet and in April 2007 (see "the catalyst" ! ) we instigated a series of comparative tests on DAB and FM antennas. Whilst we were at it can a TV aerial be used for FM or DAB ? We decided to find out, and lastly 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” (round "halo" type)
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. In the accompanying picture we have a diagonally polarised FM half wave dipole and a deliberately mis-installed 3 element DAB antenna. The latter bodge knocked about 3dB off the forward gain, which is what one would expect. Incidentally when the aerial was mounted at the top of the pole (i.e. only half of the mast is between the elements) approximately 1dB was lost (link), also see polarity of obstructions.
Or, to put it another way, almost all DAB aerials have to be end mounted, which limits how large you'd want them due to the resulting twisting forces in high winds. Thus we recommend all aerials with a windload over about 50N (at 80mph) are cradle mounted. [link]
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 a two way surface bracket.
FM and DAB aerial tests results table
(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).
Approximate relative dB readings for these tests
FM and DAB approx aerial gain differences
Such is the Black Art nature of RF all aerial test readings are approximate, in fact as far as I know no lab will guarantee accuracy better than +/- 0.5dB.
It must be admitted that these particular results are less accurate than that (unlike our TV aerial tests) but they still give a pretty reliable indication of the relative performance of the various antennas.
The “variable” figures result from the directional nature of the particular aerial. This doesn’t apply to the TV aerials, they’re variable because they’re not even designed to pick up VHF frequencies ! Note that if the 3 element aerials were pointing directly away from the transmitter you’d probably lose more than the - 5dB quoted for the FM3. Though you would be pretty unlucky if you were to align your aerial like that !
ATV's choice of FM / DAB aerials (plus musings on the above test 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 (sold here), preferably installed vertically so it will work omni directionally and will also pick up DAB pretty well too. FM gain is about 0dBd.
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.
Being a half wave dipole it naturally has a 75 Ohm impedance, i.e. matched to the cable and (most) receivers for maximum signal transfer. Thus an open dipole should not have a balun, if it does there’s something wrong somewhere !
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, generally older ones, 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 either a 2” rotating clamp or with two way surface bracket, though it should be noted that this aerial will work best when its full height is exposed to the direction of the transmitter. If mounting in a loft try to ensure the direction of the TX is through the roof rather than any end wall. Incidentally moving the aerial around its mounting 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.
Lastly, if the aerial is polarised vertically, some people have found a difference in signal depending which of the elements the cable’s centre core is attached to.
Also see wiring up an FM dipole.
It should be noted that FM folded dipoles (sold here) theoretically should have a bigger increase in gain over a half wave type than we found in our tests. In fact, there wasn’t much difference between this aerial and the FM Half Wave dipole and this is probably due to the lack of a balun in this model of aerial. A folded dipole has a 300 Ohm impedance whereas most cable and tuners are 75 Ohm, one can speculate that the increased signal from the folded dipole is to some extent lost by this mismatch, the previous (no longer available) model had a balun and about 2dB more gain on average. At some frequencies the folded dipole is a bit in front of the half wave but at some others it’s a bit behind. However, where the Folded dipole really scores is that it has probably the strongest construction of any FM aerial since it has none of the relatively weak FM directors and the dipole (being a loop) is obviously much stronger than an open half wave type. So if you live in a particularly exposed location, or you suffer from ample avians, this is the aerial for you, if not then I’d fit the Half Wave dipole.
The aerial is essentially omni-directional if installed vertically polarised and slightly directional if installed horizontally.
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.
For a higher FM signal, and increased chance of rejecting unwanted signals, go for a three element FM aerial (sold here) 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 if fitting a multi element aerial then directivity is the whole point of the exercise *.
It should be remembered that whilst the 3dBd 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. Or, put more simply, the aerial will be more directional if installed horizontally polarised.
For those who feel they really do need every last dB of signal (and most people don’t) we stock our highest gain FM antenna : the 6 element FM aerial. The aerial is similar to the 3 element version but has 3 more directors and is 6ft 10in long ! I would estimate that a 6 element FM aerial would give about 5 to 6dBd in gain. For general information on this aerial see FM3 (above).
Round FM "Omni" type antennas (also known as circular or halo aerials) do not perform as well as the half wave dipole and this reflects their design which gives a theoretical minus 3 dBd gain figure. Even so, if you’re in a strong enough signal area, you will still get reasonable FM from an Omni. However, basically, we think they’re crap so we don’t stock them. In fact I’m at a loss to understand why there are so many Omnis around.
Yes there are a lot of Omnis fitted, 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 apologies 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 and don’t forget the half wave dipole also works pretty well on DAB.
Having said all the above, which is undoubtedly true, an Omni (or even a TV aerial ! ) will still pick up DAB if your signal strength is high enough !
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 (sold here) because it does give more DAB response than the half wave FM dipole. DAB gain is about 0dBd. 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, particularly if it’s on the roof or (slightly less so) in a loft. 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 give very good FM reception 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.
This aerial is (theoretically) omni-directional.
If you’re really short of signal then you could consider a 3 element DAB aerial (sold here). Bear in mind that DAB is supposed to be a “single frequency network” (for the main stations anyway) so an omni dipole which can pick up a signal from the strongest available transmitter is preferable [but only if there is sufficient signal strength for their use], a directional aerial will obviously make that more problematic. The 3 element model is directional but does have about 3 dBd of gain.
If you’re unsure whether to go for a DAB dipole or a DAB 3 element, bear in mind that you can convert a 3 element to a dipole [which is omni directional] by simply removing the director and the reflector ! Thus the DAB 3 (an aerial which can be converted from a higher gain directional to a lower gain omni) is a good choice if you are unsure of the signal strength in your area.
For people who want as much DAB signal as possible (and most people don’t need a high gain DAB aerial) we stock a 5 element DAB aerial (sold here). I would estimate that the 5 element DAB aerial would give about 2dB more gain than a 3 element, i.e. about 5dBd.
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.
Just out of interest we tested how good TV aerials are 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.