Combined FM/DAB aerial

[Dave Plowman (News)]

Does anyone make a combined FM/DAB aerial (Band II&III) for external use? Some tuners that do both have only one aerial input.

[Andy Hall]

I looked for one a few months ago without success. Generally the DAB antennas seem to be 1 and 4 element. I ended up with a 4 element log periodic which seems to work well.

However, AFAIK, the DAB transmissions are vertical polarisation - at least that's the orientation of the antenna - whereas the Band 2 ones are usually horizontal. I suppose this would mean that a combined unit would have to have orthogonal elements.

The combining issue can be handled differently though.

When I added DAB to my distribution set up, I changed a diplexer filter which had been combining UHF signals from two antennas (via filters) with band 2 to a triplexer filter which has band pass ranges of Band 2, DAB (215-230MHz) and UHF. There are three inputs.

For the outlet plates I was able to retain diplexer outputs. These have VHF (everything below 300MHz) and UHF (everything at 470MHz and up). Some diplexers cut off VHF at 150MHz so wouldn't work well for DAB.

At any rate, I think that this could all be made to work with a triplexer, wideband amp and diplexer.

Alternatively, there are VHF combiners if you just want that. Have a look at

Product No. SCC212/DAB/FM

[john lyon]

You do not really need two aerials, one band works around 100MHz and one

200MHz, so both are harmonically related. A 1/4wave on 100 will be a 1/2wave on 200MHz. Unless you are going for 50-60miles away, local DAB stations will be picked up on a standard dipole designed for 88-108MHz with no problem. It's perfectly acceptable for receive purposes. Don't buy the nasty B&Q ones as the self tapping screws allow water in to the junction box! It's a bad design fault. You can use a dipole for 88-108 with your TV aerial in to a diplexer/distribution amp if you must, then split it in each room to either TV/VHF or even TV/VHF/DAB.

[Andy Burns]

I thought FM used mixed or slant polarisation? Or is that only local stations?

[Dave Plowman (News)]

Horizontal polarization used to be the case for main FM transmitters with vertical for local ones IIRC, but surely most are now combined for car and portable use? I'm also not much concerned with DXing distant transmitters.

[Andy Hall]

AFAIK, it's local stations

[Dave Plowman (News)]

This was originally my thoughts but with a 5 element FM aerial I'm getting occasional DAB reception issues - even although in a high strength area of South London with a direct view of both Crystal Palace and Croydon.

However, it's more complicated.

I have two aerial installations feeding different parts of the house - mainly through history. The one on the highest part of the house with just an FM aerial for radio works fine on DAB.

The one at the back, the one in question - which has easy access via the roof terrace - doesn't. (The back of the house faces south so the main building isn't in the line of sight to the transmitters)

Thinking I had simply some form of fault - although FM strengths were excellent - I removed the FM aerial and checked it. I also replaced the downlead, just to be sure. Tried vertical alignment instead of the original horizontal. No - or little difference - to the DAB signal with still the occasional 'boiling mud' issues. Of course tried the other DAB tuner which works fine on the other aerial.

Bought a DAB aerial. DAB now fine, but FM noisy. ;-)

Thing is I don't really want a rats nest of aerials so close to the roof terrace and fitting both current FM and DAB aerials would mean a longer pole to clear the chimney pots on the stack while allowing reasonable spacing. I'm wondering about cutting down the FM aerial to just the main folded dipole as that could be fitted vertically and still look ok - but was hoping for a neater all in one solution.

[RobertL]

I thought they changed to circular polarisation many years ago as a result of the increased use of FM radios in cars. Car aerials pretty much have to be vertical. That would mean that you can mount the FM aerial at any orientation.

perhaps you coul dadd some vertical 'twigs' to the FM aerial.

Robert

[Andy Hall]

As you were. I just found a BBC reference that main stations radiate with mixed polarisation.

Having said that, one doesn't see vertically oriented Band 2 antennas on houses.

[tony sayer]

In article , Andy Hall scribeth thus

No Mr Hall, there are very few Horizontally polarised transmitters left in the UK they are now predominantly Mixed polarisation at main sites, and Vertical only in the case of lowered power stations..

As the OP is a great DAB fan one wonders why he's bothering with VHF FM;?

In a lot of the UK there is sufficient signal strength so that a FM band 2 dipole vertically polarised will pick up usable DAB signals...

[Andy Hall]

That may well depend on which FM programs you want. While you might have enough signal from anything sited at Crystal Palace or Beulah Hill to operate an electric fire, the remaining FM stuff is from Wrotham, which I guess is about 30km from you

[tony sayer]

In the main BBC locals are mixed or rarely these days, slant. ILR with powers up to and about 100-200 watts then to use vertical and above that generally mixed..

[tony sayer]

In article , Andy Hall scribeth thus

Except that there is an FM relay at Crystal place in Dave's back yard;!...

[Andy Hall]

So I've just discovered.

Even so, one doesn't see band 2 antennas deployed vertically on houses. Perhaps that's habit by the installers?

[Dave Plowman (News)]

Just shows you chose not to understand *any* of the arguments.

With you - and others - DAB bad, FM always perfect.

I want both to be available under the best conditions - but really can't be arsed with another DAB v FM argument especially on a sensible group like this.

[Dave Plowman (News)]

The vast majority you see round here are those halo thingies which are only for local reception - so presumably should be vertical? But never are.

[Andy Hall]

[Dave Plowman (News)]

Pretty pathetic on a portable radio. Nothing like as powerful as the genuine local stations broadcast from the same area.

[tony sayer]

Yes, for installers .. read cowboys;-(...

[tony sayer]

No but ..sufficient..