Aerial for new Denon DM41DAB

[Andrew]

Just bought one of these from Richer Sounds (on offer).

It has a single F connector on the back panel marked FM/DAB 50 ohms. The 2006 Onkyo it replaces has separate FM coax and DAB F connectors.

In the box is an indoor aerial which is just a length of twin speaker cable, with an F plug on one end and a plastic dimple of some sort near the other where the two wires then separate and terminate in plastic tags for pinning to a door frame or whatever. The two horizontal sections are 30 cms long (each), and the manual shows it installed horizontally.

Shirley this would be a 300 ohm arrangement,and is neither correctly polarised for DAB or long enough for FM.

Interestingly it gets just as good an FM signal from Rowridge (where the loft aerial is pointing) as it does from Wrotham which is 180 degrees the other way, through the cold water tank, a cavity wall and the neighbours extension that was built with celotex insulation right up to the apex of the gable !

So, what should I do with the loft aerial to improve FM ? (R4 FM from rowridge is still poor using the horizontal loft aerial but perfect downstairs on my £5 Sony charity shop radio with its vertical telescopic aerial.).

How would you combine a DAB and an FM aerial onto one down cable (or is this a bad idea), and why does the connector on the DM41DAB say 50 ohms when coax is 75 ohms?.

Why do DAB receivers use an F connector anyway ? Isn't it just a cheaper form of coax connector ?.

[Andy Burns]

Or is the dimple a balun?

I have a loftbox that triplexes TV/FM/DAB onto one cable, and a diplex faceplate that splits TV on one BL connector and FM+DAB on another, the Pure radio is happy with FM and DAB into its single F connector.

[Chris Green]

It depends on the type of co-ax. 'Thin ethernet' co-ax is 50 ohms, I think it's just 'TV' co-ax that's 75 ohms.

Cheaper than what? F-type connectors are usually better than the nasty Belling-Lee ones that were standard in analogue days and are still quite widely used.

[Andrew]

It is only about a centimetre in diameter with a small hole in the centre where I can just make out the copper core of one of the cables. I think it is just an 'end stop' to stop the dipoles becoming longer if the the ends were pulled.

Ok, maybe I need to try that.

[Andrew]

Well, .. yes, but the only difference between an F plug and a coax connector (that I can see) that the F connector allows the cable dielectric? to almost butt up against the receiver connector, while the coax plug has a bit of polythene between the cable and the receiver.

[Harry Bloomfield, Esq.]

It happens that Andrew formulated :

They may be cheaper, but they are more reliable and much less lossy then Belling Lee, that's why satellite uses them exclusively. You will find almost all modern equipment uses F-plugs.

[Bill Wright]

Install a vertical dipole at FM length in the loft. It can be made from any reasonably stout wire (not bellwire). Don't fold the dipole. It should be 4ft 9" to 5ft overall, that sort of length, cut in the middle with the coax inner connected at the middle with the inner core connected to the top half and the outer screen connected to the bottom half. Fix it up using non-conductive rope or string. Keep it away from tanks and pipes. Take the coax away at a right angle, ie horizontally, for as reasonable distance. Use normal 75 ohm TV cable; Don't take any notice of it saying 50 ohm. That will work for DAB and FM.

Bill

[stephenten]

I can confirm that a FM dipole seems to work really well at DAB frequencies (Been there, done that, got the T shirt :-) )

Not surprising really when DAB is almost double the frequency of FM which means that a Quarter wave FM dipole approximates very well to a half wave DAB Dipole.

If like me you want to do the job properly to Professional standard, I'd lash a pole to the outside of the house, stick a vertical DAB dipole and a vertical FM dipole, use CT100/WF100 co-ax, combine the downleads using something like a Labgear PSF410, put terminators in the unused UHF and sat input, then connect with CT100/WF100 etc the combined output of the PSF410 to the DAB tuner.

[Andrew]

Thanks Bill,

slight problem, my house is 1976 with a 37 degree roof made of trusses, plus I have added 300 mm of insulation with a storage platform up on rails and home-made extender legs, so there is only 54 inches from platform to roof apex.

I already have a 4 element FM aerial up there, carefully and laboriously manoeuvred into place and horizontal (almost).

I have tilted it to 45 degrees (since rowridge is supposed to have vertical and horizontal FM transmitters. Maybe this will help the DAB side, but the signal strength went down from 8 to 6.

I'll try and make a single dipole as you suggest and see how it performs.

Someone did post here a similar design but put it inside a length of 40 mm kitchem waste pipe and used a push-fit plastic connector to join the two dipoles, but he ran his coax inside the 15 mm copper pipe that he used, and out the bottom.

Someone mentioned the lack of a balun at the time.

[stephenten]

Ah, that will be me! Its still on my pole right at the top!

Two lengths of copper tube with a plastic pipe coupler in the middle. The coax fed up the inside of the lower copper tube and connected to ends of the two tubes at the plastic pipe coupler.

Then the whole lot put in 40mm white waste pipe for weather protection. there are stop ends fitted at each end of the plastic pipe. A hole was drilled in the bottom cap for the cable. The copper aerial was about 1.5m long and the 40mm waste pipe was 2m long.

This so that I could get clearance of 0.5m between the bottom of the copper tube aerial and the top of the galvanised steel tube mast pole.

As mentioned earlier, it also makes a good DAB aerial. All DAB in the UK is vertically polarised.

in the case of FM, horizontal polarisation allows you to focus on one transmitter, If you align vertically, you then get all vertically polarised FM transmitters in all directions so good for getting more regional FM stations or having multiple sources of the R1,R2,R3, R$, R5 and Classic FM signals.

[Bill Wright]

Fix the top of the wire dipole to the highest place. If it touches the floor at the bootm end and runs off to one side a bit it doesn't matter. (In the days of VHF TV the half wave was 3 metres so we normally had to bend the bottom end of the dipole if it was in a loft.) The main thing is to take the coax away from the mid-point horizontally so it doesn't act as an unpredictable parasitic. Because there's no balun it's 'RF live' so it can cause funny effects. No, spellchecker, I did not mean Balkan.

Although this dipole is more-or-less a full wave for DAB, it shouldn't really work because the impedance at the mid point of a full wave is all wrong for 75 ohm coax. But it works well enough in practice. Obviously if you need every bit of gain you would have to use a proper DAB yagi. To generalise, the limitations to DAB reception given an aerial in the loft or on the roof is more likely to be co-channel interference than weak signal.

Bill

[The Natural Philosopher]

This works for me with TV and VHF.

I am sure labgear would do sonething. FM and DAB are close enough that one aerial will do for both

[Brian Gaff (Sofa 2)]

So many questions. The twin cable can be lower impedance if the conductors are closer together and as the temp aerial you mention is a compromise, then it probably won't matter. I suspect the F can be either 75 or 50 ohms, many connectors come in both sizes, and the difference is the internal spacing but in most domestic cases it probably would not matter if you used a couple of wires twisted together! However you mention specific transmitters. On a portable with a rod, I'd imagine this is a mono radio, and mono needs ten times less signal to make a hiss free signal than stereo does, at least that has been my finding generally when you are dealing with our crowded fm bands. You can use a diplexer on the input if you want two specific aerials, and also an amp if you want, but if there is a specific reason to get signals from a particular transmitter, much trial and error might be needed. Over the sea paths are notoriously variable, due to reflections from the sea itself changing with tide, roughness etc.

Most signals these days seem to be mixed polarity. I have a feeling that of vertical does not go as far as horizontal, not that it seems there is any proven reason, but that it just seems to be so to me! Brian

[Brian Gaff (Sofa 2)]

Actually that should in theory be a terrible mismatch since the high voltage points would be wrong when used at dab, however it does work, somehow, but goodness knows what the match and polar diagram is like in real terms. I had quite a good success with a fan dipole, ie two cut to the right length on the same coax but one fanned to the left a bit the other to the right. I guess, though when you consider its in a loft, I might just have been lucky. Brian

[Brian Gaff (Sofa 2)]

V beams can work well if you want to make it work in a particular direction as well, and of course, this saves height. Brian

[charles]

When the BBC added a vertical component to its Band 2 transmissions and called it Mixed Polarization, the vertical component seemed to travel over hills far better than its horizontal partner. DAB is VP only, probably because it was originally aimed at the motorist. At the time, most cars had VP whips for receiving aerials.

[Bill Wright]

The frequency ratio is 2:1.

Bill

[Bill Wright]

Even outdoors!

[Bill Wright]

See also

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Bill

[Andrew]

How long would those two arms (?legs) of the Y be ?