FM-Dab aerial converter

[Tim Streater]

Sure but I assume a multi-element aerial tuned for the frequency band will give a better result. And pointing at the transmitter.

[Jock]

No, radio telescopes are dishes which aren't tuned.

[Tim Streater]

Well I hope not, otherwise they'll be picking up all sorts of broadcast shit.

[williamwright]

No, not possible. It can't alter the resonance of the aerial, and it's at the wrong end of the cable to correct the matching (because an end-fed quarter wave 100MHz whip is after all an end-fed half-wave whip at 200MHz, so it's more-or-less resonant but doesn't match the 50ohm feeder very well because the characteristic impedance is in the thousands of ohms).

Bill

[williamwright]

No you'd use an ATU. Not VHF but MF, but in the 1960s I used to use an ATU in the van for reception of the pirates and it made a lot of difference. I was feeding the signal into a portable radio because for a while I didn't have a car radio.

Bill

[williamwright]

Most radio telescopes are parabolic dishes, to which the concept of tuning doesn't really apply.

Bill

[williamwright]

Yes, I made a DAB quarter-wave a few weeks ago and I double checked the calculation. It's about a foot in fact.

[williamwright]

Yes it would. But a six element yagi aerial for 200MHz would be impractical on a car. It would be too large and would need constant realignment. The gain would only be about 8dB at most if the aerial was cut for just the upper half of the DAB band. If it were cut for the whole band the gain and directivity would be poor.

Some DAB is transmitted using a single frequency network (SFN) which would make the use of a directional aerial problematic.

Bill

[RJH]

Ah OK, thanks.

I suppose I was confused by the fact that DAB portables have what *appear* to be ordinary wire or telescopic aerials, but in fact they have a particular resonance. And that resonance coincidentally works for FM, but not the other way round.

[RJH]

How is it, then, that my clock radio works perfectly well? (it has a piece of trailing wire).

If you read Bill's post further down you'll find the explanation.

[RJH]

Thanks - I think I'll give it a try, not much to lose.

[RJH]

The DAB tradio unit means the tuner itself = the bit that sits in the dashboard and you control with your hand, voice etc. 'Calls for the required frequency' means only allows the required frequency as an input. You'd need to read your own post (above), plus the post you cut, to understand what that means in context.

OK.

[Tim Streater]

I was talking about fixed installations as I mentioned in a prvious post.

[The Natural Philosopher]

All pieces of wire pick up all signals. It is merely a question of how well.

[Dave Plowman (News]

I'd guess that tells you it would work better with the correct aerial, then?

In this part of London, a TV will work with a bit of wire dangling from the aerial socket.

But when reception conditions are poor - like they often are in a car - a decent aerial makes a world of difference.

It can be quite interesting (for a nerd) to have a test route where you know there are problems. And assess different setups that way.

[charles]

It's already good - how much better? Trouble is fitting a DAB aerial, SWMBO doesn't like me working on ladders at any height. so that stops it being a DIY project

[RJH]

Indeed - which was sort of my original question - would it be 'well enough' with whatever that thing I linked to is? And the answer is 'probably not'.

The unit's due next week and I've got a lot of long drives coming up, so I'll lash up something that at least works (it comes with a screen aerial) and take it from there.

[Dave Plowman (News]

The thing is that even maker's car radio installations vary. Living in London, I have no trouble finding marginal reception places in a car. Tunnels, high buildings, etc. And my everyday car which has a windscreen aerial on FM doesn't perform as well as the old car - with an older radio, but decent roof aerial. And the old car which has DAB provides even better reception round London. Hardly a surprise, as that's what DAB was designed to do.

[williamwright]

If Charles receives all the stations he wants with no drop-out, in what way would it be better?

Bill

[williamwright]

No they aren't. Some are arrays of dipoles. An example is the Murchison Widefield Array MWA radio telescope.

It does, in that there are parameters that vary with frequency. The gain and directivity are related to the size as expressed in wavelengths. The surface, construction, and accuracy of the reflecting dish has to be appropriate to the frequency received. At one extreme it can be mesh with quite large apertures. At the other it needs to be totally smooth with absolutely no deformations.

Bill