DAB aerial

Yes.

Yes too.

Ah. It does indeed have a balun. However, it works ok when vertical for DAB but not FM. That's what is confusing me. And I don't mean DXing - just local stations.

RF is and will remain a closed book to me. ;-)

I can't find such a beast on Comet's web-site. What model?

theres' a house near me whose just had a DAB aerial installed on a pole on the roof. it's the Maplin high gain one one, mounted horizontally!

Robert

Magicbox "Imp". I don't see it on their website either, so maybe it was a last-of-line sale item in that particular store. There may still be a few left in some stores, or if not there seem to be plenty of equivalent things - just Google search for "internet radio".

Rod.

Does that get them off the Sale of Goods Act's "Fit for purpose" requirements?

I'd guess that the folded dipole will be resonant at 3x the frequency it's cut for, whereas the simple dipole has resonance at 2x, in analogy with open and closed organ pipes.

No, it is still 3x for an open dipole (and 5x, 7x etc.)

Interesting. I'd have thought that there would be a difference, due to one being a short and the other being open. Looking into it, there's not much.

I suppose in terms of signal, the 1/4 wave dipole element is equivalent to a stopped organ pipe, with the tip acting as a reflection point for the signal. Or the whole half-wave aerial acting like a bowed string, with a (current) null at each end.

This would explain why, if there's a balun to cause better matching, it doesn't work as well at 2x its cut frequency. Without a balun, it takes on some of the characteristics of a whip aerial, as the co-ax braid (the downlead is usually vertically aligned) acts as part of the aerial.

As a folded dipole absolutely requires a balun to feed to 75 ohm downlead, it's probably a different type to the 1:1 balun you'd get with an open dipole, and due to its design would as a side-effect null out the 2x harmonic.

At least they were honest..!

Ivor

Some time ago you were raving about the Triax DAB 5 element. AFAICR it was because it wasn't wideband.

I bought one and pointed it due north from my location about 30 miles south of Sutton Coldfield. It's very good, bringing in about 100 channels (some duplicates) but also stations from Shropshire and beyond, for which we are well outside the service area.

The Birmingham stations are all "off the scale" whereas previously on the internal rod antenna they weren't even being decoded.

Interesting.

Bill

In article , Bill Wright scribeth thus

Not really. The difference an outside aerial makes from an indoor bit of wire is very significant. A while ago I was playing about with a Denon tuner in a Suffolk town not renowned for its signals, even mobile which can be quite good. An ordinary vertical FM di-pole @ 10 MAGL made the band come alive!..

If everyone were to use the right aerial then we could drop the ERP's by a lot and therefore "save the planet" i.e. bailing out the titanic with a teaspoon!...

If you pull in distant (i.e. over 46 miles) signals won't that harm the BBC SFN signal from your local TX? also you'll just get "n" copies of Gold/Chill/Galaxy/Heart/Traffic/XFM

No, the local BBC/D1 transmitter's signal is about 30dB up on any of the distant ones.

Yes I do, I just ignore them.

As a matter of fact I did a job today in a village called North Wheatley, out near Gainsboro somewhere. The house was in a bit of a dip and the resident had a portable DAB radio that couldn't find a station. The building is listed so the best I could do was a loft aerial. A vertical dipole cut for 225MHz brought in four muxes at good strength.

Bill

I also found that slighly moving a DAB aerial can make or break reception. I was getting nothing on Cambridge DAB, then I moved the DAB aerial down the mast about 12 inches and signal shot up to 80%.

Marky P.

Remember that it's the signal quality as opposed to the strength that matters. The two don't always correlate.

IIRC, it's a feature of DAB that receiving a second signal from a different transmitter can be a benefit as it adds to the original - rather causing multipath problems as with FM.

Up to a point. There is a guard interval that allows multipath signals up to a certain amount of delay (hence distance). I can't remember what that distance is, but I know it is less than it would have been if they hadn't gone for the "available now" option, but waited for the chip sets that permitted more, slower carriers.