DAB via TV Aerial & DAB or Internet?

[CandT]

So I have a new Roberts Stream 93i arriving tomorrow to replace my ageing Pure Bug DAB. Obviously I'm pretty sure all DAB stations will have Internet streams, so my question one is : which of the two will be the better quality?

My second question is regarding the aerial. I discovered, like the 93i, that my Bug also has a screw-off aerial, exposing a female F Connection. I have a relatively new loft aerial with a hard connection in my office, and a spare wall connection, so I tested with the Bug, knocked up a quick F terminated cable, and I'm getting 100% signal strength on the radio. Am I ok to do this with the 93i too? I'm not somehow 'losing' stations by using the TV aerial, or is it just that the aerial and the copper cable between it and the radio is enough to pick up the DAB signal in the aether?

Just wanting to make sure I understand the mechanics of it.

This is my aerial, in case those in the know need to see it


I think this is a '5 bay x-beam' aerial. which I don't read particularly good things about, but on the TVs all around the house, I've got 92% signal strength, and 100% signal quality, so I must be in a pretty high signal area!!

 

[Lucid]

The fact that your TV aerial is on its side, and despite a loft location you're getting 100% signal strenfth tells me that you're probably in a very high signal strength area, and you're also working off a repeater TV transmitter or one of the very rare main TV transmitters that is vertically polarised.

Okay, there's a lot of information there so let's break it down. Trying to receive digital TV through slates or tiles will knock down some of the power of the signal. It will work okay though if you have good line of sight to the transmitter, which it looks like you have.

Next: polarisation. DAB signals are vertically polarised. That means the aerial must be pointing "up and down" to get the most effective reception. Your TV aerial happens to be mounted that way. If you were to turn your aerial so that the elements were horizontal then you'd lose a lot of the DAB signal strength as well as losing your TV signal.

The bit of your TV aerial that is picking up DAB is most likely to be the dipole. That's the thing that looks like a squished figure 8; and why it is picking up DAB is because the length and shape of it is close enough to a 1/4 wave DAB folded dipole to still work.

I'm not expert on making antenna. All I know is that with aerials you're trying to match the resonance of the source signal either directly or at some even fraction; so 1/2, 1/4, 3/4, 5/8 etc. The nearer the match and the more metal in the air then the stronger the potential signal. This means that a 1/2 wave dipole gives a stronger signal than a 1/4 wave, TTBOMK. The size of the receiving element is determined by dividing the speed of light by the frequency you want to receive. In the case of DAB that would give you a 1/4 wave dipole length of 37.5cm. A smaller dipole will still work but not give as much signal.

The coincidence of your aerial type and it's orientation and the high field strength where you live is what's giving you such good DAB reception.

As for internet versions of DAB stations, it all depends what sort of music rights and broadcast licence they have. Radio station DAB transmissions have a limited range due the nature of those frequencies of radio wave. If you like, you can think of it as geographic boundaries in a way. The Internet doesn't have that. So if your favourite DAB station us only licenced for music in the UK then it might not have and Internet version.

As for quality, that's anyone's guess. There are too many variables to make a broad prediction that works for every station.

 

[CandT]

That's fantastic - just the information I was looking for. In fact, I've just had a quick look at some of the surrounding houses. It's a new build estate, and there aren't that many external aerials, but all of the ones that are mounted externally are vertically polarised - we're running off Kilvey Hill here, in the high strength region for TV and DAB. I suppose the supplementary question is - will I do the 93i any harm on this aerial? (Probably a stupid question I know!)

 

[Lucid]

You'll be fine running off the TV aerial as far as DAB goes. If there are any stations where reception is a bit marginal then consider a dedicated DAB aerial.