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DAB radio works better with aerial down

Discussion in 'Music Streamers' started by ad47uk, Oct 18, 2005.

  1. ad47uk

    ad47uk
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    :confused: :confused: I have managed to get hold of a DAB radio that was cheap from work and I mean cheap. the reason ti was cheap because there was not a box and no bits for it, so no power supply either. I got it for a Tenner.


    Anyway, it is a RED DAB radio, one that was made for Sainsburys.
    It works fine and the sound is ok as far as DAB goes, but what confuses me is when I put the telescopic aerial to it's full height, I get a signal error of about 30 and lots of break up. Now when I put the aerial almost down, with just one section sticking up I get a signal error of 0 and it works fine.

    This is on the Digital 1 mux as I only got the radio to listen to Primetime. I have just tried it on the BBC network and the same thing on that.

    Now in theory, the longer aerial should work better and it should be pointing straight up. so I am now confused. :confused:
     
  2. GaryB

    GaryB
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    Aerials are one thing where biggest is not always best! All aerials work best at their resonant frequency (known as full wave). If this is not possible, which it generally isn't as this results in an aerial several metres long, then the next best thing is an aerial that is a exact multiple of the resonant frequency (half wave, quarter wave, etc.). It's possible that shortening the aerial is giving a more efficient aerial as it them becomes a multiple of the wanted frequency. This is why, for example, the elements on a TV aerial are much smaller than those on an FM aerial.

    A couple of links that explain in a bit more detail.

    http://www.radioelectronicschool.com/reading/reading37.pdf

    http://en.wikipedia.org/wiki/Antenna_(radio)#Resonant_frequency

    Google for something like "aerial resonant frequency" for more.
     
  3. Chris Muriel

    Chris Muriel
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    Gary is correct.
    Telesopic aerials should be adjusted for maximum signal which usually means amking them a quarter wavelength. For example at 200 MHz this would be 37.5 cms ; however it won't be exactly that in practice.
    The velocity factor of the aerial and parasitic effects of whatever connects it to the tuner will normally mean that the real-world optimum is a bit shorter (or in some cases longer).
    It's actually a difficult adjustment because when you put your hand on the aerial rod you detune it and also it will change due to proximity effects of surrounding objects (including especially your body and metallic objects).

    Chris Muriel, Manchester
     
  4. ad47uk

    ad47uk
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    Yes, I know what you mean, I should have remembered that from my old CB days, when I made my own aerials.

    This radio have only got a telescopic aerial, which is a bit of a shame, but then, I only want it for in here (computer room) and if it was not so cheap, I would never have bothered getting DAB.


    It is not really that great.
     
  5. gilesme

    gilesme
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    Got one of those "free" DAB little mags - it said optimal length was 31cm (IIRC ?)
     
  6. Chris Muriel

    Chris Muriel
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    Well that equates to a quarter wave at 241 MHz. Taking into account both velocity factor and parasitics it could still be correct fro some radios / tuners.

    Chris Muriel, Manchester
     
  7. ad47uk

    ad47uk
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    The best signal is when the aerial is aout 21cm and is pointing away from the transmitter, which is strange.
    But it do depend on where the radio is in the house, so I expect it is something to do with this multipath thing.
     
  8. crater

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    As quoted in the latest DAB Digital Radio magazine: "the perfect length for a Dab digital radio aerial is 31.5 cm (based on the frequency used to broadcast the national commercial digital radio stations).".

    If you get the magazine, it's on page 15.
     
  9. Chris Muriel

    Chris Muriel
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    "the perfect length for a Dab digital radio aerial is 31.5 cm (based on the frequency used to broadcast the national commercial digital radio stations).".

    In theory, but that does not take into account extra inductance between the rod and the input to the 1st active device (e.g. base or gate of a trasnsistor or FET) nor velocity factor which will vary for different methods and materials of construction.
    31.5 cms should be a quarter wavelength at the resonant frequency.
    31.5 x 4 = 1.26 metres.
    300 divide by 1.26 equates to a frequency of 238 MHz.

    The factors I mentioned above means that the actual SYSTEM resonant frequency will be lower than that.

    Chris Muriel, Manchester.
     
  10. crater

    crater
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    Well, don't that beat all!

    I just take a feed off my TV aerial and I get perfect reception on all channels. So perhaps science isn't everything!
     
  11. Nick_UK

    Nick_UK
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    Science is about experimentation, which is what you did, and got a good result.
     
  12. crater

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    Not exactly, but a nice thought. I used the same connection that I had on my old (very old!) tuner-amp for FM.
     
  13. Chris Muriel

    Chris Muriel
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    Certainly that's the best thing to try first if you have that option.
    As soon as the guarantee runs out on my DAB portable which, like most DAB portables, doesn't have an external antenna connection, I'll take it apart and add an external antenna port / socket to it.

    Chris Muriel, Manchester.
     
  14. Nick_UK

    Nick_UK
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    Let me know how you get on. I'm thinking about doing the same on my Goodmans GSR80DAB battery-muncher. It's in the bathroom, so I don't really want to use any direct connections from outside. I was thinking about making up a re-radiating aerial close to the radio (probably in the roof space) connected to a good DAB aerial and amplifer. It works for GPS aerials in cars - might be worth a try !
     
  15. Chris Muriel

    Chris Muriel
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    Mine is the Sainsbury's "RED" radio.
    The way the telesopic rod is coupled to the 1st active stage may well be different on yours. However, I will report back when I do this.

    Chris Muriel, Manchester
     
  16. ad47uk

    ad47uk
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    You have got the same radio as me then, by the seems of it.
    It works ok, but sounds better with external speakers, since there is a slight h7um from the ones that are built in.
    But for what i want it for, it works fine.

    I would like to find a way to put a lager aerial on it, but at the moment I do not want to take it apart.
     
  17. Chris Muriel

    Chris Muriel
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    When I get around to disassembling mine, I am hoping all that is required will be a simple coupling loop to be wired to a 75 ohm antenna socket.
    It depends on how the input circuit is configured , e.g. with or without a balun and whether not AC coupling is used to the 1st active stage.
    (It helps that I am an electronics engineer and have some RF testgear).

    I'll post on this forum when I do it - possibly over the Christmas break.

    Chris Muriel, Manchester.
     
  18. AcerKev

    AcerKev
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    I do the same on my DAB tuner and get both nationals and both the NDEMs (Leicestershire and Nottinghamshire).

    My Sony XDR1 is best when the aerial is pushed fully down, and laid horizontally.
     
  19. ad47uk

    ad47uk
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    It depends where I put the radio in the house.
    In here which is the front bedroom of the house, I have to do almost the same thing as you, put the aerial almost right down a lie it horizontal across the top of the radio.


    In my back bedroom, I got to put the aerial halfway up and point it in a certain direction.

    If I take the radio downstairs, which is not very often, then I have to put the aerial almost all the way up and point it in the opposite direction to what I do in the back bedroom.

    Very strange, I suppose the signal bounce affect is what causes these things.
     
  20. Nick_UK

    Nick_UK
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    Yes. The signals bounce off all sorts of things (metal objects cause the most bounce), and some of these signals arrive at the radio completely "out of phase" with the main signal, and cause an apparent "weak spot", because the two signals have a cancellation effect. Move the radio a few feet in any direction, and the phasing effect will change. Unfortunately DAB signals aren't terribly strong to start with, so the effect is more noticeable than it would be on (say) VHF/FM.
     
  21. Chris Muriel

    Chris Muriel
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    Can anyone work out how to get the **"£% thing apart.
    (I wish I was more mechanically minded)

    Chris Muriel, Manchester
     
  22. Chris Muriel

    Chris Muriel
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    Sorted it now. I particularly wanted to get a Liverpool based mux which has the 3C country music station on it.
    Best results for using / fitting an external aerial socket were with the whip aerial totally disconnected , connecting (soldering) the screened cable that originally went from the base of telescopic rod to an IEC aerial socket instead.
    There is no balun and the ex TV antenna is 75 ohms unbalanced too so there are no impedance matching problems.
    I now have the mux I wanted via an old Band III TV aerial, mounted vertically in the loft.

    Chris Muriel, south Manchester
     

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