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FM fading

Discussion in 'Radio and Wireless' started by Deigh, Apr 9, 2018.

  1. Deigh

    Deigh

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    Apr 26, 2011
    I live in a ground floor flat with a corrugated iron roof plus aluminium window and door frames. This is an ideal Faraday Cage and gives me problems with FM reception. I can't use the TV aerial because it is the wrong type so I have ended up with a wire home made contraption outside the nearest window. It works very well most of the time, but occasionally the signal fades and if I approach the set and put my hand on it the volume is restored. Obviously I am becoming the aerial. I can't find any fault with the co-ax wire to the window and obviously the home made aerial works most of the time.
    Can anyone suggest the cause of the problem and how I can overcome it? I've tried to blame sunspots and UFO activity but my wife is unimpressed with my explanations.
    Deigh
     
  2. davenn

    davenn Moderator

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    Sep 5, 2009
    so show us photo(s) of this "antenna" you made

    I'm not surprised ;)
     
  3. Deigh

    Deigh

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    8
    Apr 26, 2011
  4. davenn

    davenn Moderator

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    Sep 5, 2009
    OK

    so does any other FM radio pick up the 105.4 station OK ?
    how far is this station from your location?
    what hills other line of sight blockages are there between your location and this station ?
     
  5. Audioguru

    Audioguru

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    Sep 24, 2016
    Your antenna must be 'broadside" toward the radio station's location or the location of its signal reflection.
    Get the radio station from the internet instead.
     
  6. duke37

    duke37

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    726
    Jan 9, 2011
    The aerial is close to the window which, I think has a metal spacer separating the glass.
    The aerial is fed at a corner so the sensitivity will be maximum at 45 degrees to the horizontal. Try with the feed at the bottom or side. What polarisation does the transmitter use?

    A reflector could be added to make a cubical quad which will have a drive impedance of about 50Ω.

    The feed impedance of the loop will be about 300Ω. You can use 300Ω balanced feeder and then a balun to match the radio. Baluns were often used to match loops to portable TVs, I found one in the garden which seems to work on band II. I do not think that this will make a lot of difference.
     
  7. Deigh

    Deigh

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    Apr 26, 2011
    Thanks for that but I'm not allowed to do much interference with the outside of the flat and change the appearance. I've already had hints that the cross is 'over the top'.

    A car is an ideal Faraday cage and radio reception is obtained with a roof aerial. Would fitting a car type aerial to my porch sticking upwards overcome my reception problem?
     
    Last edited by a moderator: Apr 27, 2018
  8. duke37

    duke37

    5,256
    726
    Jan 9, 2011
    A quarter wave vertical as used on a car should be fed against an 'earth' which is the car roof.
    For a simple earial, I use a vertical dipole, fed in the centre. This will have a gain not a lot different to your loop.
    The verticle is made from 300Ω ribbon cable. Take a piece 60 inches long and put a short 4.5 inches from each end. Cut the centre of one side of the loop and connect any length of 300Ω feeder. I hang it in the loft with a drawing pin but it could be taped to a bamboo cane. A balun will help.

    The folded dipole is supposed to give a wider bandwidth than a centre fed rod but i have no means of measuring this.
     
  9. davenn

    davenn Moderator

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    Sep 5, 2009

    you didn't answer my earlier questions


     
  10. Hopup

    Hopup

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    Jul 5, 2015
    You could try using one of those cheap active antenna boosters or possibly lift the antenna higher.
     
  11. Deigh

    Deigh

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    Apr 26, 2011
    Duke37.........thanks for that ...

    Have you pix of this? Difficult to understand the 'short 4.5 inches from each end'.
    Deigh
     
    Last edited by a moderator: Apr 27, 2018
  12. duke37

    duke37

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    Jan 9, 2011
    This is just a folded dipole, the short is due to to the velocity factor of the ribbon cable
    imgFMaerial001.jpg
     
  13. davenn

    davenn Moderator

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    Sep 5, 2009

    I'm really not impressed with your lack of response !! :(
    But since I am a nice guy, I will still try and help you

    @duke37 , you don't really need to do a short in from the ends like that just do the shorts right at the end of the line at the right length :)

    to calculate a 1/2 wave dipole .......
    300 / 105MHz = 2.858m ( full wavelength)
    2.858 / 2 = 1.43m ( 1/2 wavelength)
    hat then needs to be converted to an electrical 1/2 wavelength
    1.43 x .95 ( .95 is the approx. Velocity Factor) = 1.358m

    So get some 300 Ω ladder line as Duke suggested ( sometimes the plastic is solid all the way in the middle .... it makes no difference )

    [​IMG]

    Ladderline FM Band Antenna.JPG


    Dave
     
  14. duke37

    duke37

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    Jan 9, 2011
    @davenn
    The idea of the short is that the overall length of the aerial is half a wavelength and the short is placed where the line is resonant, taking into account the velocity factor. The value I have for Vf is 0.85, this is presumably for the solid plasic type, the ladder line will have a higher Vf, you have used 0.95 so the short can be right at the end.

    The FM band is quite wide so the dimensions will be a compromise.
     
  15. KJ6EAD

    KJ6EAD

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    Aug 13, 2011
    You can buy a premade folded dipole for a few dollars new or find one at a local thrift shop for less since they're packaged with every stereo receiver sold and often donated, sometimes unused. For best reception, the top of the T needs to be perpendicular to the signal source.
     
    davenn likes this.
  16. davenn

    davenn Moderator

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    Sep 5, 2009
    If you read my post, you will see I have already taken Vf into account ...... my style is just the standard folded dipole

    either way works .... mine is just easier ;)



    not really, anything cut for mid band will work well right across the band .... after all it is receive only :)
     
  17. Deigh

    Deigh

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    8
    Apr 26, 2011
    Thanks for the drawing, will have a try at it sometime, Obviously the aerial is the weak point in my setup. Deigh
     
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