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How antennas receive AM radio signals

  1. Sep 7, 2012 #1
    The greater the amplitude of a given frequency of light hitting an antenna the greater the induced speed of electricity in the antenna? Reply in simple english please.
     
  2. jcsd
  3. Sep 7, 2012 #2

    vk6kro

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    No, that applies to hitting golf balls, but not to antennas.

    The speed of the wave in an antenna is comparable with the speed of light and it depends on the dimensions of the wire but not on the amplitude of the signal.
     
  4. Sep 7, 2012 #3
    Then how does the energy from the amplitude of the light transmit onto the antenna? I appreciate the clear writing.
     
  5. Sep 7, 2012 #4

    vk6kro

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    A stronger signal field strength results in a larger output signal from the antenna.

    So, if the field strength is varying like a sinewave then a stronger field will produce a bigger sinewave output than a weaker one.

    But the velocity of the wave in the antenna stays the same. Weak signals travel just as fast as strong ones in space and in an antenna.
     
  6. Sep 7, 2012 #5

    davenn

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    the energy of light doesnt transfer onto an antenna
    you have a little confusion I suspect

    Tho an electromagnetic wave is composed of moving photons, those photons are are not visible light photons.
    An electromagnetic E-M (radio wave) wave moving through space ( the air etc) will induce an electrical current into the metal wire/rod of the antenna ( or any other conductive surface it encounters). To get maximum efficiency of energy transfer (induced energy) the antenna length is cut so that it is resonant with the frequency of the radio wave.
    that length can be determined by the formula wavelength = speed of light / frequency
    an easy way to work that out for MHz of frequency is ....

    300 / 100MHz = 3 metres (wavelength)
    rarely are full wavelength antennas used more normally a halfwave one is used so you would divide that 3metres by 2 and end up with 1.5 metre for a halfwave antenna


    Dave
     
  7. Sep 8, 2012 #6
    If haven't noticed I don't know a lot about this science. What's meant by signal field strength? The magnetic field, electrical field, or both fields? A sine wave with a big amplitude does not always have a stronger magnetic field than a sine wave with a small amplitude? I appreciate the help a lot.
     
  8. Sep 8, 2012 #7
    Maybe transfer is the wrong word. When I wrote light I meant EMR, expecially radio waves. Why does the length of the antenna change the resonance of th antenna? So if the desired signal wavelength is 3 metres then what would be the best length for the antenna? I appreciate the help!
     
  9. Sep 8, 2012 #8

    davenn

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    Thats OK :) we all have to start some where
    3 metres is the wavelength of a frequency of a 100 MHz ( in the middle of the FM broadcast band 88 - 108 MHz) radio wave.
    I dont know what country you are in ? maybe you have the 27 MHz or 476MHz citizen bands
    ( also known as PRS ) lets, using the formula I gave you above, work out the wavelengths for each of those frequencies.

    27MHz is at the high end of the High Frequency (HF) band
    300 / 27MHz = 11.11 metres wavelength

    476 MHz is around the middle of the Ultra High Frequency (UHF) band
    300 / 476MHz = 0.630 metres = 63 cm wavelength

    when the antenna is the same length as the wavelength of the radio wave its at its most resonant and will have the greatest amount of energy induced into it

    To use a non radio wave analogy, but still the same principle, think of an opera singer who breaks a wine glass with her singing. The wine glass because of its size and construction will have its own resonant frequency. Now the singer can sing all sorts of musical notes at the glass, most wont have any effect as their frequency is too far from the resonant frequency of the glass. but as the singers musical tone approaches the resonant freq of the glass it will start to vibrate and if the amplitude ( loudness) of her singing plus producing the correct tone is just right it will cause the glass to vibrate so much it will shatter.
    At that time the frequency of the tone ( note) being produced by the singer and the resonant frequency of the glass are the same and the glass absorb the maximum amount of energy.

    does that help ?

    cheers
    Dave
     
  10. Sep 8, 2012 #9
    Yes it does help me a lot and there's always more questions so answer if only feel like it. I don't understand how a sine wave passing by an antenna at a strange angle can induce a current that can be decoded..
     
  11. Sep 8, 2012 #10

    davenn

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    OK, that leads us to the next stage. for a start dont get so wrapped up by this sine wave thing :)
    An E-M field is made up of 2 separate fields and as you may guess they are an Electric field and a Magnetic field. These 2 fields are perpendicular to each other.

    attachment.php?attachmentid=50574&stc=1&d=1347096418.jpg

    Thanks to the NZART for the graphic :)

    OK back to antennas and angles and things.....
    You may have noticed as you look around that you see antennas orientated in one of normally two different polarisations, horizontal and vertical.
    If the transmitting antenna is vertically polarised so will the receiving antenna. If the transmitting antenna was say, vertical and the receiving antenna horizontal, there would be a substantial reduction in the received signal strength because of the mismatch.

    attachment.php?attachmentid=50575&stc=1&d=1347097081.jpg

    Above is a pic of a local radio tower site, you can see there are a mass of antennas on it, and this was only the top 1/3 of the tower
    I have marked some of the antennas VP or HP to show you which ones are Vertically Polarised or Horizontally Polarised.
    You can see that they are all pointing in various directions. All point to another antenna at the other end of the link which will have the same polarisation.

    cheers
    Dave
     

    Attached Files:

  12. Sep 8, 2012 #11
    There's one that says VP and it seems to be HP to me. I notice that the VP antennas point up to the sky but in space there's no up. I'm learning so much but I don't know what I could do in return for helping me so much.
     
  13. Sep 8, 2012 #12

    davenn

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    which one are you referring to ? Maybe that vertical antenna on the left side of the mast ?

    OK I will go through the antennas I have labelled ... from left to right....

    1 -- a 9 element horizontally polarised Yagi
    2 -- a vertically polarised colinear
    3 -- a 9 element horizontally polarised Yagi ( one centre top)

    then down the right hand side from top to bottom....

    4 -- a vertically polarised dipole
    5 -- a vertically polarised dipole
    6 -- a 15 element vertically polarisd Yagi
    7 -- 2 x horizontally polarised Yagis ( line is pointing to 2 antennas at the bottom)

    Now I have introduced some new terms there for you to understand and learn

    Yagi antenna ( NOTE the capital Y), the name comes from Prof. Yagi and his assistant Mr Uda, a pair of Japanese guys who invented the Yagi system many years ago .. do a google search on his name
    A Yagi antenna consists of a number of elements from 2 to whatever, but ~ 35 is a practical limit depending in frequency.
    eg ... a 10 element Yagi on say 10 MHz would be physically huge and have really huge wind loading ( wind resistance) 3 or 4 elements for frequencies around that area are more the norm.
    but go to 100MHz and a 10 element Yagi is quite manageable
    the Yagi's on that mast are mainly around the 400 to 500 MHz range and several others that can be seen lower down ( sorta a golden colour metal) they are maybe up ~ 1000 MHz give or take a bit. A 10 to 20 element Yagi on those frequencies is both relatively small and has high gain.
    As you go higher in frequency above 1000MHz there becomes a trade off between size and gain and it becomes more practical to use a dish type antenna

    will finish this post later I need to head away from home for a few hrs :)

    Dave
     
    Last edited: Sep 8, 2012
  14. Sep 8, 2012 #13
  15. Sep 8, 2012 #14

    davenn

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    ok to continue....
    ( I know we are a long way from your original question and I hope thats has been answered successfully :) ... I'm just passing on some radio antenna basics without getting into any serious maths explanations)

    .... As you go higher in frequency above 1000MHz there becomes a trade off between size and gain and it becomes more practical to use a dish type antenna
    in this next pic there are several dish antennas....

    attachment.php?attachmentid=50600&stc=1&d=1347162328.jpg

    a radome is simply an E-M transparent shield/cover over the front of a dish to protect it from animals and the weather.

    basic dish .....

    attachment.php?attachmentid=50601&stc=1&d=1347163445.gif

    Using ray paths as you would with an optical mirror you can see the incoming ( received signal) or the outgoing, transmitted signal, being reflected off the parabolic surface of the dish coming to or from the antenna feedpoint. Dish antennas produce very hi gain as they focus the radio signal into a narrow beam

    A basic Yagi antenna....

    attachment.php?attachmentid=50604&stc=1&d=1347164588.jpg

    OK the pic above shows a simple 3 element Yagi ... the director element is at the front of the Yagi, then the driven element in the middle it is the active element and has the coax cable connected to it. Then at the back is the reflector element.
    1 -- The director(s) element(s) are shorter than the driven element, ~ 5 - 10%.
    2 -- The driven element is called a dipole and its length is a half-wavelength at the frequency its being used at, ( it is actually an electrical half wave-length, but we wont complicate the issue at the moment)
    3 -- The reflector element is longer than the driven element, again ~ 5 - 10%

    A dipole antenna on its own radiates reasonably evenly 360 degrees and is great if you want a wide coverage. But if you want to get a signal into a specific area then you use a Yagi with a reflector and one or more directors. This has the effect of confining the radio wave to a narrower beam so the signal can be aimed into a specific region.

    OK lets see how you go digesting that and what questions it generates :)

    cheers
    Dave
     

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  16. Sep 8, 2012 #15

    davenn

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  17. Sep 9, 2012 #16
    So it comes down to me trying to build an ornithopter (although it doesn't resemble a bird very much). I need it to be radio controlled and I want the radio control system inside it to be well defined for this ornithopter. I'm also interested in radio control for other reasons but I'm wondering how I could get the receiver inside to receive signals from long distances. Is it best to use an antenna or no antenna at all? Of course info on anything close to that will be appreciated.
     
  18. Sep 9, 2012 #17

    dlgoff

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    Ah. Now I can go a little off topic. :)

    When I saw this,

    I wanted to say, "I wish I owned the tower". Well maybe a lot off topic, but there's money to be made if you own one. The Department of Transportation here in Kansas has a information page as an example.

    http://www.ksdot.org/burConsMain/Connections/Radio/LeasingTowerSpace.asp

    Okay. Back to antennas.
     
  19. Sep 9, 2012 #18

    davenn

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    Hi Don
    yeah costs for leasing space on towers and hi-rise buildings, here in Australia are very expensive. On a roof top, you usually pay by the sqr metre and you are limited to height of antenna etc. On a mast you pay, not only for the area but also position on the mast, eg. the higher up the higher the lease cost. And considering the ongoing need for more equip to be located on a finite number of tower sites, its really a licence for the owners to "print money" haha

    Dave
     
  20. Sep 9, 2012 #19

    davenn

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    OK, I had to look up ornithopter just to confirm we were both on the same page haha
    There are many good radio control systems out there. All the best ones these days use 2.4 GHz and are digitally encoded to avoid multi system intereference. Any respectable model hobby shop will be able to guide you into something suitable. As for range, well out to around 1km would be an approximate maximum.
    yes, of course you will need an antenna, and again the systems for sale come with an antenna suitable for the unit you are remote controlling :)

    cheers
    Dave
     
  21. Sep 9, 2012 #20
    Actually I meant that I want to aim towards learning about that, any attempt to help me will be appreciated but it would help me most to know about how I can make the radio control. Learning about antennas is very good.
     
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