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does wavelength determine height of antenna

Discussion in 'Radio and Communications' started by bhuvaneshnick, Jun 18, 2014.

  1. bhuvaneshnick

    bhuvaneshnick Member

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    in the image 354 wavelength correspond to 840 MHz.does wavelength decides height of antenna.if that so for low frequency signal.they have to build ery taller tower.is it possible to build such high antenna
     

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  2. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    354 meter/1113 feet tower Yes I have worked on towers like that. These are used for AM radio. The tower is a antenna.
    1500 feet is the tallest I have seen. The tower was not used as a antenna. It has several FM and TV antennas on it.
     
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  3. bhuvaneshnick

    bhuvaneshnick Member

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    is that anything like fm antennas sill be small and am will be tall
     
  4. dave

    Dave New Member

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  5. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    Yes. FM is much smaller. FM usually has many antennas working together.
    upload_2014-6-18_20-48-36.png
     
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  6. bhuvaneshnick

    bhuvaneshnick Member

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    what you have shown is directional antenna right.And why am has low frequency range than fm.
     
  7. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    FM directional antenna: That antenna is not directional in the North/South/East/West directions. It is very directional in the up/down direction. There is really no reason to send FM music into space. (or down into ground)
    Don't understand

    AM radio (low frequency) travel a long distance because low frequency radio waves bends when traveling around a round world.
    FM radio (high frequency) travels in a straight line. So it does not bend beyond the horizon.
     
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  8. bhuvaneshnick

    bhuvaneshnick Member

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    you mean that fm is directional(not omni directonal)
     
  9. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    No that antenna is directional in that it focus the energy.

    Lower frequencies have the property of following the curvature of the earth. (surface wave or ground wave)
    High frequency is more like light and it will not bend to follow a curved earth.

    upload_2014-6-18_22-50-43.png
     
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  10. bhuvaneshnick

    bhuvaneshnick Member

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    thanks.now consider two things
    1)am(low frequency) antenna are taller and fm(high frequency) antenna are shorter
    2a) am haev low wavelength so it tend to follow the curvature and t has difrracted through object due it low wavelength then why the antenna need to be taller
    ab) fm is more like light does not bend.then the antenna should not not be shorter becuase it cannot bend.so its supposed to talller(like point to point communication)

    is my thought bad
     
  11. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Shorter and taller are two different things. Short/long vs lower/higher. Probably and English thing. Short/long usually refers to element length and wavelength. e.g. shortwave. Then taller/shorter can also refer to antenna height. The words by themselves are ambiguous.

    Long wavelengths bounce off of the ionosphere better.

    Tall antennas have less obstructions. FM has a tendency to bounce and thus Multipath distortion comes into play when the same signal arrives at different times. There is also dead zones. In a car stopped at a light, moving less than a wavelenth may cause the station to get much louder

    Antennas can be polarized where the orientation - horizontal/vertical or both, or circular will change the strength of the signal. TV/FM is USUALLY horizontal.

    There are some comparisons of AM/FM here: http://www.google.com/url?sa=t&rct=...=BPkzk6jBoqqBqGyWTuhD0A&bvm=bv.69411363,d.cWc
     
  12. ramuna

    ramuna Member

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    Hi bhuvaneshnick,
    First of all, a minor correction ;). In post #1, either the frequency or the wavelength is wrong. A wavelength of 354 metres corresponds to a frequency of 869 kilohertz not megahertz. Conversely, a frequency of 840 MHz corresponds to a wavelength of 0.354 metres. These are important distinctions because of the frequency dependence of radio wave propagation. And since the above correction is simply a side-issue to the main thrust of your question, ignore it:D, and instead have a look at the following pages:

    http://www.arrl.org/files/file/Technology/pdf/119962.pdf
    http://www.radio-electronics.com/info/propagation/ionospheric/hf-propagation-basics.php

    which will confirm what Ron & KISS have said so far, and may also clarify things for you a bit more.
     
  13. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    If you want a "keep it simple stupid" answer. yes you are right.

    If you want a NOT simple answer.
    1) There is a difference between "antenna" and "tower".
    Every AM broadcast station I have seen uses the tower as a antenna.
    FM radio does not use the tower as a antenna. It is not good to have a 10 foot antenna/tower. That is why FM uses a small antenna on a tall tower.
    2) The antenna needs to be a length so the electricity travels from one end of the antenna in 1/2 cycle or 1.5 cycles or 1 cycle or.....(depends on type of antenna) The antenna should resonate at the frequency you are transmitting at. Sorry for the bad pictures. Here are some examples.
    [​IMG]
    [​IMG]
    3) I do not understand.
    Here is a picture of a short antenna on a pipe on a building. The antenna pieces are about 6 inches long. (left to right)
    upload_2014-6-19_6-57-34.png
    This antenna is 6 feet long. On a pipe to hold it high in the air.
    [​IMG]
     
  14. Afterburn

    Afterburn New Member

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    The distance traveled by a radio wave depends on several things.

    1. The frequency: This determines the length of the antenna needed to transmit and receive the signal. It also effects the height above ground and that influences the take-off angle for the transmission. It also allows maximum transmitted power output.

    2. Yes. The lower the frequency, the more it can bend around the earth or reflect off the ionosphere. The higher the frequency, the shorter the antenna, the more direct the transmission. It bends less.

    3. Yes. The way the antenna is orientated provides polarization. If you want to receive the best signal from a transmitter, then, you need to have the same type of antenna and the same orientation.

    4. The closer to ground that you place your dipole, the more upwards is directed the radiated energy. And the shorter its range.

    5. Towers allow the antennae to be raised to a height where they can transmit further around the curvature of the earth.
     
  15. crutschow

    crutschow Well-Known Member Most Helpful Member

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    If you mean why is the AM band at a low frequency and the FM band at a much higher frequency, it has to do with history and bandwidth. The first commercial broadcasts were AM since AM is very simple to generate and receive. They were done at low frequency because it was easier to build low-frequency transmitters and receivers and the low frequency wave had a long reception distance as explained in post #8. The bandwidth of the AM signal was also selected to be low (adequate for voice but with corresponding low music fidelity) so many stations could be crammed into the 1 MHz width of the AM band.

    When FM went commercial in the USA it was designed for high fidelity and low noise so the bandwidth of each station became about 250kHz, 25 times that of the AM stations (thus only 4 FM stations could fit in the AM band). This required that the broadcast frequency be selected to be much higher to accommodate this higher bandwidth for a reasonable number of stations. The selected range was 87.5MHz to 108MHz or about 20 times the width of the AM band.

    A side note about long distance transmission of AM signals: When I was a kid in Wisconsin I remember that most nights I could clearly pick up KWKH in Shreveport, Louisiana on my AM clock radio (internal antenna) which is about 1000 air-miles distant. Of course KWKH is a Clear Channel station (no other station in the country on that frequency at night) broadcasting with 50,000 watts of power so that helped with the reception.
     
  16. Afterburn

    Afterburn New Member

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    Very true. But the question asked is about the length of the antenna to transmit and receive radio signals.
    You have all heard of half wave antenna, quarter wave, eighth wave etc: Well, the lower the operating frequency then the longer the wave length of the signal.
    This means that am transmissions use longer in length wires to transmit the radio frequency (rf).
    Amplitude modulated signals are found in the HF range of frequencies (3.000 to 30.000 MHz).
    Very high frequency (VHF) lies in the 30 to 300 MHz range and is used for frequency modulation.
    Now to work out the wave length of a sine wave at any frequency, we divide the speed of light by the frequency of the rf signal.
    Antenna are set up to be friendly with a particular wave length. There is a natural resonance when the wave length and the length of antenna are in agreement... are related to each other. i.e 1/2, 1/4, 1/8 etc.
    From memory, the speed of light is 3 multiplied by 10 to the eighth power or 30,000,000 meters per second.
    When you divide by the frequency (say 25,000,000 Hz or 25 to 10 by power 6 or 25 MHz... The antenna for a one wave length is 30 over 25 or 1.2 meters. The wave length of a 3 MHz signal is 12m long. This is very long; hence the use of fractional antenna.
    Omni directional antenna radiate in a full 360 degree pattern. Dipoles are bidirectional. They radiate forward and backwards equally. It is important to aim them in the direction you want to send.
    Also... and this is important, the height above the ground plane of the dipole effects the take off angle of the transmission. Hence there is a thing called near vertical transmissions.

    With HF: there are ground wave, sky wave and space wave transmissions. Hf travels the furthermost being able to circumnavigate the planet in some instances.
     
  17. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Used to listen to Radio Luxembourg as a kid, you could hear it 'come to life' in the early evening as the 'bounce' became the right distance :D
     
  18. JimB

    JimB Super Moderator Most Helpful Member

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    A few errors there Afterburn.

    OK

    Half wave and quarter wave - Yes
    Eighth wave - No

    You are off by a factor of 10 here.
    The speed of light is 300,000,000 metres per second, (3x10^8). Corrected as per MikeML comment - Thanks Mike.
    So 3Mhz has a wavelength of 100 metres.

    JimB
     
    Last edited: Jul 22, 2014
  19. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    Speed of light

    3*10^8 / 3*10^6 = 100

    The way I rember it is that 150MegHz has a wavelength of 2 meters
     
  20. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    In the UK the LW radio transmitter for BBC Radio 2 used to be on 1500 meters or 200KHz - unfortunately they moved it a number of years ago :(

    Your way is more 'radio ham' as the 2m amateur band is 144-146MHz (in the UK - don't the USA go to 148?).
     
  21. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    Yep, our local ragchew repeater output is 147.220MHz (input is 147.820Mhz). During my ham career, I have tuned dozens of 1/2λ 2m-band antennas using an swr bridge or impedance analyzer. They always end up being just under 1m long at resonance (~38 inches). 1/4λ antennas such as are mounted on a vehicle end up just under 50cm (19 inches).
     

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