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Large diameter of dipole antenna's rod....Can I use?

Discussion in 'Radio and Communications' started by Willen, Jul 27, 2012.

  1. Willen

    Willen Well-Known Member

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    I have a long and 1.8 cm diameter aluminium pipe. Actually it is a Boom of Yagi Uda. I want to use it as a straight Dipole antenna rod. But it has large diameter of 1.8 cm. If I use it on my 100MHz FM Tx, will it be perfect radiator?
    I will cut these rod in exact length by calculating the formula of dipole rod length including A or K (diameter of rod) factor. (do you have any simple formula for A/K factor?)
     
  2. Sceadwian

    Sceadwian Banned

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    This link shows

    It tweaks the resonant frequency a little causing you to have to use a slightly longer cut, but it increases the bandwidth. It's difficult to find exact numbers at least with a quick Google search, but I'd throw in a random guesstimate to increase the length by 10% of normal, it won't be so drastic as to make the antenna unusable either way, and the only real way to test it is with a network analyzer.
     
  3. Willen

    Willen Well-Known Member

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    searching simple, fix and practical answer...
     
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  4. dave

    Dave New Member

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  5. Sceadwian

    Sceadwian Banned

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    Return answer.... "Just use it" don't over analyze; you probably have more important things to deal with than the specific antenna efficiency to 3 decimal places...
     
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  6. Willen

    Willen Well-Known Member

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    Cannot understand....:(
    I am little beginner so i don't want to experement because I won't be able to solve if it is in mistake and it frustates me. So asking to experience people before using 1.8 cm diameter pipe as a dipole rod.
     
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  7. Sceadwian

    Sceadwian Banned

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    What part of JUST USE IT did you not understand? It will function within approximately acceptable boundaries for typical usage.
     
  8. WTP Pepper

    WTP Pepper Active Member

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    It will work fine. Atmospheric conditions will have a bigger impact.
     
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  9. davenn

    davenn Active Member

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    Hi Willen,
    it really isnt that critical, its not going to make any operation difference, to if the elements are 1cm or 5cm in diameter. The larger diameter will give better bandwidth as the others have already indicated. That article that Sceadwian linked to is just what you need to read over and over and come to understand :)

    I have to agree with him, just use it you are not going to see any other detrimental effects.
    Some antennas I build I purposely make large diameter elements, cuz i want good tuning bandwidth across a wide bandwidth without the need for retuning eg. the 2 metre ham band 144 to 148 MHz ( 4 MHz bandwidth) or the 432 to 450 MHz band (8 MHz bandwidth) I can produce an antenna that the SWR doesnt go above 1.5:1 across the whole band
    Thats a bit more difficult on our 23cm band, 1250 to 1300 MHz, covering 50MHz is nigh impossible and keeping the SWR low. Generally we will peak the atenna across maybe the 4 - 10 MHz where most of the transmissions are done and if the SWR is a littlebit higher in the receive sections its not really an issue.
    On this band the repeater TX and RX frequencies have a 20 MHz split, its virtually impossible to get an antenna to have flat SWR across 20MHz on that band

    cheers
    Dave
     
    Last edited: Jul 30, 2012
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  10. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    I have used 100mhz FM antennas with a diameter of 4cm and 8cm. My power level was very high.

    Armature Radio is a good source of antenna theory. I have some of their books.
    http://www.arrl.org/antennas
     
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  11. JimB

    JimB Super Moderator Most Helpful Member

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    ronsimpson wrote:

    Is that for those of us who are all wound up and spin round and round?:rolleyes::rolleyes:

    I think you really mean "Amateur Radio" :D

    JimB
     
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  12. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    LOL I have a spell checker that leads me astray.
     
  13. Willen

    Willen Well-Known Member

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    -Can I use such larger diameter pipe as a antenna rod in high power transmitter like 25 watt or 100watt? Are there any different and special formula for accurate length of dipole?
    - Actually this pipe was Boom of Yagi-Uda. So there are some holes in pipe to attache directors & reflectors. Is it harmful for rediation in my dipole?
     
    Last edited: Jul 31, 2012
  14. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    Use your pipe. It will work. Not harmful. You have been told this before.
    You want formulas.....there are all over the internet.
    25 or 100 watts = high power??? If the pipe did not have holes in it I think it will work to 1000 watts.
     
  15. Willen

    Willen Well-Known Member

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    You are a experience person of 100,000 watt transmitter so 100 watt is not ''high power'' for you. I am use to play with 5mW to 20mW FM Tx, so how I can take to 100 watt, you can imagine!
    Anyway, nobody had told me before ''holes are not harmful''. Ok i will. Thank you
     
    Last edited: Aug 1, 2012
  16. Sceadwian

    Sceadwian Banned

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    Willen, power is nothing for an antenna, it just boils down to using thicker materials. Copper pipe would be better but I doubt you'll see an losses from resistance.
     
  17. RCinFLA

    RCinFLA Well-Known Member

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    If you think of the integral segment by segment coupling between the two dipole rods, larger diameter reduces the outwardly series inductance and increases the rod to rod coupling capacitance. This has three dominant effects.

    First, it will require shortened rods. You will likely need to shorten them to about 0.80 to 0.85 of the normal half wavelength, rod end tip to end tip. Second, it will drive down the impedance at the feed point. If just a dipole arrangement you will move from about 70 ohms for thin rods to about 50 ohm. This may actual be an advantage if you are feeding with a 50 ohm coax. Lastly the bandwidth of the dipole will widen out, again likely an advantage.

    If you are putting this in a yagi array, things change significantly. Generally, a yagi drives the input impedance down significantly for driven element but this depends on reflector and director(s) spacing from driven element. Generally, larger diameter rods in a yagi requires the rods to be shortened and brought closer together because inter-element coupling is reduced by the lower impedance, larger diameter rods. Basically, a totally new yagi dimensional design.
     
    Last edited: Aug 1, 2012
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  18. Willen

    Willen Well-Known Member

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    0.80-0.85 percent? inch? Feet? I think there are some formula to shortning rod according to diameter.
     
    Last edited: Aug 1, 2012
  19. RCinFLA

    RCinFLA Well-Known Member

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    Just looked up formula and I gave you a bit excessive shorting. Looks like it should be about 95% of half wavelength for dipole length, end to end with 1.8 cm diameter rods. This is net tip to tip, including any gap you might have at feed point on boom.

    For 100 MHz, that is (2.998e10 / 100MHz) X 0.5 X 0.95 = 142.4 cm. or 56 inches tip to tip.

    If your dipole is attached at the center to a metal mast, the metal mast capacitance to the antenna rods will shorten it a bit more. Take off another 1% for a correction factor of 94%.

    If you are going to a 50 ohm coax make sure you use a balun to balance the feed point. A 23.6 inch coax shield connected to feed point coax center conductor at the dipole center feed, taped in parallel with feed coax and grounded to feed coax shield at far end of 23.6 inch coax will act as balun. (takes unbalanced coax feed to balanced feed for dipole). It forms a parallel line of quarter wavelength.
     
    Last edited: Aug 1, 2012
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  20. Willen

    Willen Well-Known Member

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    I am using Bambo, or Plastic Pipe Mast.
    -Can't I make Coax Coiled Balun?
    -In your equation, what is '0.5' ?
     
  21. RCinFLA

    RCinFLA Well-Known Member

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    - I would not recommend coiled balun based on your experience level and the fact it is 100 MHz. Straight line taped to feed line is only 23.6 inches.
    - That's why it is called a half wavelength ( 0.5 wavelength) dipole.
     

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