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Building a half wavelength dipole antenna

  1. Mar 31, 2015 #1
    Alright so I am building a few half wavelength dipole antennas for a project and was wondering how large the space should be between each side of the dipole antenna. I have asked a radio forum and they could not help me so I came here. Thanks.

    Also I herd that an 1 and a half wavelength antennas are stronger, is this true?

    Also how would wires in parallel work here such as a twin lead wire?
     
    Last edited: Mar 31, 2015
  2. jcsd
  3. Mar 31, 2015 #2

    davenn

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    hi there
    welcome to PF

    Tell us more about these dipoles ... are they part of a phased array or individual dipoles of different frequencies ?

    Are they going to be vertical or horizontally polarised ?
    if vertically polarised, are they going to be side by side or stacked above each other ?

    same questions for horizontal dipoles

    all these variations make a huge difference


    cheers
    Dave
     
  4. Mar 31, 2015 #3

    davenn

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    what for ? the feedline or the antennas ?
     
  5. Mar 31, 2015 #4
    Sorry i am a novice with these sort of projects. Anyway they will be individual dipoles with different frequencies that will arranged horizontally. There will also only be one up at a time. The parallel wires will be for the feedline, and what I was wondering on that aspect is the difference of Coaxial Wire and twin lead wire.

    Thanks
    -Ethan
     
  6. Mar 31, 2015 #5

    davenn

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    Hi Ethan OK

    OK I did misunderstand what you initially said

    you haven't said what frequencies you are using or if this is for transmitting, receiving or both ??
    here's a pic of an HF freq dipole feed, for 30MHZ

    images.jpg

    the gap between those ends will be around 4 - 5 cm ( ~ 2 inches)
    This gap will get smaller as you go up in freq,
    at VHF say 144MHz ham band they will be around 3 cm
    at UHF say 432MHz ham band they will be around 2 cm
    at SHF say 1296MHz ham band they will be around 1 cm

    the distance isn't absolutely critical and as long as you are close to those measurements you will be OK
    its more critical for transmitting than receiving ( the gap distance will affect the impedance of the feedpoint)


    personally I wouldn't use parallel wire feedlines just too difficult a job mounting them on mast etc and you will need BALUNs as they wont have the correct impedance. A straight dipole has an impedance of 75 Ohms so would normally require a 75 Ohm impedance feedline, say RG59 coax ( if for transmitting, for lower powers (up to ~ 100W) RG6 for higher powers ... several 100 W) tho you can get away with using 50 Ohm coax say RG58 or RG8, RG213.

    EDIT addition, you want to be building 1/2 wave dipoles ... each side will be 1/4 wavelength long


    does that help ? :smile:
     
  7. Mar 31, 2015 #6

    berkeman

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    What frequencies are these antennas going to be receiving on? Are you planning on transmitting as well? If so, what are you using for your transmitter?

    Coax (50 Ohm or 75 Ohm) is an "unbalanced" transmission line, because the 2 conductors are not symmetric. You use a "balun" at the feedpoint of the antenna to convert from the unbalanced coax to the balanced 75 Ohm input of the dipole. If you are using a "balanced" twin-lead feedline, you will still need something like a balun at the feedpoint to convert from the ~300 Ohm characteristic impedance of the twin-lead feedline to the 75 Ohm input impedance of the dipole antenna.

    http://en.wikipedia.org/wiki/Feed_line

    :smile:

    EDIT -- Dave beats me to the punch yet again!
     
  8. Mar 31, 2015 #7

    davenn

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    I didn't get a chance to get to that point
    that is definitely an important consideration


    Thanks for your addition :smile:

    here's a drawing of a 1/4 wave sleeve BALUN for use with a coax feed

    untitled.png
     
  9. Apr 2, 2015 #8

    sophiecentaur

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    You are talking about antenna gain, which is really to do with the directivity of the pattern. How important is gain for you? Designing and building antennas is quite hard unless you stick to well established types and the half wave dipole is best unless you really need it to be clever. Coax is obviously the thing to go for and there are plenty of designs of balun arrangement that will give a balanced drive and help with the match at the same time. If you are transmitting, the match needs to be better.
    I am surprised that a "radio forum" didn't shower you with alternatives for a home made dipole design. Have you tried Amateur Radio Society sites? They are usually good for practical help. What operating frequencies are you planning to use? Is the spread so wide that you can't use a single dipole?
    As usual, I think you need to provide more specific information about this project so that answers can be targeted better to your needs.
     
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