10 meter J-Pole antenna

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by N7USR, Aug 11, 2009.

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  1. N7USR

    N7USR Ham Member QRZ Page

    I'm beginning construction of a 10 meter antenna for installation onto the top of my palm tree. It will end up being about 35 feet above ground level to the tip of the antenna.

    I have enough wire to make a 1/2 wave upper section but I am a little stumped on the bottom section. I plan to use 300 ohm twin lead for it. I need to know if simply making a 1/4 wave section using the 300 ohm twin lead, connecting it to the 1/2 wave section above it, and then simply tuning it by finding a resonant point near the bottom of the "J" section is all I need to do.

    Is it that simple? I have made a 2 meter J-pole and it was that easy using all 1/2" copper pipe. I guess I just need to know if the 300 ohm twin lead will function as the "J" section.

    Thanks and 73's de N7USR Greg
  2. WB2UAQ

    WB2UAQ Ham Member QRZ Page

    Hello Greq,

    The matching system below the the half wave radiator is not exactly a quarter wave long but close to it plus a shorter shorted stub across the coax. The high impedance at the end of the radiator is transformed down to a lower impedance (50 Ohms) about an electrical quarter wave down the 300 ohm ribbon plus some capacitive reactance that you tune out with the short shorted stub. So two things need to be juggled- the tap point down about a quarter wave and then the length of the shorted stub. Basically you adjust for 50 Ohms with some remaining reactance (Z = 50 - jx )that gets tuned-out by the shorted stub (inductive reactance) at the bottom. I think the shorted stub is about 0.015 wave lengths long ( a shorted stub less than 1/4 wavelength is an inductive reactance so X is minus jX. The result is 50-jX+jX resulting in 50 Ohms for a very nice match). It takes a bit of tinkering but the results will be satisfying:) This is a good learning project to help understand the Smith Chart. When you understand the Smith Chart, you can visualize the whole matching process in your head. 73,
  3. NL7W

    NL7W Ham Member QRZ Page

  4. N7USR

    N7USR Ham Member QRZ Page

    Thanks a million !

    Hey thanks ! I appreciate the replies.

    I'll have a look at them and see if this helps me. I have seen the Smith chart in books and other places but never had anyone help me understand it. I guess now is the time.


  5. N4AEQ

    N4AEQ Ham Member QRZ Page

    I used this link to build one

    Sense only the top 1/2 wave does the radiating you can make the matching network out of coax instead of 300ohm, I worked great for me:


    1/2 wave aluminum tubing is alot easier to deal with than a full length J pole

    Have fun N4aeq
  6. RU9CA

    RU9CA Ham Member QRZ Page

    Can you are useful such scheme of the co-ordination?
    This is realized on resounding coaxial "glass".
    The whole this system tunes in in 2 stages:
    1 - At first, tunes in the glass - is simply changed its long. First, it is necessary to take the cable, little longer of design, but afterwards checking on SWR-meter it is necessary to find the minimum. Abbreviating glass - adjust the minimum on necessary frequency.
    Then, similarly, possible select the vibrator long .
    The not bad antenna is got. :)
  7. K7FE

    K7FE QRZ Lifetime Member #1 QRZ Page

  8. RU9CA

    RU9CA Ham Member QRZ Page

    This truth. Possible do and so, what has done Mike. I did so, when did such antenna on 2 metres. The more large antenna also possible to do so, but this for 10 metres - an unnecessary consuption metallic tube.
    I did such antenna with cable "glass' on 160 metres - a vibrator was long 246 feets, "glass" - approximately 88.6 feets.
    On 10 metres - a vibrator must be 16.7 feet, "glass" - only 6 feet. 6 feets of the coaxial cable for not bad aligner - a small expenses. :)
    What see - an antenna is got rather compact, at small consuption material.
    The radiating part - in both variant has alike long - 16.7 feet.
    Last edited: Sep 6, 2009
  9. K2TY

    K2TY Ham Member QRZ Page

    That's a sizable vibrator.
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