"Copper Cactus" J-Pole

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W0PWS, Mar 2, 2008.

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  1. W0PWS

    W0PWS Ham Member QRZ Page

    Thanks. I've certainly learned a lot by posing this question to this forum.

    I really appreciate all the information.
     
  2. KA4DPO

    KA4DPO Platinum Subscriber Platinum Subscriber QRZ Page

    I don't know where your getting that from Terry but a J-pole is a linearly loaded half wave antenna and unless I was sleeping through my Fields and Waves class it should not need a counterpoise to exibit a cardioid pattern.

    By the way, when the current maximum is at 90 degrees on any half wave antenna the current at each end is zero just like a dipole and it doesn't matter if it's end fed or center fed...
     
    Last edited: Mar 3, 2008
  3. K7FE

    K7FE QRZ Lifetime Member #1 Platinum Subscriber Life Member QRZ Page

    A J-pole is not “linear loaded” or loaded in any other way because it is already a full size half wave or 180 degree antenna and does not need electrical length. Loading is used to make up the difference when the antenna is electrically "shorter" than needed.

    73,
    Terry, K7FE
     
  4. K0CMH

    K0CMH Ham Member QRZ Page

    Then it would seem that the coax shield is radiating, thus RF in the shack and on the chassis of the radio.

    I will be reading my ARRL antenna book tonight to see what it has to say.

    EDIT: some posts got in before I got this in. My post was in the context that the shield is working as the counter poise.

    Also, I was of the understanding that DPO stated. That is why a half wave works so well for mobile, the ground plane is not critical. Yes, I know that the half wave does not have as desirable angle of radiation as a quarter wave ground plane, but it seems a good compromise for mobile applications.
     
    Last edited: Mar 3, 2008
  5. W0PWS

    W0PWS Ham Member QRZ Page

    When I said I'm poor ... I meant I have kids. ;)

    I went out and bought a Diamond SWR meter today at my local HRO.

    I made some slight adjustments on the "sliders" on each vertical and got the SWR down to around 1.2:1. The 1.2:1 reading is actually on my rig's Medium power setting. SWR is 1:1 on both Low and High power outputs. (Yes, I changed the setting on the meter when I changed the power output from the radio.) BTW, the choke seemed to make little difference in the reflected power I saw ...

    Low = 5
    Med/Low =10
    Med = 25
    High = 65

    Thanks again for all the input.
     
  6. KA4DPO

    KA4DPO Platinum Subscriber Platinum Subscriber QRZ Page


    OK, poor choice of words. The quarter wave stub is a matching section that transforms the high impedance (voltage feed ) to a low inpedance that will match the feedline. It's a linear matching section..

    Sorry for the mental hicup......
     
  7. KA4DPO

    KA4DPO Platinum Subscriber Platinum Subscriber QRZ Page


    A 1.2 to 1 SWR is excellent. Sounds like you have it whipped.
     
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    ::power level can't change the SWR. It is what it is. If you see a change in SWR with power level, either the meter is acting poorly or maybe your hand or body influenced the measurement.

    Now that you have an SWR meter, to determine if your coax is "hot" or not (it shouldn't be), just transmit and watch SWR as you grab the coax loosely and run your hand up and down the coaxial cable, grasping it with light tension. If you see the SWR change, then your coaxial transmission line indeed is not well decoupled from the antenna, and acting like part of the antenna.

    Ideally, nothing would change at all.

    WB2WIK/6
     
  9. W0PWS

    W0PWS Ham Member QRZ Page

    I understand. I think.
    It took me some time to understand how to read the meter. The SWR does not change , the forward and reflected power changes based on the power used. Right? From those readings, comes the SWR reading.
     
  10. KG6YTZ

    KG6YTZ Ham Member QRZ Page

    Hm... Nice trick. I'll have to try that with mine. My Arrow OSJ-146/440:

    [​IMG]

    That's the way KE6WMG [who had the tools and the big ladder] put it up. As you can see, there's no coax coil, just a couple of long turns around the mast [steel chain-link-fencing pipe]. The coax is LMR-400, which isn't really made for tight coils... :) SWR is higher at the bottom of 2m than at the top, but still acceptable. If I can do something to improve things a bit, though, it might be worth a try. Ferrites just below the PL-259?
     
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