"Copper Cactus" J-Pole

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

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  1. NW9T

    NW9T Ham Member QRZ Page

    I See One Problem With The J Pole Antenna

    Looking at the pictures at the beginning of the thread, it appears you have the feedline hooked up backwards. The Braid of the coax goes to the short end, and the center feed to the long section of the antenna. That's how I constructed the three variations of J Poles I have and they all work excellent. Or are my eyes deceiving me on your pictures?
     
  2. KB8QWN

    KB8QWN Ham Member QRZ Page

    Those J-poles also work fairly well on 440mhz as well...

    All the J-poles I've ever built, and I have one over in the corner currently until I get something up on the roof, but they've all had the braid going to the long end and the center feed to the short end. Although, if yours were opposite, perhaps it doesn't matter, since if you look at the antenna, it should be a dead short. That could be the beauty of the J-pole. :)
     
    Last edited: Apr 17, 2008
  3. NA5Z

    NA5Z Ham Member QRZ Page

    By the way, don't let all the ground plane guys erode your enthusiasm. It's always fun to build and try new antennas and to get a feel for them on your own. I ran a J-pole for 6m last year just because I wanted to try one and it did all right.

    If you do feel the urge to experiment more in the future, though, I 'd suggest keeping an eye on swap ads or the hamfests and pick up a VHF VSWR meter or analyzer so you won't be flying blind all the time.

    FYI, all through college, I used to rely on a little ground plane. I called it my Sputnik antenna and made it mostly out of heavy aluminum wire from the hardware store (I think it was clothesline wire?). Basically had a small tab of fiber board (kind of like wood paneling, but much much cheaper)... PL259 screwed onto one end of it. Radials stubbed out of the screws. The whip was also an aluminum chunk screwed down to the tab with a very short jumper of copper to the center conductor on the 259. Not ideal, but it worked ok for a starving college kid. heh

    That little turkey stayed with me for many many years, too... heck of well behaved antenna that really irritated my Ringo Ranger loving classmate because it pretty much always did as well as his nice shiny new antenna. :D
     
    Last edited: Apr 17, 2008
  4. KF4HAY

    KF4HAY Ham Member QRZ Page

    Made a 2 meter J pole out 5/8's copper pipe last year for a friend. It uned easy at a low swr, he clamped it to a 2x4 edgewise and put it as a standoff about 20 feet up his tower. It worked very well and he was pleased with it, it would have done better higher up. He used some low loss coax as well. He is 35 miles from me and we could simplex easy.
    I might be wrong, but I thought a J pole was a 5/8 or 3/4 wave antenna, it sure worked a lot better than a vertical 1/2 wave dipole antenna we made and used for awhile.

    http://www.hamuniverse.com/antennas.html
     
  5. N4AUD

    N4AUD Ham Member QRZ Page

    Why would that make a difference?
     
  6. KG6YTZ

    KG6YTZ Ham Member QRZ Page

    'cause if you get the polarity wrong, your signal is gonna be upside-down, and nobody will be able to hear you because of the 180-degree phase cancellation. :D :D :D

    [Disclaimer: No, of course I don't actually believe that.]
     
  7. KI4OXD

    KI4OXD Ham Member QRZ Page

    And if the signal is upside down it lets all the SWRs leak out. :rolleyes:
     
  8. NA5Z

    NA5Z Ham Member QRZ Page

    You just have to turn the antenna upside down and it'll be fine. ;)
     
  9. NW9T

    NW9T Ham Member QRZ Page

    From the webpage http://www.cebik.com/vhf/jp1.html

    "Theoretically, it should make no difference to which side of the matching section legs one connects the coaxial cable center conductor. However, because currents are not balanced perfectly, it may in some cases make a difference. Most builders of J-poles tend to try the connection both ways, opting for whichever they discover or believe to provide superior performance."

    As he said in theory it shouldn't make a difference, but theory and reality don't always go hand in hand. If you look around online you'll see the vast majority of designs for J Poles specifically state to hook the center feed to the long segment. From personal experience I've built mine (5 to date) following that rule and have never had a problem with tuning and have had excellent receive and transmit on all of them.
     
  10. K7FE

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

    It really does make a difference which side of the J-pole you connect the center conductor to. W8JI's web page shows low angle gain is 5dB lower when the center conductor is connected to the short element. http://w8ji.com/end-fed_vertical_j-pole_and_horizontal_zepp.htm


    [​IMG]



    This is the pattern of a J-pole with the center conductor connected to the longer element.
    [​IMG]

    Note the GAIN figures for the two feed methods, 2.37dBi vs -3.17dBi = 5.54dB difference. The two J-pole patterns are somewhat different.



    You can see the more uniform pattern of a 1/4 wave ground plane. Additionally the gain at 4 deg. elevation is almost identical to the "best" of the two J-pole feed configurations. The ground plane pattern improves beyond what is shown here with a good balun and more than four radials.
    [​IMG]

    The same results of a 1/4 ground plane performing about as well as a J-pole were documented by Tech Bench Elmers Amateur Radio Society in their test of 1993 - 1995
    http://www.geocities.com/SiliconValley/2775/anttest.html

    J-pole construction can be a fun project, just do not expect superior performance over a 1/4 wave ground plane.

    Radials at the base of the J-pole (despite old wives tales) will help balance the J-pole and reduce common mode currents on the outside of the coax shield and conductive mast.

    If you decide to build a J-pole without radials, do not use a balun because that isolates the antenna from it's needed counterpoise, the coax shield. The mast, if conductive, will also act as a counterpoise as the antenna seeks balance.

    End fed antennas are notorious for common mode currents in the shack. Just ask anyone who has used a long wire antenna. That is because the antenna is "out of balance." You may have a mike that "bites you" or erroneous readings on your VSWR or watt meters. Center fed antennas do not have that problem, they are "balanced", thus minimal RF in the shack, because the common mode currents are low.

    This information is not magic and is common sense if you think about it.

    73,
    Terry, K7FE
     
    Last edited: Apr 18, 2008
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