"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

    I need some feedback on a 2 meter J Pole I built today. I had no 2 meter antenna and very little cash on hand, so I decided to build a J Pole. I have tried to get feedback on the air ... either no one could hear me, or no one was around. The repeater responded ... so I assume I was transmitting.

    Like I said I'm poor ... I know there's no substitute for an SWR meter. I just want to get up and running.

    I used 1/2 inch Type M copper pipe, 1/2 inch copper caps for the vertical sections and 10 gauge solid copper wire between the vertical sections. Connector is a SO-239 post from Radio Shack. I am using an ICOM IC-2200H radio, transmitting mostly with 5 and 10 watts, but occasionally want to use up to 65 watts for simplex QSO's.


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    Last edited: Mar 2, 2008
  2. KI4RVH

    KI4RVH Ham Member QRZ Page

    It looks a lot like the one I built. It should work fine. I have built mine exactly like the plans and SWR was 1.2 to 1.

    73

    Chris
     
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    We're all poor in some way. I want to live in the White House and have a private jet, but neither has happened yet. I'm still hoping.:p

    Anyway, the j-pole looks fine except the copper wire between the SO-239 and the opposite pole adds a lot of inductance to the system, which will require capacitance to tune out the resulting reactance. That is definitely possible. If you happened to stumble on a match point by accident and measurement, that's great, but it's not that likely.

    Normally you'd need to adjust the tap points for "both sides" of the feedline (coax) up and down to find the correct non-reactive tap point to feed the antenna and make it resonant. I see you have an "adjustable" tap point for the end of the copper wire (hose clamp connection) but no corresponding adjustment for the outer conductor because that's the flange of the SO-239 which appears to be fixed in position (maybe it's not -- hard to tell).

    If the SO-239 is not adjustable "up and down" on the j-pole, then any tap point you find for the wire will never be quite optimum. Both conductors of the coax really should be movable "up and down" so they are always parallel to each other for optimum results.

    A lot of guys who have built j-poles and written construction articles about them have no idea how they work: One problem with the internet.

    If you can borrow an SWR bridge or an antenna analyzer that would be very helpful. Once the j-pole is "tuned" to resonance, it's a decent antenna. A j-pole that is untuned can be an awful antenna, and the tuning adjustment is critical.

    In 100% of *all* cases, I can outperform anybody's homebrew j-pole with my homebrew ground plane that requires no tuning of any kind and is actually half the height of the j-pole. No exceptions to this, the test proves it every single time.

    WB2WIK/6
     
  4. W0PWS

    W0PWS Ham Member QRZ Page

    The SO-239 is not "fixed". It's tightened into place with copper pipe wrap ... and as such, can be adjusted. Thanks for the info. I certainly have no idea what I'm dealing with ... having an antenna was my goal today. I've accomplished that goal. Tuning it will be a whole different project altogether.

    I have since made contacts on the local repeaters. I've been advised of some white noise, but was easily able to carry on a conversation.
    Of course I don't know what the SWR is for the time being ... but for now I can communicate which is cool with me.

    I *was* using a scanner antenna ... *yikes* -- while specified as usable ... it was a horrible choice for an antenna.
     
    Last edited: Mar 2, 2008
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    ::It was wise of you to make the SO-239 position adjustable! Good for you, and good thinking! I don't know if copper pipe wrap is good enough to make a good "RF" connection or not, though. Hopefully, it is!

    Keep working on it and I'm sure you'll succeed.

    WB2WIK/6
     
  6. W0PWS

    W0PWS Ham Member QRZ Page

    The pipe wrap is tight ... really tight. I used a brass bolt and nut to secure it around the main vertical.
     
  7. KA4DPO

    KA4DPO Platinum Subscriber Platinum Subscriber QRZ Page

    One neat way to make adjustable taps is to use copper pipe straps. These are used to fasten copper pipe to house framing. By putting screws through the holes two of these make an ideal moveable tap. They cost about 20 cents each at home depot.

    If you can find someone with an antenna analyzer it would really help you to tune it correctly.
     
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    That's good, and probably good enough assuming there's no oxidation between the "wrap" and the other items requiring connection.

    Copper oxide, which occurs in about one second when copper is exposed to air, is not a good conductor. That's why most connections are soldered using flux to remove the oxide layer.

    But anyway, assuming you have a terrific connection there, the fact is the tap points on both the shell of the SO-239 and the center conductor copper conductor which taps the opposing pole should be at the same height. If they're not, you have an imbalance that will be hard to perfect.

    Use an SWR bridge to tune this thing up, and use a coaxial current balun (RF choke balun) to help decouple what is below the antenna from what's above the feedpoints, and it will work pretty well.

    Trust me, though: The j-pole is a real "compromise" antenna that really isn't worth the effort, considering the alternatives that work better and are easier to build.

    WB2WIK/6
     
  9. W0PWS

    W0PWS Ham Member QRZ Page

    KA4DPO: That's the work I was looking for ... pipe straps! That's what I used.

    The hose clamp is adjustable and the pipe strap with brass bolt and nut are adjustable on the other side.

    WB2WIK: Too late! I already built it. I needed an antenna ... and I learned a lot in the process. A lot of the plans that I saw for a lot of different antenna's were either way too cryptic (circa 1930) or just wouldn't work for me in my situation. I'm still just a newbie here. Easy is key. Easy for me is defined as readable material with a nice list of supplies. I really wasn't looking for a whole lot of antenna theory ... which is what I found. I would love to spend some time learning some serious antenna theory ... I just don't have time right now.
     
    Last edited: Mar 2, 2008
  10. K0CMH

    K0CMH Ham Member QRZ Page

    I have made a number of the copper j-poles, and a few other other with Aluminum rod. What I have found is that if you follow the directions, and make feed attachments at the measurements the plan calls for, your SWR should be within 1.5:1. It isn't worth much to work hard to get below 1.5:1.

    One of the reasons for this is that the diameter of the copper pipe is so large with respect to the wavelength of 2 meters, that there is a large "band width" for the antennal. It is tolerant of some less-than-perfect percision.

    I would suggest contacting some local Hams, (maybe visit a club or two) and find someone who can lend you a SWR meter (but be ready to replace it if it gets damaged) or who will come to your location and check it out.

    Good luck and you probably have the antenna made just fine.

    I always solder my feed attachment points once I find they have good SWR numbers. Then I don't have to worry about corrosion, etc.

    Good luck.
     
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