J-Pole Problem

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W4XKE, Apr 21, 2012.

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  1. W4XKE

    W4XKE Ham Member QRZ Page

    Today I built a J-Pole antenna and I have some strange results. Nobody around here so I thought I'd ask to see if somebody could shed some light.

    I built the standard J-Pole / 18 1/2 inch stub and a 39 1/2 inch radiator. Most people find a sweet feedpoint about 1 1/2 inches up from the bottom. My SWR was almost 3 at this position so I started sliding it up and found the spot about 9 inches up from the bottom.

    The SWR at resonance (146.400 MHz) was still around 1:2.7. The tuning curve wasn't critical at all. A lot of adjustment yielded little change.

    I tweaked and pruned until I finally achieved a 1:2.5 still not very good but the best I could do.

    Then someting strange appeared. While the antenna was still mounted and transmitting from my MFJ-249 I inadvertently took hold of it and the SWR went down to 1:1.0. Pretty near perfect.

    I took my hand away it it jumped back up to 1:2.5

    By placing a thumb and forefinger across the radiator and the stub at about 14" up from the bottom it attained a perfect match. (???)

    I tried grounding the bottom of the antenna and it got worse.
    I tried hanging a length of wire as from the bottom and it was no better. So I took the wire off.

    The only thing I can imagine is that my body capacitance is acting favorably on the antenna (but I'm not going to hold onto it there so I can key down at a perfect match.... LOL)

    Next brainstorm was to put a small ceramic capacitor across the radiator and stub at the place where my finger and thumb found perfection but that made things worse again.

    What would you suggest that I use to replace a finger and thumb effect? (Sometimes I wish that I were an RF engineer so I'd know what was happening. My wife came out into the back yard and asked, "Hi Honey. Whatcha doin'?" I told her, "I wish I knew!"
     
  2. W4OP

    W4OP Ham Member QRZ Page

    The J pole will radiate from the coaxial shield and any support mast. In order to do any real analysis, you need to put a choke on the feedline and go to an insulated mast. Once you have the RF off the coax and the mast, try adjusting the stub length and the feedpoint.

    Dale W4OP
     
  3. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    If you're describing a 2 meter j-pole the dimensions you gave in the post were really far off...

    The two elements of the "J" should be 1/4-wavelength and 3/4-wavelengths -- NOT 1/4 WL and 1/2 WL, that won't work worth a darn.

    The shorter element is usually 19" and the longer one 57" (not 39-1/2" -- where did you get that from?).

    If your longer element is really 39-1/2" I cannot imagine how that could work on 2m.
     
  4. W4XKE

    W4XKE Ham Member QRZ Page

    Did that. I used a coil of coax as the choke. With or without was no change. The mast is PVC. After all the tweaking, pruning and tuning it only yielded "the agony of defeat".

    WIK... Here's the site that I used for my antenna design...

    http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=8274

    Okay... the radiator is closer to 58" when describing it from top to bottom. The designer used the 39 1/2 measurement describing the length above the stub.
     
  5. VU2NAN

    VU2NAN Ham Member QRZ Page

    Hi OM Johnny,

    1. First ensure you are using 50 ohm coax.

    2. Then, that the length of coax from the SWR Meter to the antenna is an odd multiple of half wavelength and not an odd multiple of quarter wavelength.
    For more info please refer
    http://books.google.co.in/books?id=... an ODD multiple of a half-wavelength&f=false

    3. Now adjust the feedpoint for minimum SWR.

    Good luck!

    73

    Nandu.
     
  6. W4OP

    W4OP Ham Member QRZ Page

    Another Cb myth. Assuming you do not have common mode on the coax, changing the coaxial cable length does NOT alter the VSWR. Look on a Smith chart- varying the length of the coax simply revolves you around a constant VSWR circle. Using an electrical half wavelength (or multiple of 1/2) simply repeats the Z that one would see at the antenna.
    That article is filled with errors.

    Dale W4OP
     
  7. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    What W4OP posted is true. SWR doesn't change with coax length, but the components of the complex impedance will. SWR always stays the same except for being reduced by the loss of the coax, as you get closer to the transmitter.

    J-poles are one of the silliest designs ever. I never use them for anything, as almost anything I can build works better than a j-pole.:eek:

    Actually, a 1/4-wave ground plane with at least four radials (preferably 6 or 8) will almost always outperform a j-pole at equivalent mounting height, and it's shorter so it takes up less space.
     
  8. N5MDT

    N5MDT Premium Subscriber QRZ Page

    Wait a sec... are you saying you took hold of the insulated 450 ohm ladder line and it changed the SWR, or did you make your J-pole out of something else, using the measurements for the ladder line J-pole?

    Are you sure you shorted the two at the base?
     
  9. W4XKE

    W4XKE Ham Member QRZ Page

    The J-Pole that I attempted to make is a single length of #12 bare copper wire bent into the proper shape and supported by taping it onto opposite sides of a piece of 1/2" PVC tubing. It is pretty darn near the distance apart as 450 Ladder.

    I left ample length of the copper wire to adjust as necessary since it does differ slightly from the specifics on the web site. No amount of trimming or pruning or moving the feedpoint would improve it to less than a 1:2.5 no matter what.

    Well, except when I'd place a finger and thumb across the stub and the radiator, touching the bare wire. Then it was perfect. (???) That tended to make me think that perhaps it needed some capacitance across that point to make it work. (???)

    I believe I'll take WIK's advice and scrap the whole J-Pole idea. Enough time spent already. Good thing wire, tape and plastic tubing is cheap. Not much monetary loss. I'd prefer to have learned something out of all this but the main thing I learned is how little I know about the physics of RF.

    The whole endeavor is an effort to have some 2-meter communications while on camping trips with my tow-behind camper. My wife is the camper enthusiast. I end up sitting in a chair outside the camper for 3-4 days, being bored out of my skull while she reads her Kindle. I have no reason to be there and have no purpose for being there so it's a terrible experience every time.

    I've tried to kill time by talking to some other campers but they usually want to be left alone. If they want company and conversation they bring a group of their family with their trailers. Others will come in groups of friends and associates of their own and select adjacent campsites. Either way, it's a closed society so I just go back to sit in my chair for a couple more days.

    Anyway, I thought I'd talk on my 2-meter rig to kill some of the time but the mobile antenna on my pickup truck limits the coverage severely, especially when we're parked down in a hole by a lake or pond. That's when I decided to try building a J-Pole so I could pull the thing up into a nearby tree and make a few CQ calls on 520.

    The J-Pole project was a flop so anyway, I tried. They'd probably complain if they saw an antenna hanging from one of their trees anyway. The rules say I can't wash my camper or my truck so that's out. I can fish in their pond but they own all the fish in there and they have to be returned. (I choose to fish someplace where any fish I catch is MY fish to decide whether to keep it or release it. If the fish belong to somebody else, I see no point in causing all the pain & suffering in the futile catches.) Did I mention that I HATE camping? What a god-awful waste of time and money to be miserable!

    Now that I know that I can't make a J-Pole work, I think I'll just leave the radio at home and just watch the birds and the squirrels and make imaginary pictures out of the clouds. We did that a lot in the army. I used to be quite good at waiting for days doing nothing.
     
  10. KL7AJ

    KL7AJ Premium Subscriber QRZ Page

    Although there is theoretically a place you can attach the coax that will achieve zero feedline radiation, I always like to use some sort of a baun or choke near the feedpoint, just out of principle. At 2 meters, all you need is a single turn of 6" or so.

    eric
     
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