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I recently made a "deseret j-pole" dual band antenna for basic 2m and 70cm transmitting and receiving use. I have it on a large metal tube behind my house, connected through a 50 ohm pl-259 cable, and adapted to sma-female (the one required by my baofeng UV-5RA). here's the problem, when it is fully connected, it doesn't transmit or receive clearly (if at all). it receives best when not even connected, but when I hold the center prong right next to the center hole on the adaper, but never touch them. what's the problem? any help is appreciated.

here's a crude depiction of how I need to hold the cables for best connection

enter image description here

sorry if I'm unclear.

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  • $\begingroup$ Do you have another radio you can try the antenna with, perhaps a friend's or club member's? And I assume your radio works with its original antenna, but do you have another antenna or cable you can test with it? $\endgroup$
    – MoTLD
    Commented Mar 30, 2017 at 4:22
  • $\begingroup$ Yes, I tried my dad's radio, same problem. However it's the same kind of radio, so that's not completely out. $\endgroup$
    – Trevader2413
    Commented Mar 30, 2017 at 13:21
  • $\begingroup$ At least that rules out your radio being somehow broken. $\endgroup$
    – MoTLD
    Commented Mar 30, 2017 at 16:48
  • $\begingroup$ Have you measured the SWR? If you didn't build the antenna correctly, you could have a horrible SWR mismatch that could cause the symptoms you describe. $\endgroup$
    – rclocher3
    Commented Mar 31, 2017 at 1:03
  • $\begingroup$ Yes, when it was built six months ago or so, we measured it's swr as about 1.1 though I have no way to test it now. $\endgroup$
    – Trevader2413
    Commented Apr 3, 2017 at 1:40

2 Answers 2

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You mention that it's mounted to metal - is it electrically connected to that mount? A J-pole should be ungrounded and isolated from anything metal. Also helpful is a ferrite choke around the coax near where it feeds the antenna, but most important is that it be mounted with non-conductive hardware.

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  • $\begingroup$ Would a j-pole such as this one link ( the one on the right) also require NOT to be grounded? Mine looks nearly identical and I notice it says it does not need to be grounded (implying it can be). If that's the case I guess I have a trip to me roof in the near future. Thanks. EDIT: forgot to mention, the metal ground of the antenna is in direct contact with the metal pole, so yes, electrically connecte. $\endgroup$
    – Trevader2413
    Commented Mar 30, 2017 at 13:19
  • $\begingroup$ You're right, the wording is ambiguous, but that looks like a pretty standard J-pole. I would not ground it. $\endgroup$
    – MoTLD
    Commented Mar 30, 2017 at 16:52
  • $\begingroup$ If a J-pole needs to be ungrounded and isolated from anything metal, how do you connect coax to it without developing a common-mode current on the shield? $\endgroup$
    – Phil Frost - W8II
    Commented Apr 1, 2017 at 18:31
  • $\begingroup$ Thanks, I wrapped the pole in vinyl electrical tape, then more rugged duct tape on top of that, then mounted the antenna to that. it's not permanent, however, the antenna seems to be receiving great now! (well, not on my radio, but my dad's radio. but my radio still works just fine with the normal HT nagoya antenna for it, but that'll be another question. $\endgroup$
    – Trevader2413
    Commented Apr 6, 2017 at 2:44
  • $\begingroup$ @PhilFrost-W8II My understanding is that common-mode current on the shield is an inherent downside to the coax-fed j-pole design because of its unbalanced-to-balanced-to-unbalanced nature and the best you can do is the ferrite choke I mentioned. I've read that a carefully designed ground plane can also help but still doesn't eliminate the problem, but I have no experience with that. In general, when someone is having j-pole troubles and I've suggested they isolate it from the mounting, it's helped them. $\endgroup$
    – MoTLD
    Commented Apr 15, 2017 at 23:06
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In general, if making an incomplete connection to your antenna works somewhat but a complete one is entirely silent, that means you have an open or short circuit somewhere in the antenna system.

The reason the incomplete connection works somewhat is because you have contact (or in your particular case capacitive coupling) with a large piece of metal and no shield connection. Thus, you have basically a weirdly-shaped monopole antenna consisting of the feed line and everything attached to it, and the radio's chassis as the "counterpoise" side of things.

Once you make both connections solidly, the coax shield starts functioning as intended and the signal cannot pass through it. So if you have a short or open circuit anywhere along the coax or at the antenna feedback, you have basically a coax stub, which is not an antenna.

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  • $\begingroup$ shortly after setting up the antenna and found it working wrong, my dad and i talked to some buddies and tested the cable for shorts. we jumpered the atenna end (attached the center to the shield, and tested continuity with a multimeter at the other end, and it worked perfectly. I'm not ruling out a short though, I'm going to have to test that as soon as my roof dries (it just rained, and it's a metal roof). thanks for the suggestion, I'll test out what I have so far. $\endgroup$
    – Trevader2413
    Commented Mar 30, 2017 at 22:46

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