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I've been designing a "Super J-Pole" for 10M in MMANA-GAL, but I'm designing it having two 5/8λ sections. However, I came across this picture: Various J-Poles types diagram indicating the phasing stub for a colinear 5/8λ antenna is a λ/4, is that right because I've had been modeling the antenna with a 3/8λ phasing stub? When modeling the λ/4 phasing stub as suggested by the diagram, the pure resistance drops to 72 ohms with the λ/4 stub, the antenna loses 1dB of gain, the max gain lobe narrows and the angle increases by almost a degree, and the 2D diagram shows the high angle lobes as large "rabbit ears". See the pattern below. (The teal color is the overlapping of the two patterns) Combined Radiation Pattern

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    $\begingroup$ J-poles are hard to simulate in EM packages costing less than an annual salary. Closely-spaced conductors, short segments and small loops mean that NEC-based models are far from accurate. NEC doesn't model two-wire transmission lines well. I had better luck in FEKO (mid-price... months). I suspect most longer J-poles in the field have been tuned for best SWR, but have current on the coax and don't radiate optimally. Test your model well, vary the segmentation and shape of the feed area, and be prepared to measure the beam tilt when you build it, and tune for that before matching. $\endgroup$
    – tomnexus
    Nov 7 at 15:15
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    $\begingroup$ Your simulations look like they're over ground - if it's a mast-mounted "ground-independent" antenna, maybe start simulating it in free space? The danger is that you will mix up the natural shape of a perfect dipole over lossy ground (try it quickly), and the breakup of the pattern of the antenna itself. Of course if it touches the ground or uses a ground stake as a counterpoise, then you have no choice. $\endgroup$
    – tomnexus
    Nov 7 at 15:19
  • $\begingroup$ Thanks tomnexus, a few clarifications to my first post. First, I'm modeling the antenna without the λ/4 matching section that creates the J-pole, I'm modeling this as simply an end-fed antenna, and the program is using 400 segments to do the calculations. Secondly, I'm modeling it at 10M (30+ feet) above "real ground", and the actual antenna will be suspended from a tree branch with the lowest part at approximately the modeled height, so no part of the tuning stub will be grounded. $\endgroup$ Nov 7 at 15:35
  • $\begingroup$ Finally, I plan on using a 1:1 balun at the feed points to minimize currents on the coax shield. Please feel free to add any other comments based on this information. $\endgroup$ Nov 7 at 15:35
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    $\begingroup$ OK. 1) 400 segments sounds too Many. miniNEC needs 20/wavelength, up to maybe 100, so for a 1-2 wave structure I'd say 50-100 segments max. 2) balun is good (if the antenna still works, ha ha) but it doesn't prevent current on the coax. Fields are strong near the tip of the dipole and will couple to the coax anyway. Best to try the model with/without coax. 3) I still think you should simulate in free space. Ground will do what it does to the patterns, but won't affect the antenna, so don't pollute the results with reflections. Sorry I can't recommend anything about the phasing section itself. $\endgroup$
    – tomnexus
    Nov 7 at 17:19
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I think it's just a typo in that diagram. If you look at this page on the same site, it gives the source of the 2x 5/8-wave J-pole design as KB1DIG (wayback machine archive). And the dimensions in that diagram are for a 3/8-wave phasing stub. And 3/8 makes good sense.

You could ask John — he does respond to comments on his site if they're intelligent.

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  • $\begingroup$ IMO there's nothing magical about antennas, @LouisSeaman, other than the fact that they work at all of course. If you modeled the antenna and it works better with a 3λ/8 phasing stub than a λ/4 phasing stub, then you've proved your point. Besides, thinking about how the antenna works, the phasing stub is meant to make the top 5λ/8 section be in-phase with the bottom 5λ/8 section; a λ/4 phasing stub can't do that because the phase at the top of the lower 5λ/8 section isn't the same as the phase at the bottom. (I'd think λ/8 for the stub would work, but 3λ/8 must work better somehow.) $\endgroup$
    – rclocher3
    Nov 24 at 16:43

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