Trying to understand how the J-pole works

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Trying to understand how the J-pole works

Postby Alexandre » Sun May 22, 2011 9:55 pm

Hi all,

I am still unclear about how the J-pole antenna works. I would visualize the part above the feed point as a dipole antenna, and the bottom part as a shorted stub for impedance matching. However, since the elements are folded on each other, it does not seem to be a real dipole. My question is, what's the function of the shorter element? To cancel 1/4 wavelength of radiation from the longer element? But then, for what purpose? If we lengthen or shorten it(the shorter element), what will be the effect on the antenna?



Alex, Va2vpa
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Re: Trying to understand how the J-pole works

Postby Eamon » Sun May 22, 2011 10:43 pm

Alex,

there are better explanations on the web, but a couple of points to reply to your questions:

I am not sure what you mean by "folded back on each other".

The 1/4 wave section is there to match the 50 ohm coax impedance, to the very high impedance of the end-fed half-wave antenna (maybe it's better to call it a 1/2 wave rather than a dipole, because "dipole" may suggest centre-fed).

The power, where it feeds into the matching section is in a low-impedance form, i.e. relatively high current and low voltage. The stub acts as an impedance transformer. At the end of the stub, it is relatively high voltage and low current, which lets the power transfer efficiently to into the high impedance END of the half-wave (or dipole) antenna.

Here is a general discussion of the end-fed half-wave antenna, which develops the concept through the J-Pole: http://www.aa5tb.com/efha.html

Here is a very complete 4-part treatment of the J-Pole by the late legendary L. B. Cebik. You need to register (for free) on the web site to view it. The name of the series is "Some J-Poles That I Have Known"

http://www.cebik.com/content/a10/vhf/jp1.html
http://www.cebik.com/content/a10/vhf/jp2.html
http://www.cebik.com/content/a10/vhf/jp3.html
http://www.cebik.com/content/a10/vhf/jp4.html

If you want to take it further, you could download some free antenna modelling software. The 4nec2 package includes a J-Pole model. I should probably do a seminar on antenna modelling for anyone who's interested.

- Eamon
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Re: Trying to understand how the J-pole works

Postby Eamon » Sun May 22, 2011 11:09 pm

Here's another good discussion from the QRZ forum archives...

It talks about the fact that the matching point is quite critical on a j-pole, as opposed to the overall length of the antenna.

It also points out that you can expect reasonably good SWR from a 2m jpole on 440, although since it is a 3/2 lambda antenna, the radiation pattern is not very flat.

http://forums.qrz.com/archive/index.php/t-79529.html

- Eamon
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Re: Trying to understand how the J-pole works

Postby vernerle » Thu May 26, 2011 9:01 pm

I **used** to own a 1/2 wave whip antenna that was a clip-on to the car window and the swr was always high even though it seemed to work well. i got rid of it when the whip failed one day going into an underground garage. I have the mount still and might even build a1/4 whip one of these days when I can find a high quality TNC (threaded BNC) male connector.

Vernon - VE2MBS/VE2QQ
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