J-Pole Problem

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

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  1. KA4DPO

    KA4DPO Premium Subscriber QRZ Page

    I guess Kraus covered it as well and so did several other texts in Fields and Waves class. A half wave element does not need a radial system to provide low angle radiation nor will radials improve it's performance. It can be grounded for lightning protection but the return path for RF is through the matching section. Look at a trailing ZEP, it did not need any counterpoise and the matching sections only return was the transmitter not the frame of the airship. You may want to check several articles on copper cactus J-poles. The bottom of the stub is shorted and grounded to a metal mast. The coax (with a choke at the feed point) runs parallel to the mast so which one carries the common mode current? how would radials improve it's performance?

    So I will agree to to disagree with you and you might also explain why a center fed half wave vertical antenna does not require radials but an end fed does and how it improves efficiency.
     
  2. W9XMT

    W9XMT Ham Member QRZ Page

    The NEC-2D analysis linked below compares the free space elevation patterns, directivities and feedpoint impedances for 1/4-wave and 5/8-wave ground planes, 1/2-wave center fed dipoles, and end-fed 1/2-wave monopoles.

    The biggest difference among all of them is their feedpoint impedance, which in some cases would be difficult to match to normal transmission lines.

    In reality the end-fed 1/2-wave monopole would have a higher feedpoint impedance than shown if fed at the extreme end. NEC forces a feedpoint away from the open end of a conductor, and in this case it was located on the 2nd of 30 segments in the monopole.

    If the end-fed monopole was driven against a set of four horizontal, 1/2-wave radials (making it a ground plane), then its feedpoint impedance would have much less reactance, and be easier to match.

    But, assuming matching networks with the same loss were used with each of the radiators compared, the fields they would radiate in the horizontal plane for a given applied power would essentially be equal.

    http://www.box.com/s/ec014bce87732e921d9c
     
  3. KA4DPO

    KA4DPO Premium Subscriber QRZ Page

    Thanks for adding that. The impedance at the end of the J-pole is not infinite, end effect and loading of the matching section lower it somewhat but you would need a network analyzer to measure it. The quarter wave matching section, if properly matched, is nearly lossless so the pattern for a vertical half wave monopople and a center fed half wave dipole are virtually identical. Assuming the end fed antenna is properly loaded and matched to the feedline, adding radials will not improve it's performance. I don't know where these things come from but they cause people to make some very incorrect assumptions.
     
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