A viewer asks why I solder my solder the feed point connection on my J-Pole Antennas with the center feed going to the short side of the antenna. I take you through the reasons why this is a better way to do it.
Transcript: Keith’s got a question about the feed point on the J-Pole antenna: Is there a reason for the way you connect the feed line to the center conductor on the short stub?
Yes there is and the reason it is that it creates a better way in which to attach my feed point. I’m directly soldering the SO-239 connector to the the metal pipe itself. Unlike other antennas, I’m not using any kind of rivets or straps or anything of that nature. I’m doing direct soldering for a better connection. If I solder the edge of that UHF connector onto the longer piece I think that makes for a more mechanically sound connection than doing it the other way around.
But actually it doesn’t really make a difference if you’re feeding into the short part the matching section part or if you’re feeding into to the longer part because with the J-Pole style antenna it’s comprised of two components: the halfwave radiator and at the base is the quarter wave matching section. The quarter wave parts work as two parallel segments and each segment is of the opposite phase of the other, so they cancel each other out.
The key point of the matching section is to provide the proper 50 Ohm impedance to the feed line. As the base of the section the impedance is at zero and at the top of the matching section the impedance is infinitely high. As you move from the bottom to the top, you will find a spot where the impedance is 50 ohms. That’s the point where the feed point connector is attached.
In Rothammels Antenna Book 13th edition page 777 the author states that feeding both the short and the long sides of the antenna are electrically equivalent yet a configuration where the short side is direct fed provides improved over voltage protection because the halfwave section is directly grounded via the outer cable conductor so parenthetically this method also helps reduce overall noise of the antenna by dissipating static to ground.
KB9VBR J-Pole Antennas: https://www.jpole-antenna.com
Rothammels Antenna Book (13th ed, English translation): https://rothammel.com/Rothammels-Antenna-Book
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