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I have as assignment to make a half wave dipole antenna, that radiates at 433 MHz. I am trying to figure out how to use a coaxial cable as transmission line. I know that the coaxial cable is unbalanced and an ideal half wave dipole is balanced. To properly connect these two is a balun required.

The main question I have about the coaxial cable is how do I connect the coaxial cable to the dipole when only the inner conductor transfers the signal? What is the best way to connect it together, taking in consideration that I also need a 180 degrees shifted signal connected to one of the arms of the dipole?

Glenn W9IQ
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Mike576
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  • Have you looked online for what a balun looks like electrically? Or is your question about how to actually manufacturer one? Also, please elaborate on the "180 degree shifted signal" - what's that mean? – mike65535 Jun 13 '18 at 12:02
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    https://ham.stackexchange.com/questions/538/using-a-balun-with-a-resonant-dipole – Phil Frost - W8II Jun 13 '18 at 12:51

1 Answers1

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How do I connect the coaxial cable to the dipole

Using a balun. That's its job.

when only the inner conductor transfers the signal

This is an incorrect assumption. Current always flows in loops, so there is always an equal and opposite current somewhere. In a coaxial cable, the signal is carried by both the inner conductor and the outer conductor (or, in some perspectives, “in the space between them”).

If you connect the inner and outer conductor of a coaxial cable to the two arms of a dipole then the dipole will in fact work as an antenna — but the outer conductor of the cable will also radiate/receive. This is the problem a balun solves. If you ignore it, you will still have an antenna, but possibly not the antenna you intended to design.

Kevin Reid AG6YO
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    In fact, there are a whole lot of inverted-V antennas on HF that are built exactly this way (including mine) with the center conductor soldered to one leg of the dipole and the shield soldered to the other. Of course it's not optimal, but it's still a useful antenna. – Duston Jun 13 '18 at 16:34
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    +1 but perhaps a current balun to be susinct? Any hints on construction at 433 MHz? – Glenn W9IQ Jun 13 '18 at 16:36
  • With an unbalanced antenna system using coaxial cable, there are three currents: (1) center conductor current; (2) inside of outer conductor; (3) outside of outer conductor. The difference in currents (2) and (3) is due to the skin affect that allows them to be considered as separate currents. It is that outer current in an unbalanced system, also known as common mode current, that will radiate. A properly terminated coaxial cable will not radiate. Yes, a balun should be used. – K7PEH Jun 13 '18 at 23:48
  • Oke then i think i can change my question a bit. Could you maybe explain to me how the outer conductor starts conducting if the inner conductor transfers the signal and should the outer conductor be grounded? – Mike576 Jun 14 '18 at 07:31
  • @Mike576 That's a very different question. You'll get better answers if you post it as a new question for everyone to see. – Kevin Reid AG6YO Jun 14 '18 at 14:23