I'm attempting to follow this white paper in order to build an antenna. This is my first antenna and as such I have a rather simple question.

DIY 137 MHz APT Weather satellite antenna by Adam-9A4QV


Should one of these legs go to ground and the other to signal from the coaxial cable? It's unclear to me from the diagram:

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Whether or not the terminal block is conductive through the center horizontal plane. IE is the top and bottom legs of the antenna separated and thus one goes to signal and one to ground or are they connected in which case they both are connected to both (which I admit makes no sense)?

up vote 7 down vote accepted

One leg of the antenna goes to the center conductor and the other leg of the antenna goes to the braided shield of the coaxial cable. The terminal block is just a way to make the connections, and it has two electrically separate positions. You can use any means of making the connection you have handy, as long as it is small compared to the wavelength.

(Vocabulary tip: the junction between the cable (feed line) and the antenna is called the feed point.)

However, it's worth noting that this is a poor design. Specifically, it has no balun — a device for converting between a balanced device (the antenna, which as you can see is symmetric) and an unbalanced one (the coaxial cable, which has an inside and outside conductor that are very different). This means that the shield of the coaxial cable will act as part of the antenna, and the performance will vary depending on how the cable is routed. It may pick up common-mode noise traveling from your receiving equipment along the cable, which can be a consideration when trying to receive weak signals from sources such as satellites.

You can read more about what a balun is and how it works in this application in the question Using a balun with a resonant dipole.

  • So something like this at the end? amazon.com/CESS-Connector-Antenna-Matching-Transformer/dp/… – Cody Smith Nov 14 at 20:53
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    @CodySmith That is a balun, yes. If you have one lying around it might be worth experimenting with. In the end, do what empirically improves your signal-to-noise ratio, not what someone else's plans tell you. – Kevin Reid AG6YO Nov 14 at 22:07
  • Would this style balun be better suited to what he's trying to do here? amazon.com/RCA-VH54R-Matching-Transformer-VH54R/dp/B00005T3EY – Lance Nov 15 at 21:17
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    @Lance They're both TV baluns, intended for TV frequencies and 75 Ω coaxial cable. The only difference is the connector polarity, and the quality of any given unit's circuit. (None of them would be fit for transmitting, but that's not the application here.) – Kevin Reid AG6YO Nov 15 at 21:51
  • @KevinReid I was speaking more to the physical configuration given the diagram presented. – Lance Nov 16 at 1:18

It doesn't matter which wire is connected to which side of the V-dipole, as they both carry the same signal information (just 180 degrees out of phase with each other).

  • While true, that's not answering what he's asking. – Lance Nov 15 at 21:18
  • @Lance I don't understand your reasoning. I thought Rich gave a good answer to at least part of the OQ. – Mike Waters Nov 15 at 21:24
  • The question, located in the paragraph below the diagram doesn't ask anything about which should go where. It asks if the two legs are isolated from each other through the terminal block or if the terminal clock connects all 4 screws, basically. The answer here is accurate, but doesn't apply. – Lance Nov 16 at 1:23
  • @Lance Perhaps so. – Mike Waters Nov 16 at 3:03
  • From the OP: "Should one of these legs go to ground and the other to signal from the coaxial cable? It's unclear to me from the diagram:..." Lance then comments, "The question, located in the paragraph below the diagram doesn't ask anything about which should go where." --- Please note that my answer DID respond to the question quoted from the OP, above the diagram. – Richard Fry Nov 16 at 20:57

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