Tube or Rod or Wire?

One of my newest Elmerees is now intensely interested in antenna making. He’s making a series of J-poles and dipoles and who knows what else. On Saturday, down at the museum, we got into a discussion about the different types of materials he could use to build antennas. I repeated to him the old chestnut, “The larger the diameter of the material uses to make the antenna, the wider the bandwidth will be.”

Now, I’m not sure exactly where this bit of advice comes from and what the theory is behind it. Can anyone point me towards a discussion of why this is so?

After having said all this, I got an e-mail from my Elmeree this afternoon. He asked, “So, which is better for a vertical antenna, a solid rod or a tube?”

My answer, “Whichever material you have on hand or whichever is cheaper.”

Comments

  1. Elwood Downey, WB0OEW says:

    Try thinking of it this way. Image you are at the center of a dipole looking into the ends of the two wires that spread away in each direction. Think of these ends as the two plates of a capacitor. As the wires get thicker, the plates grow larger and the capacitance increases. Now recall that Xc = 1 / (2 pi f C). If you know a little calculus you can compute dXc / df = -1 / (2 pi f^2 C). In words, this says that the change in Xc for a given change in f is inversely proportional to C. That is, as C gets larger (thicker wires) the reactance changes less for a given change in frequency. So the take-away is that the bandwidth is larger for thicker wires because the capacitance is larger.

    Regarding the tubes, you might also take the elmeree’s question as a point to mention skin effect. At radio frequencies the current flows only very near the surface, so a solid conductor is no better than a thin tube as far as radiating a signal is concerned. Of course, the solid tube is stronger mechanically which can be a separate advantage, so it’s also a good lesson in the many trade offs that are required when building a real antenna.

    73,

  2. David Brodbeck N8SRE says:

    Not sure about verticals, but isn’t it the opposite for loop antennas? Larger diameter = lower resistance = higher Q = narrower bandwidth? For other shapes it’s usually practical considerations that dictate the conductor size, of course.

  3. Dan KB6NU says:

    Excellent explanations. Thanks!

    And, yes, we did discuss skin effect when discussing this phenomenon.

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