[HCARC] Parasitic Elements

Kerry Sandstrom kerryk5ks at hughes.net
Sun Nov 11 20:14:22 EST 2012


Gary,

I thought I'd give you a few words on parasitic elements so the guy wires and arrays might make more sense.

  Basically if you have some thing metal that is resonant, it will extract power from an electromagnetic (EM) wave and then re-radiate that power.  The phase difference between the original EM wave and the reradiated EM wave is determined by the length of the metal thing.  When you "tune" the elements of a Yagi, you are adjusting the phase of the re-radiated signal from each parasitic element.  You can adjust for maximum forward gain, minimum backlobe or minimum sidelobes, but not all.  For parasitic elements, the length is approximately half a wavelength.

If you have guy wires, you should break them up with an egg insulator in lengths less than half a wavelength of the highest frequency you intend to use.  If a guy wire or other metal structure is near resonance, it will act as a parasitic element and distort your antenna pattern.

If you have metal objects that are too close to your driven element, there will be capacitive and/or inductive coupling in addition to coupling from the EM wave.  The capacitive and inductive coupling messes up the phase relationship you try to establish between the radiated and re-radiated wave.  With parasitic elements spaced too close, the antenna doesn't work as well as it should.

With quarter wave vertical elements over a ground plane, you really have half wave resonant elements due to the image of the quarter wave element created by the ground.

Just a little background that might make the topic seem a little more sensible.

Kerry


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