Demystifying SWR: Ask Dave #28

SWR, or standing wave ratio, always comes up when discussing antennas and feedlines. Although it’s by no means the most important element of antenna performance, it’s the easiest to measure, and thus probably the parameter most talked about. This video explores SWR in some detail.

You can find a complete list of the Ask Dave videos, as well as a form to use to ask questions, by clicking here. The tip jar is available by clicking here.

If you want to play with the spreadsheets, you can download them by clicking here. This downloads a zip file. Note that the spreadsheets are in Libre Office format. The spreadsheets were primarily designed simply to create graphics for the video. Note that they’re probably only accurate for mild excursions from the default values.

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4 Responses to Demystifying SWR: Ask Dave #28

  1. Dave says:

    Any antenna that is an odd multiple of a dipole (odd multiple of half-wavelength) is a complete antenna. All others need some sort of compensation for the missing part of the dipole, such as a counterpoise or radials. If the vertical is ground mounted, as is mine, the length of radials is not critical—just put out as many as you can. If the base of the vertical is elevated, then length matters; generally they should be 1/4-wavelength for whatever band you’ll operate on. For example, if you have a vertical that works on 40, 20, and 10 meters, then you likely need at least six radials; two each for each of the bands. 73

  2. Dave says:

    Hi David. A dipole antenna, whether oriented vertically or horizontally, is a complete antenna—no further parts are required. However, a quarter-wave vertical antenna is only half of an antenna; the other “half” is made up of the ground plane or radials. J-poles are actually end-fed half-wave antennas and don’t need radials. Note that the visible feedpoint (where the coax connects) is not the electrical feedpoint. 73

  3. David DO1DAW says:

    A fellow OM explained to me why a half wave vertical antenna needs no radials but a 5/8 wave length goes. I didn ‘t understand his answer so I hope you can help me. I have been reading about j-pole antennas that are actually 3/4 wave. I am confused about lengths radials (how many what length). I follow your videos and I hsve learned much from them. I hope xou csn enlighten me and other OMs on this.

    73 de David DO1DAW Germany

  4. David DO1DAW says:

    My ham buddy told me that a vertical half wave antenna needs no radials but a 5/8 antenna does. I didn’t understand hus explanation so I thought you might help me out. I was aldo reading about j-pole antennas that are 3/4 wave long. now I sm really confused. After your excellent discussion of swr I hope you will be able to enlighten me and other OM’s on antenna length.

    73 de David DO1DAW, Germany

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