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Off center fed dipole input impedance vs height above ground

  1. Jul 20, 2017 #1
    I have a question for the community. I am doing a further study and trying to locate any information regarding input impedance of an off center fed dipole at different heights above ground. A simple dipole targets 73.1 ohms for 1 wave length or higher above ground. I am not sure if the same would hold true for an off center fed dipole. For an example: If I designed my antenna for 3.6 MHz my half wave antenna would be around 134ft. If my input placement were at 90 ft from one end and 44 ft from the other, my calculated input impedance would be around 97 ohms. Would this impedance change be similar to that of a regular dipole? Any data on this would be appreciated.

    JR Richter
  2. jcsd
  3. Jul 20, 2017 #2


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    Staff: Mentor

    There are very helpful free simulation programs for these types of antennas (usually posted by HAMs). Have you looked into downloading any of those? I can try to search for the sources if you need help. There are generally free program versions with limited resolution, and modestly priced versions with more features.
  4. Jul 20, 2017 #3
    I have been searching for awhile now and have not used any of the antenna modeling software yet. I have a lot of data on regular dipoles but none on off center type. I realize that antenna's near field is the major cause of the impedance variations, but will it be the same for the off-center fed type? Any help here would be appreciated.
    JR Richter
  5. Jul 21, 2017 #4


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    The off-centre feed is effectively just an impedance transformer added to an ordinary dipole, so I would expect the result to simply scale up and down as for the normal case. By the way, may I mention that for a very low dipole over imperfect ground, the impedance rises above 73 Ohms, whereas over a sheet of copper it is less than 73 Ohms. I am not sure why accurate impedance matching is of great concern for HF operation, where feeder losses can be made very small.
  6. Jul 22, 2017 #5
    If that's the case, then what would be the input impedance at .35 wavelengths high of an ocf antenna? If a normal center-fed dipole's input impedance (73.1 ohms)would rise close to 100 ohms (3.6 MHz) then would an off-center fed input impedance of 97 ohms rise 30 ohms higher or in this case to 127 ohms?
    In reference to your question, this antenna has been used now with good results on multiple bands. It has been used successfully 3.6 MHz, 7.2 MHz, 14.4 MHz, 18 MHz, 25.5 MHz, 28.8 MHz, 36 MHz and 39 MHz. When plotting input impedance vs different feed locations you will see that the data shows that if the feed point is located at the 33.3% from the end of a normalized dipole @ 3.6 MHz, a multiband usage can be obtained with a current balun of (4:1) put at that point. I know this antenna is not the most efficient but working on that now to see If I can make it better.
    Thanks JR Richter
  7. Jul 22, 2017 #6


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    I think it's to limit the peak volts on the output valve and the feeder plus insulators. They can be pretty fearsome for a 100kW Transmitter say 300Ω feeder (>50kV RMS with a SWR of 1, even). But I only know about Broadcast stuff, not Amateur systems.
  8. Jul 22, 2017 #7
    Thanks Sophie for the input but this antenna is fed at pretty much a lower impedance point, making it a high current and lower voltage input. When fed with a (4:1 ) current balun a fairly good match can be obtained with feed point impedance of 200 ohms at the antenna terminals and 50 ohm impedance at the feed line. The max power is only 1500 watts PEP.
  9. Jul 28, 2017 #8


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    My comment applies whatever Power Device is being used. (Amateurs are just as cost conscious as Broadcasters). No one wants to spend money on a device that can handle an extra 50% of volts / current when the correct matching will allow a cheaper one to be used.
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