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Fan Dipoles for non-harmonically related frequencies

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W0RIO, Apr 10, 2020.

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  1. W0RIO

    W0RIO Ham Member QRZ Page

    From what I've read about fan dipoles, one of their major shortcomings is that they are really good
    at radiating undesired harmonics. For example, a fan dipole for 40m, 20m, and 15m used on
    40m would radiate harmonics nicely on the 2nd and 3rd harmonics of 7Mhz.

    That got me thinking that a good fan dipole design could be made for non-harmonically related frequencies.

    A few of the many possible non-resonant dipole band combinations could be:
    20m, 17m, 15m
    60m, 40m, 30m
    15m, 12m, 10m

    I'm sure this has been tried before, has anybody here worked with such a design?

    Google found a link to a 20m, 17m, 15m fan dipole by VE2XIP, but the article is no longer online.
  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    I've built many fan dipoles with different bands depending on the needs at that station. I've definitely built the 15m, 12m, 10m version you mention and have also built an all WARC version (30m, 17m, 12m) and many other variations and they all have worked fine.

    Harmonic radiation isn't a problem unless you run a rig or amplifier with poor harmonic suppression. Sure, that was an issue in early ham rigs but decently designed rigs of the last 75 years or so shouldn't really have a problem with delivering harmonic energy to the antenna system.
    K3RW, N1VAU, WB5YUZ and 2 others like this.
  3. W4HWD

    W4HWD Platinum Subscriber Platinum Subscriber QRZ Page

    ANY antenna can radiate harmonics if hooked to a nasty transmitter. Are you bored?

    I can hear the 6th (yes, the s-i-x-t-h) harmonic of a nasty aeronautical beacon on 160m, and that beacon is 30 miles away - and I’d bet the antenna is not a fan dipole.
    Last edited: Apr 10, 2020
    AK5B likes this.
  4. AK5B

    AK5B Ham Member QRZ Page

    FWIW, I enjoy my 4-band fan Marconi well (20-17-15-10m) and haven't noticed any ill effects even with close element spacing.
  5. KX4OM

    KX4OM Ham Member QRZ Page

    The harmonics generated by the transmitter must be at least 43 dB below the frequency of the fundamental (FCC requirement). The antenna does not generate any additional harmonics. If the power radiated on 40m=100W, the power radiated on 20m must be <.005W. The 3rd and 4th harmonics will be even lower. Et cetera.

    Ted, KX4OM
    AK5B likes this.
  6. W0RIO

    W0RIO Ham Member QRZ Page

    True, but fan dipoles tend to be more efficient radiators right around the
    harmonic frequencies, so they enhance any harmonic radiation coming from
    the transmitter.

    I'm aware that a 7Mhz dipole will already radiate well around 21Mhz,
    but it will radiate poorly at 14Mhz. A 40m-20m fan dipole would also
    radiate well at 14Mhz.

    And yes, I'm bored, heh.
    Last edited: Apr 10, 2020
  7. W4HWD

    W4HWD Platinum Subscriber Platinum Subscriber QRZ Page

    I’ve never heard that, so, under the mantra of “you learn something new everyday”, please explain how a fan dipole is better at radiating harmonics than any other antenna - especially a gain antenna.
  8. AG6QR

    AG6QR Premium Subscriber QRZ Page

    More efficient than what?

    A single dipole doesn't work well at even harmonics, so a fan dipole may radiate those frequencies better than a single dipole radiates those frequencies.

    But it typically won't radiate harmonics better than it radiates the fundamental. So the radiated harmonics should be as far below the fundamental as they were when the signal left the transmitter.
  9. W4HWD

    W4HWD Platinum Subscriber Platinum Subscriber QRZ Page

    Also, if it’s true a fan dipole does all this and more, then a multi-band yagi should radiate harmonics even better...?
    AK5B likes this.
  10. NK7Z

    NK7Z XML Subscriber QRZ Page

    Hi Ted,
    I have been looking for a citation along that line, showing actual numbers, for a while now. Could you point me at the FCC Part number that covers that please.

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