Antennas and Harmonics

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KL7AJ, Jan 10, 2018.

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  1. KL7AJ

    KL7AJ Premium Subscriber QRZ Page

    Once upon a time, all amateur bands were harmonically related....for a reason. Not only could you build simpler transmitters, using frequency multipliers for the higher bands....but most fundamental antennas work on harmonic frequencies. Since the WARC bands, which are not harmonically related, came into existence, this is no longer the case.

    Unlike in days of yore, when an end fed dipole could be operated on every ham band (though presenting a very high resistive component on each one), there is no simple antenna that will work on both "standard" harmonic bands and WARC bands....at least without an antenna tuner.

    By the way, the end fed dipole, using a broadband transformer, of which a number of commercial units are available, does NOT work well when there's a lot of reactance. It CAN work admirably on a number of harmonic bands, however. Like any other good antenna or accessory, success depends on using it in its original intent. (One reason the G5RV seems to fairly universally suck as an all-band antenna is that it was originally designed as a 20 meter antenna, on which it performs in sterling fashion. It was NEVER intended as an all band antenna).

    It must also be re-emphasized that resonating and impedance matching are two very different functions, and need to be separated in one's thinking. As a general rule, if you start with a perfectly matched antenna, and then move off of its design frequency, the REACTANCE changes much more rapidly than the radiation resistance. This is why, if you're building a multiband antenna....like a trap dipole or trap vertical...it's much more important to worry about tuning and resonating at your desired operating frequencies than it is to worry about a perfect resistive match. A few moments' fiddling with a Smith chart will verify this.....especially when dealing with a high-impedance feedpoint such as in an end-fed dipole. The reactance oscillates very quickly between nearly infinite capacitance to nearly infinite inductance with a very minuscule frequency change...while the resistance remains fairly consistently high.

    Stay tuned.
     
    KD0CAC likes this.
  2. WA7ARK

    WA7ARK Ham Member QRZ Page

    I recently put up a coax-fed 558 ft long horizontal loop that exhibits resonance on 160m (1 lambda), 80m (2 lambda), 60m(3 lambda), 40m(4 lambda), 20m(8 lambda), 17m(10 lambda), 15m(12 lambda), 12m(14 lambda) and 10m(16 lambda). The only HF band that it misses is 30m. Does that count?
     
    Last edited: Jan 11, 2018 at 12:31 AM
    KU3X likes this.
  3. N0TZU

    N0TZU Premium Subscriber QRZ Page

    Another reason for harmonically related bands was that transmitter harmonics would land in ham bands and not disturb other services.

    (As an antenna is used on higher and higher harmonics, the radiation pattern changes dramatically and can become less useful than expected).
     
    NH7RO likes this.
  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    Absolutely, however, what is better? Having a 12meter antenna that has a bunch of lobes off in random directions, or not having an antenna for that band at all? I can get on 12m and make contacts...
     
  5. N0TZU

    N0TZU Premium Subscriber QRZ Page

    It was a parenthetic comment, just noting a disadvantage of a multi-harmonic antenna. All antennas have advantages and disadvantages. I was not criticizing your particular antenna or use of it. If it works for you, great!
     
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  6. NH7RO

    NH7RO Ham Member QRZ Page

    I used to play my harmonica in a band back in my high school/SWL days---and I had antennas strung around my parent's house then---but I don't think that's quite the same thing Al's talking about here. :)
     
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  7. K2LCK

    K2LCK Ham Member QRZ Page

    'tennas work at odd multiples of 1/4 wave....... 40 works on 15 , 2 works on 450,etc........
     
  8. G3TXQ

    G3TXQ Ham Member QRZ Page

    I see the myth continues to be promulgated ;)

    The title of Varney's original 1958 article was "An Effective Multiband Aerial of Simple Construction":
    http://www.k6mhe.com/n7ws/G5RV.PDF

    His later 1984 article was entitled "G5RV Multi-Band Antenna" which begins "The G5RV Antenna, with its special feeder arrangement, is a multiband centre-fed antenna capable of very efficient operation on all hf bands from 3.5 to 28mhz, ...."

    Where did the idea arise that he originally designed it as a monoband 20m antenna?

    Steve G3TXQ
     
    Last edited: Jan 11, 2018 at 2:13 PM
    KD0CAC likes this.
  9. KL7AJ

    KL7AJ Premium Subscriber QRZ Page

    That specifically applies to CENTER FED antennas. The true Windom was the first one to work reasonably well on EVEN harmonics.
     
  10. K2LCK

    K2LCK Ham Member QRZ Page

    yeh, a windom is at best a compromise......but it is still my primary HF antenna below 20 mtrs....
     

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