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Tuning Multi-Band Dipole

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by N4MTB, Sep 17, 2010.

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  1. N4MTB

    N4MTB Ham Member QRZ Page

    Building a 10-15-20 "catwisker" type dipole. What element sould I tune first??? I don't want this to turn into an exercise in Frustration !!! As we all know this can turn into fast. Thanks to all that reply...
  2. KL7AJ

    KL7AJ Ham Member QRZ Page

    You always start from the inner elements and work out, whether it's a trap dipole or trap vertical. (Highest frequency band and work down). Generally this goes very quickly.

  3. KE5FRF

    KE5FRF Ham Member QRZ Page

    Another name for these type of antennas is "Fan dipole" or parallel dipole. I've never heard catwisker but thats very discriptive.

    It is best to tune the LOWEST band first, or LONGEST WAVELENGTH. This is only logical if you think about it. The longer the wavelength, the less precise you have to be with antenna length. Let me explain.

    Lets take 80 meters for example. One half wave on 80 meters is approximately 132 feet. A basic dipole is a 1/2 wavelength antenna. 1/2 wave on 75 meters is approximately 123 feet. This means **the difference** between THE ends of the 75/80 phone and CW band is about 9 feet of wire.

    I'm not using math with engineering level accuracy here, but stay with me.

    On 20 meters a 1/2 wavelength is approximately 33 feet. Now, what we call the "20 meter" band is actually nestled between 20 and 21 meters. THE ENTIRE BANDWIDTH of that band is only a portion of ONE METERS WORTH of bandwidth, whereas the 75/80 meter band has 5 meters worth of bandwidth.

    The difference between the ends of the band on 20 meters would ONLY BE ABOUT THREE FEET of wire.

    As the frequency goes up, the less wire that it takes to make a significant change in an antenna.

    If we got down to the 50 Mhz band, we might be talking about less than ONE FOOT worth of wire to seriously change the resonant point on an antenna.

    THEREFORE, to answer your question with logic and simple math, it is best to tune your lowest frequency, longest wavelength elements first and work your way upn in frequency to tune the next. This is because the shorter elements are more difficult to tune and the effects of coupling to the other wires are more pronounced. Therefore, cutting a couple of inches off of your 10 meter wires will have very little effect on the 15 and 20 meter elements, whereas cutting the 20 meter element, if done LAST, might have a serious effect on the 10 meter and possibly 15 meter elements.

    Tune lowest band first.
  4. KE5FRF

    KE5FRF Ham Member QRZ Page

    Eric, to avoid confusion, I do not believe he is referring to a trap dipole...By "cat whisker" I am thinking he means a fan dipole (if my visual of what cat whiskers look like is correct).

    If this is what he means, then I believe the very opposite advice would be true and he should start out with his low band wires first.
  5. N4MTB

    N4MTB Ham Member QRZ Page

    Sorry for the poor terminology , but I am refering to a fan dipole. Specifically for 10-15-20 meter phone.
  6. KC2UGV

    KC2UGV Ham Member QRZ Page

    I've always trimmed the higher-frequency elements first, then worked out. Always a "tuned once, and worked". I've never had to double-back on other elements. I've only built like 3 of them, so YMMV.
  7. N4UM

    N4UM Ham Member QRZ Page

    I've built a number of them and always tuner the lower frequency (longer) elements first. In my experience, the lower frequency elements affect the tuning of the higher frequency elements more than vice versa.
  8. G3TXQ

    G3TXQ Ham Member QRZ Page

    Yes, trim the lowest frequency dipole first; then move up in frequency etc.

    The potentially larger dimensional changes on the longer dipoles have more effect on the shorter dipoles than vice-versa; so you want to do the longer dipoles first. You can still expect a few iterations.

    Eric's advice is fine for trapped antennas, but not for a fan dipole.

    Steve G3TXQ
  9. AE5JU

    AE5JU Ham Member QRZ Page

    Yes, trim the lower frequency pair first.

    Also, the wider the elements are separated, the less interaction.
  10. NA0AA

    NA0AA Ham Member QRZ Page

    I'm in the longest to shortest camp myself, but I found that with 6" wire spacing I needed several trips up and down with the last tri-bander [also a 10/15/20 meter]. But once tuned, they are great antennas. I love the coaxial cable savings!
  11. KL7AJ

    KL7AJ Ham Member QRZ Page

    I might mention that interaction isn't necessarily bad; but reducing it just makes tuning more straightforward.

    If you have infinite patience and a network analyzer, it's amazing what you can do with a fan dipole by taking ADVANTAGE of interaction. (I have the former asset....but, alas, I no longer have the latter at my disposal).

    For this one experiment, I started out with a two band fan....80 and 75! I cut one dipole for exactly 3.5 and another for 4.0. At right angles, they acted totally independently. As I reduced the angle between the two antennas, the two resonant "humps" moved closer together, and at one point I got to where I could cover the entire band with no more than 1.7:1 Swr anywhere. This confirmed some very old publications of this antenna.

    I decided to add 40 meters, again with top and bottom band dipoles, (7.0 and 7.3 Mhz). I was then able to cover all of 40 with less than 1.4:1, using the same twiddle method..

    But curiously enough, I also found a PERFECT match up on 20 meters, near the middle of the band! I never would have even thought to CHECK there if I didn't have a network analyzer! Alas, I neglected to log the actual orientation that gave me this, since it was more of a curiosity than a need at the time, since we were at the bottom of the solar cycle.

    Anyway, this merits a lot more investigation. It CAN be done with just an SWR meter, but now that there are some "ham grade" network analyzers out there, it would be a fun thing to pursue. Just do what I didn't do...TAKE NOTES!

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