3 band FAN dipole

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by WL7PM, Oct 14, 2021.

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  1. WL7PM

    WL7PM Ham Member QRZ Page

    Plan to construct a 3 band FAN dipole for 80 -40 -30 meters.
    I realize the wires will interact to a degree.
    What is the "rule of thumb" regards the order in which one prunes the wire pairs ?
    Start with the lowest frequency ( 80 meters) and then 40, and 30 last ?
    Are 30 and 40 meters too close together to get along in a common feed arrangement ?
    .
    Thank you for any tips, hints, or information.
     
  2. AI5DH

    AI5DH Ham Member QRZ Page

    All you want to know is just a simple GOOGLE search and you get this.
     
  3. RW4HFN

    RW4HFN Ham Member QRZ Page

  4. K1VW

    K1VW Premium Subscriber QRZ Page

    I've got a couple of 2 element fan dipoles and a 3 element dipole. Mine are real "fans" with some angular space between them, not just a few inches in parallel. In general I have not found "I adjusted this one and now that one is off". It just didn't seem like a problem. A bigger problem was I tuned it at 30' and then raised it to 50 or 60'... and where I was previously tuned at the low end of the band, now it's tuned to the high end of the band. That can happen with any dipole, but I'm just saying that was a bigger issue than any kind of cyclical retuning between elements.
     
  5. K9UR

    K9UR Subscriber QRZ Page


    Yes, this is a good point -- as a dipole is raised, the impact of additional loading due to proximity to ground decreases. Resonance will go up. I always tune dipoles that are close to the ground about 2% lower in frequency than where I want them once at 60 ft. It's pretty easy to calculate and makes for less re-adjustment once at the final height.

    I also don't see much interaction band to band although I did have a hard time getting a fan dipole for 30M and for 75M (resonant at 3800) to work well. 75M was on the money but 30M was 1.5:1 and that was best i could do... yes, a close 3rd harmonic issue perhaps. I also thought about adding some capacitance loading wires to the 75M dipole instead (to get it to resonate down at 10.110 Mhz) but got lazy.... That's an old 40M/15M dual band dipole trick....figured it might work on 75/30 too...
     
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  6. N4UFO

    N4UFO Ham Member QRZ Page

    As someone who knows a little bit about education himself and is married to an educator with a Master's degree and 27 years experience, let me politely inform you that not everyone learns in the same way. There are combinations of styles, methods, etc. with terms like abstract, concrete, visual, aural, kinaesthetic, social, spatial and others used to describe it. But rather than elaborate and explain all that, I'll just cut to the chase... not everyone learns the same way. I'm sure the guy has heard of google before. It may be that reading a website is just not a good way for him to absorb material and he does better by asking questions. So, he comes to a forum on the topic of desire and posts his questions. And that IS what these forums are for, right? Answering questions and educating other hams?

    Instead of just assuming it's laziness or similar on his part and assume the role of a parent and/or educator instructing him on how to research something and/or the type and extent of effort to put into such... how about you either answer the questions he has asked or JUST NOT POST.

    To post in the manner you tend to do is condescending and shows a tendency on your part to demonstrate dominance and a need to denigrate. Please stop it.
     
    Last edited: Oct 15, 2021
    N9JCQ, N5PNZ, KG4DYN and 11 others like this.
  7. N4UFO

    N4UFO Ham Member QRZ Page

    I realize the wires will interact to a degree. - As others have posted, if the wires are parallel they will interact more than if they are truly 'fanned out' at angles.

    What is the "rule of thumb" regards the order in which one prunes the wire pairs ?
    Start with the lowest frequency ( 80 meters) and then 40, and 30 last ?


    Yes, BUT... try getting them in the ballpark, cut a bit long, and then fold back the wires a bit at a time... first 80, then 40, then 30. Then you go back and check 80 again, check 40 again, check 30 again, adjusting each time. If you decide to cut the folded back parts of the wire off, realize that if the wires are insulated, you won't want to cut them ALL the way off. Because even with them folded back, they are still adding some to the antennas. I try cutting about half off and rechecking... What some guys do is run the wire through an insulator and leave a bit of excess hang down. You just fold back and/or trim on that excess. That said... if this is bare wire, when you fold it back, you are pretty close to where you want to cut.

    Are 30 and 40 meters too close together to get along in a common feed arrangement ?
    - Not at all... especially if truly fanned out and not parallel. Personally, I have always done the parallel wires given the situations I have had. Presently I have a set up parallel vertical wires up. (See my QRZ page) In the past I had inverted vees up with some parallel and some separated. They were one set 80 & 30m, one set 40, 20, 10m and a third set was 17, 12, 6m. Worked fairly well.

    squalo.JPG


    Thank you for any tips, hints, or information.
    - Hope my answers helped... any more questions, just fire away! :) 73, Kevin N4UFO
     
    WL7PM likes this.
  8. RW4HFN

    RW4HFN Ham Member QRZ Page

  9. NN4RH

    NN4RH Premium Subscriber QRZ Page

    .
     
    Last edited: Oct 15, 2021
  10. WA7ARK

    WA7ARK Ham Member QRZ Page

    Here are the reasons why multiple center-fed dipoles sharing a common feedpoint (fan dipoles) interact:

    1. Lack of symmetry (unbalance) with respect to the center (feedpoint). The biggest contributor to lack of symmetry is if you leave off a Common Mode choke (~10 to 14 turns of RG400 coax wound through a FT240 type 31 ferrite core) at the feedpoint. The reason that causes interaction is because without the choke, that unbalances the entire antenna by providing a reactive conductor (which can store energy as well as radiate) for CM current along the coax shield in parallel with only the dipole halves on one side of the feedpoint.

    2. Physical lack of symmetry, such as the coax not coming away from the feedpoint at right angles, or installing the dipoles so that they are sloped with respect to the earth below also also causes unbalance.

    3. Avoiding frequency combinations where F2 is nearly exactly 3xF1 (or possibly 5xF1). This is likely to happen if you include 40m/15m or 80m/30m... This is close to your specific case because if you tune one dipole to 10.17 MHz, the closer the 80m dipole is tuned to 3.5MHz, the closer it gets to the problem area.

    If you follow the above rules, the interaction is so small so as not be noticed. That is not to say that you can expect a fan dipole to have a perfect unity swr50 on all design frequencies. You can make the dipoles be resonant on their respective frequencies (ie jX=0), but that does not necessarily mean that the R term will be exactly 50 Ohms. Remember that the height agl of any dipole effects its feedpoint R. A practical 80/40/30 fan (eg H=40ft agl) has the 80m dipole low relative to its length, while the 30m dipole is high relative to its length, so the Rs deviate from 50 Ohms.

    I have a parametric AutoEz model of a three-band, coax-fed, properly choked coax, narrow-spaced, parallel wire dipole I can run for you. Here are the variables of that model:
    upload_2021-10-15_8-16-7.png

    It is the tuning (by varying K, L, and M) of each dipole that moves the Swr minimum to a specific design frequency by minimizing jX. Adding a length of feeding coax transforms both the R and jX, but leaves the Swr minimum at the original frequency, and slightly lowers the Swr50 seen at the feeding end due to coax loss.

    If you do encounter some interaction, tune the longest dipole first and the shortest last. You can only tell if there is interaction by hoisting the antenna to its final deployment position each time you change something. It is really tedious to make the antenna work that way, so by following the rules above, you should be able to get to a useful tuning with only one or two tries...

    If you specify frequencies A, B, and D, height H, wire spacing Q, and coax length X, and tell me what type and gauge of wire you plan to use (bare or insulated), what the earth below the antenna is like, what specific type of coax you will use, I can optimize the design, and tell you wire lengths.
     
    Last edited: Oct 15, 2021
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