First attempt at a dipole

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KF7YED, Sep 12, 2012.

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  1. KF7YED

    KF7YED Ham Member QRZ Page

    I've been doing a lot of reading about antennas and this is my first attempt at building one. (Newly licensed and have only been listening on a random wire until now...) I've cut three segments for a 10-20-40meter multiband dipole (since 80m-lengths won't fit in my attic) and added a 1:1 balun at the feedpoint. If these pictures come through, can you guys tell me if it'll work before I climb all over the attic installing it... :) The segments are cut at 8'3" for 10m, 16'7" for 20m, and 32'11" for 40m, all measured from the point where they exit the center insulator to the point where they enter the end insulator. I added 5" - 6" extra to wrap back on itself at the ends. The 1:1 balun is about 14 turns (about 20 ft.) of RG8U on a 4" PVC pipe. I'd rather have used RG8x but this is what I had already. I'll use RG8U again to run down into the shack, again, because its what I have.

    I only have the built-in SWR meter on my Icom 718 to judge the effectiveness of the match, so if I'm in the ballpark I can fiddle with it later. Or build a better one with the proper cable (-8x or 213?)

    Thanks in advance for help for the new guy...

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  2. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    RG8 is fine.

    Your measurements sound fine, too. How well it actually works depends entirely on how high above ground it will be and what the attic and roof construction materials are.

    I don't know how good or bad the SWR metering in the IC-718 is, but it's better than nothing. Get it up, separate all the ends as much as possible from each other, run the whole affair so the antenna is in as much of a "straight line" (from one end to the other) as possible, and measure the results! Normally I'd make the 40m dipole the "top" one, and let the other two droop down a bit below that, so the 15m and 10m are like "inverted vee" configuration, under the "flattop" 40m antenna. Tie them all off to supports, don't let the ends of the antenna wires come too close to anything, and let 'er rip!
     
  3. KF7YED

    KF7YED Ham Member QRZ Page

    That's about how I envisioned it, too: the 40m straight out and the 10/20 legs at a slight downward angle. Actually, I think the 40m might have to bend a little at each end or maybe not be right along the roof peak but offset a little "downhill" on one side and "downhill" on the other side of the peak to squeeze into my 65' attic length.

    Thanks for the quick reply!
     
  4. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Don't worry about small stuff like that, it should be fine.

    More important is "height" and "materials" around it. Obviously, the higher the better, and the best materials around it would be plain old wood (plywood, etc) without any foil-backed insulation, cement, bricks, stucco, etc.

    Good luck!
     
  5. KB4QAA

    KB4QAA XML Subscriber QRZ Page

    Looks good for a first antenna!
     
  6. K9ZMD

    K9ZMD Subscriber QRZ Page

    The total length of a half wave antenna is measured from one extreme end of the wire to the other extreme end of the wire. That includes the length of the center insulator and the length of the little loop through each end insulator.

    I suspect that your antenna elements, as you've described them, might be a bit long (resonant somewhat lower on each band than you intended). Add to this, detuning by nearby stuff in the attic (likely to lower the resonant frequency, too) and you will probably need at least one more trip to the attic to prune wires to resonance.

    On a positive note, having the elements too long is not such a bad thing; it is easy to prune wire. In contrast, it is a bear to add wire if the elements are initially too short.

    Oh, there is still another positive note: Interaction between fan dipole elements is highly variable and unique to each installation. It could happen that your own arrangement of fan dipole elements will interact to increase resonant frequency on each band, thus counteracting other factors and resulting in resonance that is spot on for each band. That would be nice.

    Back to reality though, Murphy's law sez it won't happen that way. But remember, you will build experience and character each time you climb into the attic.

    Tip: If resonance on any band is x percent low, then your antenna is x percent too long. With recorded dimensions and the means* to locate actual resonance, you can calculate exactly how much wire must be pruned to shift resonance up to the desired frequency. Same goes for resonance that is x percent too high (except ;) it is lots easier to prune elements than to lengthen elements).

    * Owning an antenna analyzer, or having a buddy who owns one, is a blessing because you can pinpoint where the antenna is resonant even if that spot is out of the ham band.

    You are a bit more limited with just an SWR meter because that requires you to transmit a signal (legal only within your frequency authorizations). You can still get 'er done, but takes a little common sense & knowledge.

    Easiest is when resonance is off, but still within your authorizations so you can legally locate the lowest SWR reading and take it from there. Too low means prune the wire. Too high means add some wire.

    It gets harder if initial resonance seems to be somewhere out of your authorization and your only hint is lower SWR at the low freq edge or at the high freq edge.

    Worst is when the SWR is way high over the entire authorization and you can't determine if one edge or the other has a lower reading; this is one time you would be happy to know the wire is probably too long. The solution is to cut and measure, cut and measure, cut and measure, etc (remember, you measure the antenna in the final position). This is most often the circumstance that motivates hams to buy an antenna analyzer or get real cozy with a local ham who owns one. :)
     
  7. KC8VWM

    KC8VWM Premium Subscriber QRZ Page

    Bending the ends to fit is perfectly fine. Doesn't alter performance much if anything at all.
     
  8. WA4OTD

    WA4OTD XML Subscriber QRZ Page

    Nice looking for the attic! Will you pull the PVC pipe to the top of the roof pitch somehow? I would recommend that. The elements can be pulled any direction but keep the wires for any dipole as close to 180 degrees as possible leaving the PVC pipe.

    How high is the attic?
     
  9. KF7YED

    KF7YED Ham Member QRZ Page

    OTD-- Yes, I will hang the PVC up in the peak as close as I can. I used a piece of suspended ceiling wire to make a hanger for it: drilled two holes and run the rod through and bend a loop in the top of it. It only adds about 3 inches on top of the tube. Oh-- The attic peak is unfortunately only about 20-something feet from the ground. Not much I can do about that...

    The roof is asphalt shingle/plywood/blown insulation, etc. Almost no duct work to speak of- just a couple vent pipes but they're nowhere near the antenna. I do have aluminum gutters but again, not near the antenna elements. I know "near" is a relative term but it'll probably be at least 15' from the nearest gutters. I'm hoping I can get this strung up one morning this week before the attic gets too hot. But we have company coming next weekend so.... :)

    Thanks for the input!
     
  10. NA0AA

    NA0AA Ham Member QRZ Page

    I think that is RG-58, but still, it will work fine.

    If you can hang all the element pairs 180 degrees to each other but not to the other wires, it will tend to reduce the interaction, if space allows - if not, don't sweat it.

    But I concur with the comment above that finding another amatetur with an analyzer will make tuning this antenna much faster and more accurate, minizing the time you spend in the attic and insulation.

    My experience with parallel wire fan dipoles - what you have built - is that it can be a challenge to get all three bands down to a 1.5 to 1 or better match due to the interaction with the wires - I've built a few of them. However, an SWR of 2.5 is not all that lossy and any tuner makes short work of making your tranciever happy. IOW, if you have a tuner, don't sweat the match down too far.

    BTW, nice first antenna - we should all do so good.
     
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