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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

    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...

    DSC01635.JPG

    DSC01638.JPG
     
  2. WB2WIK

    WB2WIK Premium Subscriber

    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

    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

    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

    Looks good for a first antenna!
     
  6. K9ZMD

    K9ZMD Subscriber

    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 Ham Member

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

    WA4OTD XML Subscriber

    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

    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

    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.
     
  11. KF7YED

    KF7YED Ham Member

    Thanks for the compliment. I tend to think things through too much sometimes, building it in my head so many times before I actually pick up a piece of wire.
    Yes-- an analyzer would be great. I'm in the middle of Nowhere, MT so I'll have to add it to my wish list. But if I get a tuner first, might that also help a lot if I'm far off? I had a tuner on my wish list but closer to the top... :)
     
  12. KF5FEI

    KF5FEI Ham Member

    Before you start cutting and trimming, create a spreadsheet and record / graph the SWR versus frequency for each band to get an idea of where you are at length-wise. The lowest attainable SWR reading will vary according to whether or not the dipole is set up as a flat-top or an inverted V -- inverted Vs tend to have a feedpoint impedance closer to 50 ohms. The height may also be an issue at 40m, but you should be able to get it all to work.

    The only issue you might run across is if you have a metal vent on the peak of the roof or if you have a cast iron vent pipe nearby. Some modern roofs also have little aluminum clips locking the decking together, but they are spaced apart pretty well.
     
  13. KF7YED

    KF7YED Ham Member

    Nope, no ridge vent. No gable vents, either. Its all soffit-vented around the perimeter. The galvanized exhaust vent from the kitchen stove runs up past where the antennas will pass, though. Maybe 8-12 inches away. Any other vents are over 8-10 feet away. No clips on the plywood, either.

    Good idea re: the spreadsheet!
     
  14. NA0AA

    NA0AA Ham Member

    What you are likely to find out using only an SWR meter and your legal frequencies is that you are off frequency but may not know which way, so take a series of readings starting at one end of band and working down - you can use low power AM and voice ID if that's easier for you. You numbers will give you a trend to tell you if your wire is too long or too short, but not how much. If the band edge is 2:1 on 40, I'd make a 6" adjustment then check again, maybe a foot if it's 3:1, then re-measure to see if you are closer.

    Since each band interacts, you should first tune 40, then 20 then 10, then go back and check - since each band interacts. I always attach the end insulators with split bolts or other temporary fastners while I do all this, then finalize after I get the match working to my satisfaction.

    If you are serious about building antennas, you are going to need a good SWR measuring device anyway. A lot of us have the MFJ's and they work well, another new option seems to be from Ten-Tec, the YouKits FG-01 which even has a color screen and looks like it might be a good choice for HF. There are a couple other options from other vendors.

    I feel the ability to go outside amateur band limits for tuning - which can be handy when doing utility listening for example - is a good arguement for the analyizer over the ordinary SWR meter.
     
  15. KF5FEI

    KF5FEI Ham Member

    Yep, what NA0AA said.

    For years I used an SWR meter, and looked at analyzers as an expensive toy for folks who couldn't use an SWR meter. However, as I got into modeling antennas and seeing what the SWR looked like plotted over a large range of frequencies and how angles and lengths affected the readings, I decided an analyzer would be nice. I chose the AA-54, as it had a graph and worked very similar to the modelling software as far as the SWR display. It has been great for setting up taps and working out counterpoise lengths on a Buddistick, and when we needed to measure some stub filters, it was definitely very useful. Something to consider once you catch the antenna building bug.
     
  16. K8JD

    K8JD Ham Member

    tuning a dipole or other antenna

    I agree with making a plot across the whole band with as little power as the meter in your rig will allow a reading.
    My TS440 SWR meter reading can be done with about 1 watt. Start at the low end of the band, take a reading and move up 25k and take another until you have the band plotted.
    If you can get each reading in a second or so of transmission it will never be heard by anyone.
    I usually use 8 x 11" sheets of 1/4 inch graph paper and plot the freq across the bottom in 25 kHz increments and SWR readings going up the left side of the graph. Make a page for each band. This will show you a "bell" curve and tell if the dipole is too long or short. Trim and replot if you are not in the right part of the band. Save the charts for future antenna modifications.

    Have fun.
     
    Last edited: Sep 13, 2012
  17. KF7YED

    KF7YED Ham Member

    Hm. I like that idea of plotting in 25kHz steps. Lots of work but it sounds like it will yield a better picture. I strung the whole thing up this morning while the attic was cool enough. Starting to play with it now. The first run through showed SWR a little too high at the lower ends of each band and little higher at the high ends. That was before i read the suggestion about plotting the whole band. Back to work! :)
     
  18. KF7YED

    KF7YED Ham Member

    OK. Its up. SWR looks high and just about every band...
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    When its more then 3:1 I can only guess since its the built-in SWR meter. I'll climb back up and start fiddling with the lengths (after company leaves next week). Starting with the 10m segment. Right now the ends are fed through the plastic insulators and wrapped back around themselves 2 or 3 or 4 turns. If I want to shorten them, should I wrap them more or just fold them back along the line, sort of an elongated "U"? In other words, slip the line into the insulator and fold it back on itself straight along its length and secure it with a zip tie or tape... I don't want to physically cut until I know I'm on the right track.
     
  19. AE5JU

    AE5JU Ham Member

    All looks well done!

    And you should be able to work 15 meters off the 40 m elements. You will need a tuner to touch up the 15 m band, but it will work nicely.

    For the 718 (I have two, one for portable/field day, one up at the club station) let me highly recommend the LDG IT-100. They work and play well together.

    "just fold them back along the line, sort of an elongated "U"?" Yes, exactly.

    Small cable clamps on the ends will let you quickly and easily take in, or let out wire for tuning.

    Looks like all of the elements are too long. Start tuning with the longest, 40 m elements. Your 20 meter elements are not too far off, so shorten them just a little, a few inches each end.

    A few years ago an Autek RF-1 Analyzer showed up in my Christmas stocking. Works well, and more than good enough for what you are doing. Small, slips in my shirt pocket, very handy for Field Day, etc. And uses a common 9v battery.

    73,
    Paul - AE5JU
     
    Last edited: Sep 17, 2012
  20. KF5FEI

    KF5FEI Ham Member

    As Paul mentioned, it looks like they are all a bit long. As you trim, you should start seeing the SWR going back up on the lower frequencies -- the lowest point you see above this will be the resonant frequency. If you want to use the antenna for the whole band, center it within the band -- if you want to work only the CW or phone sections, center it within this part of the band.
     
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