Building a junkbox dipole tomorrow, feel free to make suggestions.

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by NW9M, May 4, 2012.

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  1. NW9M

    NW9M Ham Member QRZ Page

    Hello all,

    I'm going to build a dipole tomorrow with my kids, here are the materials I have:

    A large spool of RG6 CATV coax, E83032

    A quantity of 12 ga THHN

    F connectors, a crimp tool, and a F to PL259 adapter.

    I am still thinking, but I believe I will make it for 40 meters, trimming for lowest SWR at 7.150.

    Antenna will be suspended approximately 60 feet in the air. I am planning on using 20 feet of coax wrapped around a 4" core for a balun, and will have about 40 feet of coax after that to the radio. The center of the coax is copper plated steel so that should solder OK, and I am intending to use a crimp sleeve to connect the aluminum shield to the other leg of the dipole. I am planning on using some 3m Skotchkote for weatherproofing. I am planning on suspending it about 15 feet off the ground and tuning it there, then raising it to 60 feet.

    At any rate, does anyone have any suggestions, particularly about the balun, should I use a different diameter and a particular number of turns instead? All helpful advice is appreciated!


    Thanks,

    Mark NW9M
     
  2. NA7U

    NA7U Ham Member QRZ Page

    What a great project with the kids! Seriously, I think you've got it well in hand. 12 turns for the balun should be good (about 15 ft.). I'm sure you'll receive many fine-tuning suggestions, but it should work as you've planned. Be sure to give it a whirl on 15 meters as well, since it'll be 3/2 wl for that band. Have fun, I know you will!
     
  3. KA4DPO

    KA4DPO Ham Member QRZ Page

    Sounds like a fun project but you might skip the choke, you probably won't need it and it will save weight at the feed point. Putting it elsewhere doesn't do much good and unless you have problems with common mode you won't need it.

    Just a suggestion.
     
  4. NW9M

    NW9M Ham Member QRZ Page

    Thanks for the replies gentlemen, unfortunately as is my usual luck we have had thunderstorms all morning so it is looking like Monday instead of today.

    KA4DPO, I am somewhat of a neutral opinion about the balun/choke too but my main purpose is to give some experience to the kids which is the main reason it is being included- it may not help but it won't hurt- and it is a helpful lead in to explaining about antenna feedlines.

    Additionally, I think that we'll be adding some 20M elements to it now, so we will actually be making a fan antenna. I am thinking that for spacing the elements we will just use nylon cord tied to both elements at whatever width and then super glued so they will not slip, and then when the 40M wires are tied up into the trees we can tie off the ends of the lower 20M wires as well to keep everything somewhat stretched into the proper shape.
     
  5. K8ERV

    K8ERV Ham Member QRZ Page

    You are going to build a dipole with your kids? I think wire will work better, and not eat as much.

    TOM K8ERV Montrose Colo
     
  6. W9GB

    W9GB Ham Member QRZ Page

    I watched that line of thunderstorms yesterday ... you might get a break in the weather this weekend.

    WHAT are you using for END and CENTER insulators?
    http://www.eham.net/articles/12399

    A half-wavelength horizontal (dipole) antenna cut for 40 meters at 7.150 MHz
    will also be resonant as a 3/2-wavelength antenna for 15 meters at 21.450 MHz.
    That was a popular and inexpensive Novice class (license) antenna in 1960s and 1970s.

    Fan Dipole Design
    http://www.hamuniverse.com/multidipole.html
    DIY Build
    http://www.hamuniverse.com/ae5jumultibanddipole.html

    w9gb
     
    Last edited: May 4, 2012
  7. K9ZMD

    K9ZMD Subscriber QRZ Page

    This is a great project for you and the kids. It reminds me of wire antenna experimentation I did with help from my (then) 14 year-old son. We were in military housing at the time, so part of the fun was betting on how soon the first RFI complainant would trace the feed line back to our quarters.

    Let us know how your antenna works out. In particular, I'm curious about any measured difference in resonant frequency when elevated to 60 feet after being pruned/tuned at 15 feet. Maybe some of our modeling gurus could make a prediction so we can see how that compares to your real-world installation.
     
  8. NW9M

    NW9M Ham Member QRZ Page

    I have a bunch of 1" PVC pipe that I cut into 3" pieces and drill holes in the ends.

    The center insulator will be a combination of a larger section of PVC (most likely, but just depends what gets dug up first) for the core of the balun and then I will have about a 7" piece of 1" PVC to attach the dipole wires to.

    Additionally, I believe I read that a multi band dipole has less harmonic issues if the feed points are kept at least 5 1/2" apart, so I am thinking of having the 40M dipole section coming from the top and the 20M section coming from the bottom, so it will look like a sideways H with the balun in the center.
     
  9. NW9M

    NW9M Ham Member QRZ Page

    You would think that, but in practice you find that at the end of the week you can look in your wallet and there is still money in there! And, when you get into your car and turn the key the gas gauge pops up to 3/4 of a tank, right where you left it and not the customary 1/8 tank that you have been conditioned to!

    Granted, it is a bit of a hassle bringing food out in the afternoon and evening and waiting for them to lower the bucket on the rope, and due to complaints I had to sell my linear but as you know they say that all antennas are a tradeoff in one way or another, you just have to find one where the advantages outweigh the disadvantages in your particular environment.....
     
  10. K0RGR

    K0RGR Premium Subscriber QRZ Page

    I like using PVC 'T' connectors for the center insulator, and PVC pipe couplers at the ends. Those are quite a bit heavier than most PVC pipe, and stand up to more abuse. I've ripped the ends out of PVC pipe 'insulators' before, but I don't recall ever breaking a T or a coupler.

    A 2 liter Pepsi bottle makes an interesting, lightweight form for a feedpoint balun. Your neighbors will all want to know what that Pepsi bottle is doing way up there... somewhere here on the Zed, W8JI published some actual measurements on coax baluns.
     
  11. NW9M

    NW9M Ham Member QRZ Page

    I like my insulators a little bit longer than what pipe couplers are. My reasoning is that they will work fine in dry weather, but when they are an inch or two longer I suspect they may be less prone to moisture induced leakage if it is rainy or wet outside.
     
  12. WA4OTD

    WA4OTD XML Subscriber QRZ Page

    Have fun! BUilding dipoles is always fun.
     
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