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DIPOLE HELP

Discussion in 'Home Brew' started by Windwalker1000, Jun 16, 2015.

  1. Windwalker1000

    Windwalker1000
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    For saving space, and some experimenting, I've been thinking about creating a compact 80 and 40 meter dipole pair of antennas. I was considering using sections of heavy paper carpet center tubes, winding the legs of the dipole on them and gluing the wire in place. Then, I'd cover the whole thing with clear shellac to waterproof it. For materials, I have a 30mHz balun, 14 gauge stranded wire with insulation, and RG-8 coax. My big question to everyone is: Will this thing work as pictured in my mind? It would resemble a "T" when finished, would be about 4' long, each side of the dipole would be wound in a tight coil around the tube except for the last foot or two. Am I missing something obvious here while thinking this up? Would it be directional and, if high enough, transmit as far as a normal, strung-out dipole? Any input positive or negative would be appreciated!---Thanks, Rick


     

  2. Road Squawker

    Road Squawker
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    I don't understand what you mean by "pair", are you talking two separate antennas, one on 80 and the other on 40?

    why not just load the 80 meter antenna up on boht bands with a tuner?

    any way, a really short antenna on 80 meters will more than likely sux (highly technical term) performance wise,... you never said the actual length of the wire.

    below is a 20 meter design of what you are thinking about,..... notice the hairpin matching section.

    https://www.google.com/url?sa=t&rct...nbLIDQ&usg=AFQjCNHlCWuVKqkWFi9GjdIrCo_9eT3DOw


    [​IMG]


    will it "work"? sure, have fun building it.
     
  3. RatsoW8

    RatsoW8
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    A normal 80m dipole cut for 3.7 MHz would be about 124 ft. total length and your compacting that into a 4 ft. coil? Try it and see for yourself how it performs then report your findings..
     
  4. Captain Kilowatt

    Captain Kilowatt
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    What you are proposing will not work good at all. It will have an extremely low feedpoint impedance making it very hard to obtain a good match. It will be extremely narrow band meaning that once you do find a good match it will be way off only 10 or 20 KHz away meaning it will have to be retuned. It will have a very high feedpoint current meaning that any and all connections will have to be extra good or there will be losses. The helical elements will act like Tesla coils with even a smal amount of power and will develop extremely high RF voltages on the ends. Because of the extreme shortening of the elements it will have very poor efficiency on both RX and TX.

    Will it work? Damn sure it will work if all you want is to radiate some RF somewhere but is it worth it?
     
  5. wavrider

    wavrider
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    The answer would be YES it will work.
    IF you get the input impedance to match 50 ohm's.

    The bandwidth would be very narrow.

    I would think your antenna would have negative gain, perhaps several DBD of negative gain.

    ERP may actually be 10% of transmitter output, same with receive.

    It will work, but I would think it would only work within a few miles of your QTH.

    If space is limited then perhaps some ham sticks or screwdriver type mobile antenna installed.
     
  6. wavrider

    wavrider
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    CK we were replying at the same time and you beat me to the post button:D
     
  7. Captain Kilowatt

    Captain Kilowatt
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    At least it doesn't look like I was copying off you. :D

    BTW he would be LUCKY to see 10% efficiency.
     
    wavrider likes this.
  8. BJ radionut

    BJ radionut
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  9. Windwalker1000

    Windwalker1000
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    Thanks for all the *badly needed* input on the dipole design--firstly, yes, I was talking about building 2 separate antennas, secondly, space is somewhat limited--the only other viable option is the standard inverted V dipole--I'll just have to be really careful where I mount these things--I live in a farm house but, the way the trees are set around the house and the lack of trees in the large sections of the yards will make finding a good location interesting. The design I was mulling over in the beginning was simply for saving space(i.e. compactness) and hopefully would still get halfway decent performance--after reading the replies, I now know that it would be a waste of materials and time. I've got a lot of electronics experience but haven't messed with hardcore RF in quite a while(many years) I'm having to re-learn a lot so please be patient with me--I'm studying hard again to re-acquire a general class ticket--and yes, I feel really stupid--this wasn't as hard when I was in my teens! Things have changed sooo much in the years gone by---Thanks again for the responses, both positive and neg.----Rick
     
  10. Decoder

    Decoder
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    if you are doing what I think you are doing and that is to take the 124 ft and coil it to about 4 ft in lenth. You will have a very nice 80 dummy load, it will work but very little distance,
     
  11. DainBramage

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    if you have the room,........... start with a doublet.

    125 +/- feet of wire, fed with 450 Ohm ladder line, as high as you can get it, use a tuner on the higher bands

    have fun
     

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