VERTICAL DOUBLET 20-10

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by G5TM, Jun 8, 2018.

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  1. G5TM

    G5TM Premium Subscriber QRZ Page

    Hi all

    Looking to try out one of these in my incredibly small yard space. No room for radials or much of a horizontal antenna (15 feet of space). I can however go vertical and rather than spend loads of cash on an off the shelf all band all singing all dancing multiband (too good to be true?), I have decided to home-brew.

    My choice is a 33foot vertical doublet, center-fed with 300 ohm ladder-line. I only have a 10m fiberglass pole, meaning that the very bottom of the dipole will touch or be very near to the ground.

    I am right (I believe) in thinking this would introduce loss, and there is no opportunity to get a longer pole (money) or add a support for the existing one to create greater height (neighbors).

    So - if I leave the top element as vertical and allow the bottom one to be installed at an angle of 45 degrees, how would that influence things? Would I need to alter the angle of the feed-line? Would this create a great bias in gain towards the sloping bottom element at 45 degrees, at the expense of the vertical top element?

    Anyone have any experience or knowledge on this?

    Thanks guys.
    Tim - De M6JVI
     
  2. KC3RN

    KC3RN Ham Member QRZ Page

    A 33' end fed might be a better choice. You'll need a matching unit at the feed point (probably 9:1). A few random length radials and you should be good to go. The radials can be folded/turned to fit your available space. It will also give you better performance on 40m than you'll get with a 33' doublet.
     
  3. G5TM

    G5TM Premium Subscriber QRZ Page

    Yes I am aware of the 9:1 option but I find them noisy and inefficient. Moreover, as I stated I do not have room for radials in my v small space. 33 feet is also a half wave on 20 and will provide very high impedance which will be beyond the scope of the 9:1 to transform to a manageable impedance. Given that the doublet will be fed by ladderline rather than coax I also believe the feedline losses should be lower. Thanks for reading and replying. 73
     
  4. HB9PJT

    HB9PJT Ham Member QRZ Page

    I built some vertical doublets with 300 Ohm line for portable. Also I did calculate them with NEC 2 for MMANA. They work very well and calculation indicate it is a good antenna. You need a good tuner. I did place a common mode coil where the 300 Ohm line is connected to the antenna (FT140-43 and 8 turns). I would make the doublet 9.5 m when the pole is 10 m. For 14-30 MHz that is a very fine antenna.

    73, Peter - HB9PJT
     
    K2CAJ and G5TM like this.
  5. G5TM

    G5TM Premium Subscriber QRZ Page

    Thanks very much for the reply Peter. Would having the end of the bottom element around 0.5m from the ground create any issues do you think?
    73 Tim
     
  6. HB9PJT

    HB9PJT Ham Member QRZ Page

    No issue.
     
    G5TM likes this.
  7. G5TM

    G5TM Premium Subscriber QRZ Page

    Thank you for taking the time to reply, 73.
     
  8. W2VW

    W2VW Ham Member QRZ Page

    I've done this several times but always kept the bottom end a couple meters above ground. Did not have RF in the shack and the voltage max point was away from the dog.

    Used TV twinlead held perpendicular by string.

    It would still work fine if shortened a few feet.

    You probably know this but resist the temptation to use a balun to a piece of coax. Instead run the feeders all the way to a wide range balanced tuner.
     
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  9. G5TM

    G5TM Premium Subscriber QRZ Page

    Thank you for your reply. How did you find it worked on 10m?
     
  10. KF5PAL

    KF5PAL Ham Member QRZ Page

    Try an EFHW. You can home brew the autotransformer using an FT140-43 toroid. No radials needed. Cut it for 20m. You can add a coil and a bit of wire (~7ft) to get some bandwidth on 40m as well. if you use a shack tuner, it should be happy on 40,20,15 & 10.
     
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