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How is my doublet doing.

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by 2E0LPL, Jul 11, 2019 at 9:19 AM.

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  1. W4KJG

    W4KJG Subscriber QRZ Page

    Something just ain't right. The resonance point seems a little low. I would have expected it to be closer to 3500 kHz, but that could be due to the height above ground, the feedline, or ...

    The impedance of 32 ohms also seems a little low at that resonant point, but that may be due to using an unbalanced antenna analyzer on the balanced feedline, plus the feedline length. Feeding your analyzer directly with the ribbon line is probably unbalancing the line. That is where a 1:1 balun would be very helpful. It would decouple you and the analyzer from one side of the feedline. Using W5DXP's calculator would be helpful also.

    I've put up a lot of doublets over the last 60 years. Most have been about the same length as yours, but typically with 35-50 feet of home made ladder line, typically spaced about 3 inches/7.5 cm. They have been pretty tame and easy to tune through homebrew 1:1 current baluns and simple homebrew unbalanced L-network tuners.

    Ken

    I doubled with WA7ARK when posting. But I fully agree with his post.
     
  2. AI3V

    AI3V Ham Member QRZ Page

    How lossy do you figure?

    Did you happen to note his impedances did not change by a factor of 4? ;)

    Rege
     
    Last edited: Jul 12, 2019 at 1:23 AM
  3. NH7RO

    NH7RO Ham Member QRZ Page

    The numbers look better and better after the cork is pulled regardless of the SWR (Sobriety/Wine Ratio).
     
    AI3V likes this.
  4. AI3V

    AI3V Ham Member QRZ Page

    Consider how close to both the ground, the house, and the wiring that is certain to be in the top floor of his house....

    Like the wires running to the ceiling light a meter away....

    Not going to get textbook impedance or resonance numbers.

    (Just what is the dielectric constant of a English house? :) )

    Trying for a particular feedpoint z of such a installation is strictly a cut and try affair- dare I say "fiddly bits"!? :)

    I also think that a battery operated analizer, sitting on a non conductive surface should have no trouble giving a accurate reading +/- 50% or so :)

    Rege
     
  5. WA7ARK

    WA7ARK Ham Member QRZ Page

    A sure indication that his 1:4 (probably voltage) balun is not operating in a place where it is happy...
     
  6. WA7F

    WA7F Premium Subscriber QRZ Page

    I had a similar situation with my very first antenna, a ZS6BKW. One side was in clear air at about 35’ and the other side was about 5’ over the peak of the roof for most of its length. After countless hours trying to tune it the numbers did not match the designer’s. In hindsight, it was probably unbalanced from the run over the house. Once it was finally tuned to my satisfaction and I gave it a try and RX was extremely noisy. The noise was probably from the run over the house. The ZS6BKW went in a box because there was no clear area to install it. I then installed a remote tuned Inv-L at the same location and it served me very well.

    I’m willing to bet that you will see different measurements and better performance if you can re-route your doublet to avoid the house.
     
    Last edited: Jul 12, 2019 at 7:21 AM
  7. 2E0LPL

    2E0LPL Ham Member QRZ Page

    Hi guys...one of my problems is I am not very far up the ladder of experience...and still have lots to learn.

    I was hoping a doublet would be a comparatively easy antenna to rig.

    Seem maybe not.

    I am not sure now what direction to go...I cannot get to deeply into the tech side of this topic as I just don't have that knowledge yet.

    But I am keen to learn...and I'm sure I wil learn from this exercise...if I can keep up.

    So I need to ask some basic questions....if I experiment with a 1 to 1 balan were would it be fitted...I assume at the feed point.

    I suppose an easy starting point for me is to vary the feedline length....from its short 20ft length...I presume going longer would be better

    The tuner I use is an MFJ 941e....

    https://www.google.co.uk/search?q=m...egQ9QEwCXoECAQQBg#imgrc=joPNJsozfmD29M:&vet=1
     
  8. W5DXP

    W5DXP Ham Member QRZ Page

    Not at the feedpoint. A dipole is balanced and window line is balanced so there's no need for a balun there. The 1:1 balun-choke goes where the window line and coax are connected together. Window line is BALanced and coax is UNbalanced so that's where the 1:1 BALUN is located if you have an unbalanced tuner. If you have a balanced tuner, you may not need a balun.
     
  9. 2E0LPL

    2E0LPL Ham Member QRZ Page

    Ok I dont know if this is correct but fitted a 1-1 current balun to the shack end of the ribbon today.

    From the balun I fitted a 1ft coax link to the ATU...via one of its 2 coax antenna inputs.

    Had a quick tune up across a few bands but did not have any QSO...just some quick tuning tests.

    The double indeed tune easy 1-1 on the band I tried...80/40/20m.

    I have a field strength meter mounted 6in away from the junction of the ribbon/balun/coax link and I see a lot RF reading on the meter....if I move the meter 3ft away from that point...the becomes reading is very low.

    No idea how well or not the setup gets out but love to hear your views.
     
  10. W4KJG

    W4KJG Subscriber QRZ Page

    Nick,

    Here are some links that might help a little bit. Dont give up on the doublet. The length and the feedline you have should provide a very versatile antenna.

    This is what I've been using with my doublets for many years. It goes at the end of the open wire feedline, usually just outside the shack) to a short piece of coax. The tuner has few problems matching it to ax1:1 VSWR. There will be some loss in the coax, but very low loss through the balun to the open wire feedline.

    https://vk6ysf.com/balun_1-1.htm

    Good general info here:

    http://vk5ajl.com/projects/baluns.php#voltage_tfmr

    https://www.balundesigns.com/blog/baluns-for-multiband-antennas-fed-with-open-wire-or-ladder-line/


    I looked at your QRZ page. You make me very envious.:D One day you may just find a Yank knocking on your door for a tour. That Tiger Moth is beautiful. My brother had a Great Lakes for many years. It was a lot of fun. Many of my friends are into ultralights.

    Ken
     

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