Balun advice, 1:1 common mode choke to remove CM noise from ladderline doublet

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by N1VAU, Jan 7, 2020.

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  1. N1VAU

    N1VAU XML Subscriber QRZ Page

    I need some advice in my quest to get the best possible performance out of my 135' doublet that is fed with 4" spaced (~600 ohm, supposedly) home brew ladder line.

    Background:
    It's up about 50' at the center. The ends are 65' + 75'. It's hung from pine trees and the 135' wire is strung over smaller trees below. The doublet and feed line wires don't come close to touching any trees at all.
    Feed line originally connected to the internal 4:1 unun in the Ten-Tec 238B

    Original problem:
    noise on 12 + 10 meters, like S-5/6 steady when the bands aren't open. I tried adding twist to the LL with no noticeable change.

    Solution: (partial?)
    I gave up on the tuner internal 4:1 unun and installed Radiowave B14CF Ferrite Core BiFilier Dual Core Guanella 4:1 current balun I had on the shelf, that dropped the noise floor substantially, to S-1 or less. It is still not as low as my nearby coax fed fan dipole on a 90 degree axis compare to the doublet.

    While S-1 noise floor is an improvement, I still want to get the most performance I can out of this antenna system.

    I've searched the all the massive amount of antenna books I have, the internet, this forum and I have determined there is much conflicting information out there. (understatement)

    My question:
    I want to get the best 1:1 balun to see if it will choke off even more of the noise floor. I wanted to build one or buy one if I have to.

    I haven't found any concise information on what to buy, what to build or how to build it.

    Palstar's offerings are ~$100, very little specs besides power handling capacity.

    DX Engineering's high powered offerings look pretty sweet, but again do not supply the actual lab tested specifications of how well they work. They cost $175-$195 (that's a high price if they don't work better than everything else)

    Anyone have any real world experience on what 1:1 balun I can build or buy that will handle legal limit and will shed the most common mode current for 10-80 meters?
     
  2. K7URX

    K7URX Ham Member QRZ Page

    I use the same antenna here. Its just up a bit higher. A johnson matchbox has always worked well for me with the doublet. Used, they cost no more than a balun. A link coupled tuner is probably your best bet with that antenna. My doublet is always quieter than my coax fed dipoles. I found that grounding the johnson matchbox made for difficult tuning at times so mine floats from ground. Sorry that this doesn't answer your question directly but I do believe a link coupled tuner is the way to go with that antenna. Good luck.
     
    AG5CK likes this.
  3. K7URX

    K7URX Ham Member QRZ Page

    Heres mine, the coil makes for easy tuning on 40 without messing around with feedlline length.
     
  4. K7URX

    K7URX Ham Member QRZ Page

    sorry pic wont post
     
  5. WA7ARK

    WA7ARK Ham Member QRZ Page

    Have you done the "shut off power to the whole house test and run you receiver on batteries test" yet?

    I predict you will find that the section of ladder-line between the wall of your house and the balun/tuner is picking up RFI that originates in your own house, likely even inside the ham shack, if you have any computer equipment close to the operating position.

    That the "balanced" nature of ladder-line offers any immunity to near-field RFI is a myth! Coax shielding, especially foil-shielded coax running from rig to the antenna works, provided prevent the RFI from travelling up the coax as a CM noise current so that it can enter the antenna at the feedpoint.

    Coax and CommonMode Chokes are your friend. Ladder-Line became obsolete in 1937.
     
  6. VA3VF

    VA3VF Ham Member QRZ Page

    KM4BDH, AF4RK and K2XT like this.
  7. WA7ARK

    WA7ARK Ham Member QRZ Page

    Since we dont have the "edit" button, let me modify post #5:

    Change "in 1937" to "when all consumer equipment became powered with switching power supplies by about 1995"
     
  8. W1VT

    W1VT Ham Member QRZ Page

    http://www.karinya.net/g3txq/chokes/
    I use two designs on this web page. For 10 through 40 meters I use 11 turns on two stacked FT-240-52 cores. For 160 through 20 meters I use 17 turns on a single FT-240-43 core. RG-400 Teflon coax will handle high power.
    This page also describes how to measure 1:1 baluns with a 2 port VNA. There are cheap imports on ebay in the $50 range.

    Zak W1VT
     
    N1VAU and K7TRF like this.
  9. K7URX

    K7URX Ham Member QRZ Page

    I should have also mentioned that I had the same problems here until I got the matchbox. Open wire feed directly to back of matchbox that is 6 feet away from computer. No RFI No noise. That antenna design was meant to be used with a link coupled tuner from the beginning. Its a fine antenna and if situated correctly, can have some very nice patterns.
     
    AG5CK likes this.
  10. K7TRF

    K7TRF Premium Subscriber QRZ Page

    No harm in adding more choking behind your 4:1 Guanella which might help.

    But your higher noise on the Doublet may not be stemming from common mode issues or be resolved with more choking. You've got two antennas at 90 degrees to one another and one is a 135' Doublet and the other a fan dipole presumably covering the bands where you see less noise on the fan dipole. The antenna patterns are different, the antenna gains in their peak directions will be different and one may run closer to local noise sources than the other.

    When you see higher noise on the Doublet do you also see higher signal strengths or do you see better S/N on the fan dipole and if so is that in directions you'd expect to be covered fairly well by both antennas (e.g. not directly off the ends of either antenna)?

    Sure, adding some chokes may help you figure out why the Doublet seems to be noisier but it's hard to compare very different antennas, running in different directions and on bands where the Doublet's pattern will be quite different than the halfwave Dipole's pattern.
     
    K2XT, N1VAU, W1PEP and 2 others like this.

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