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End fed and common mode

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K6SRF, Jan 11, 2018 at 12:27 AM.

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  1. K6SRF

    K6SRF XML Subscriber QRZ Page

    Since an end fed wire is the best antenna that I can have at my location, I'm trying to understand better how it works, so that I can hopefully improve its performance. It does "work", but the term is relative. It kills my computer USB keyboard sometimes, which I believe is due to common mode current.

    Subsequently, I thought I would implement a 1:1 unun common choke, but I don't know where. I read online that a good place is between the radio and the 9:1 unun, right before the 9:1. In other places I read that no, that's not a good place, because by killing the common mode current there, it will also impact the "useful" current. Or something like that... I'm still drinking from a fire hose.

    Others say to use a tuner right before the 9:1, and at least a radial. Maybe use additionally the 1:1 common choke at the radio.

    I understand that the coax becomes part of the entire setup, and I would like to eliminate that, or at least minimize the effects. I would appreciate any input.
     
  2. KP4SX

    KP4SX Subscriber QRZ Page

    Try to use a length that is not resonant in a ham band at half-wave. An end fed wire presents a very high impedance if its a half-wave multiple. Odd multiples of quarter-wave are less problematic.
     
    WA7PRC likes this.
  3. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    My dog drinks from a toilet sometimes so a fire hose isn't that bad.

    To make any useful suggestions, I'd sure want to know:

    -How long is the end-fed?
    -What band(s) are you using?
    -How is the end-fed installed? That is, how high above ground is each end, and where is it fed (other than at "the end" -- I mean is it fed indoors, outdoors, where? And how high above ground and how far "outdoors" away from the house is it fed)?
    -Describe the 9:1 unun. Is it commercially made or homebrew? If commercial what's the model? If homebrew, what's the design?
    -How long is the coax from the rig to the unun?
    -What is the "second terminal" of the unun connected to? Ground? Nothing?
     
  4. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Odd multiples of a 1/4-wave can be matched a lot easier, but then I'd use a 1:1 unun, not 9:1.

    If it's an EFHW or half wave on any band I'm using, a 9:1 isn't even sufficient: A 49:1 works better.:)
     
    WA7PRC likes this.
  5. W5DXP

    W5DXP Ham Member QRZ Page

    For any current flowing into the 9:1 balun, there must be an equal magnitude and opposite phase current flowing somewhere else into a counterpoise. If no physical counterpoise is provided, the feedline will be used as a counterpoise. Any choking attempt to keep the current from flowing into the feedline-counterpoise will tend to drive the antenna feedpoint current to zero. The radial is a good idea to provide a physical counterpoise. The result of any choking action will then cause current to flow in the radial rather than in the feedline.
     
    WA7PRC likes this.
  6. K6SRF

    K6SRF XML Subscriber QRZ Page

    My answers are below, in bold.


    -How long is the end-fed?
    About 130 feet
    -What band(s) are you using?
    Mainly 20 meters. SWR is 1.8 there, but sometimes I use 40 meters with a tuner
    -How is the end-fed installed? That is, how high above ground is each end, and where is it fed (other than at "the end" -- I mean is it fed indoors, outdoors, where? And how high above ground and how far "outdoors" away from the house is it fed)?
    It's sloped, from 100 feet to about 15 feet. Te feed point is at 15 feet, outdoors on the deck. It's right next to the house.
    -Describe the 9:1 unun. Is it commercially made or homebrew? If commercial what's the model? If homebrew, what's the design?
    It's commercial, the one coming with this end fed (http://www.ni4l.com/portable-end-fed-40-6-meters-dipole-antenna/) I replaced the wire with a longer one.
    -How long is the coax from the rig to the unun?
    25 feet
    -What is the "second terminal" of the unun connected to? Ground? Nothing?
    Nothing.
     
  7. K6SRF

    K6SRF XML Subscriber QRZ Page

    Thank you, that's actually a very good summary (and confirmation) of hours of reading :)

    Should I used a quarter wave counterpoise? Is it ok if it's on the 2nd floor deck, and not on the ground? If I use one, can/should I then put a 1:1 common choke right before the 9:1? Sorry for the many questions...
     
  8. KL7AJ

    KL7AJ Premium Subscriber QRZ Page

    W8JI did a pretty good dissertation on the EFHW wire on his website. While you do need SOMETHING resembling a counterpoise, you really don't need much....five feet of wire running off in the opposite direction works pretty well on most HF bands. It's certainly not ideal, but no end fed wire is!
     
    WA7PRC likes this.
  9. K6SRF

    K6SRF XML Subscriber QRZ Page

    It is indeed very good, and every time I read it I understand a little more. To my surprise though, he doesn't say anything about a common mode choke. I thought that a choke is an agreed upon solution, or part of a solution.

    He says "The only way to minimize common mode on the feed line is to pick the "lucky feed line length" or feed line grounding point" That's it? No other way? That's a bit surprising.
     
  10. K7TRF

    K7TRF Premium Subscriber QRZ Page

    A good common mode choke can help IF there's a suitable RF return in the form of radials, counterpoise, even the coax shield itself (assuming you place the choke near the rig end of the coax allowing the coax to implicitly form the RF return/counterpoise). But a choke alone, near the end fed feedpoint in a system that doesn't have some explicit counterpoise won't do much.

    The antenna needs something to serve as the "other half", as pointed out in W8JI's links it doesn't have to be a tuned quarter wave radial or anywhere near half of the antenna but you'll need something to source and sink charge carriers. Assuming you have something fulfilling that role then a well placed choke can help reduce any remaining common mode on the feedline.

    So your basic options are:
    - Run the end fed with no explicit counterpoise and no choke and the coax shield will serve as the counterpoise. A lot of folks, especially running QRP power levels do this all the time but it can become problematic at higher power levels and may or may not lead to elevated receive noise levels.

    - Run the end fed with no explicit counterpoise but choke the coax back at the shack end near the rig. This allows the coax shield to become an implicit counterpoise but chokes the common mode currents before they get to your rig or into your home wiring. This method is advocated by some including Palomar Engineers: http://palomar-engineers.com/tech-support/tech-topics/best-end-fed-antenna-for-ham-and-swl

    - Run the system above but with some explicit counterpoise/radials. This is shown as an option in Palomar's diagram linked above. In this case the explicit counterpoise should limit the amount of common mode on the coax shield but the choke can still clean things up a bit before the coax enters your shack.

    - Run explicit counterpoise/radials, choke where the coax feeds the antenna system. Basically you deploy a full antenna system with both a radiating element and an explicit RF return and then choke the feed line at the feed point to minimize common mode currents at the source.

    - Same as above but add another choke at the shack end of the coax run which is a belt and suspenders approach to limiting common mode on your feed line. This can help in situations where the coax ends up running somewhat parallel to the antenna itself. IOW, even if you provide a good RF return in the form of radials or counterpoise you could still couple common mode currents onto your coax shield and the second choke can minimize those coupled currents as well before they enter your shack.
     
    WA7PRC, WA7ARK and K6SRF like this.

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