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Link Coupling

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K9WW, Oct 17, 2021.

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  1. K9WW

    K9WW Ham Member QRZ Page

    Hello all,

    I am planning to build a link-coupled matching unit. My question is, which is more efficient, a link coil outside of a continuous secondary coil, or a split secondary coil with the link between the secondary halves?

    If built as either, will making a tap in the link coil cause a disruption of the symmetry between the two?


    Kirk K9WW
  2. K9WW

    K9WW Ham Member QRZ Page

    I should have asked which is better, a link coil positioned coaxially around the secondary coil, or between two sections of a split secondary coil. Thanks.

    Kirk K9WW
  3. KH6AQ

    KH6AQ XML Subscriber QRZ Page

    A search under the term link coupled antenna tuner finds many photos and schematics; most show a link coil positioned coaxially around a continuous, symmetrically tapped secondary coil.
    K0UO likes this.
  4. K8DO

    K8DO Ham Member QRZ Page

    As a guy who has used seen/used both methods for decades I do not see an efficiency difference. Whatever is best for you will work.
    Some of my WWII gear (BC610 and others - now long gone) used a swinging link that fit into a gap in the center of the secondary so you could vary the coupling for tuning.
    Stuff I have built in recent years has the link coil on the inside. An outside link is easier to make for those who cannot machine a set of spreader bars that are split so you can trap the windings without having to thread them around and around through the bars. (tough to do with quarter inch copper tube)
    K0UO likes this.
  5. WA7ARK

    WA7ARK Ham Member QRZ Page

    Since the coupling coefficient is likely to be much less than 1, either way, I dont think it matters. The losses have to do with I^2 * R and will be determined primarily by the RF current in the inductive part of the resonant circuit, just as it is with any tuner, balanced or unbalanced.

    I dont see that a link-coupled tuner has any efficiency advantage over one where one side of the tuning network is grounded.
    Last edited: Oct 17, 2021
    K0UO likes this.
  6. K9WW

    K9WW Ham Member QRZ Page

    Thank you all for the replies. When you say that the link-coupled network does not have an efficiency advantage over a tuning network with one side grounded I assume you mean a network like a Pi, CLC or T type network. But from what I've read here on the Zed, doesn't the link-coupled balanced network have a wider matching range?


    Kirk K9WW
  7. W2VW

    W2VW Ham Member QRZ Page

    I don't know the answer to the original question but I do know there are times when a link coupled tuner will need to be configured between series and parallel tuning depending on the load.

    Construction similar to the B&W HDVL etc. series of coils allows this easier than many other form factors.

    One can argue that link coupling is not the best way to homebrew as mentioned. This subject boarders on religion.
    K0UO likes this.
  8. WA7ARK

    WA7ARK Ham Member QRZ Page

    Matching range is determined by the adjustment range of the L and C components, and by the HV breakdown voltages of the tuning capacitor(s).

    The only advantage that a link-coupled tuner has is that the balun is built-in ;)
    K0UO likes this.
  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I think the "split coil" with the link between halves can allow for coupling adjustment by moving the link in and out of the coil, whereas otherwise the position of the link is fixed and thus you lose one adjustable element.

    Of course, something could break and then we could complain it's "the missing link."
    K0UO likes this.
  10. K9WW

    K9WW Ham Member QRZ Page

    Thanks for the additional info, and yes, I'm married and have been called that a time or two. Seriously though, I have seen photos of vintage transmitters and amps with the "swinging link" but am not RF savvy enough to know what affect its adjustment would have in the impedance transformation other than to reduce the coupling between the input (transmitter) side and the output (feedline) side of the matching unit thus reducing the amount of RF energy delivered to the feedline and, ultimately, the antenna. Please enlighten me but keep it simple enough for a humble non-EE guy.

    Thanks again for the responses and the University of QRZ.

    Kirk K9WW
    K0UO likes this.

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