ham radio: chokes & baluns

[last updated: 2024-06-20]
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go to: chokes & baluns - Schematics
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On This Page:
• Introduction:
• Background:
• Function in Ham Radio:
• Balanced vs. Un-balanced Devices:
• Voltage vs. Current Baluns:
• Common-mode current:
• Toroid Cores:

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• Introduction:
  • Source Material:
    Unless otherwise noted, content here came from the web. I've tried to include links to sources where possible.
    It happens, as is commonly known by anyone who spends any time surfing the web, that there is often ambiguity and even contradictions between various posts on the web.
    In the context of this subject, people sometimes use the terms "balun" and "choke" as if they're synonyms. In fact there are forum threads complaining about people who call a given device a balun when in fact it's only a choke.

    
    The situation is surely exacerbated by the nebulous subject matter - it's often impossible, for me at least, to look at the schematic of a device and quickly determine with confidence what kind of device it is. This isn't always true, of course, and hopefully after going through the examples and schematics here, you'll be able to correctly identify at least the more common configurations.

  • In General:
    Baluns and Chokes are coils of wire that are inserted into the signal path between a transmitter and an antenna.

  • Definitions:
    • A Balun is a device that connects a Balanced device (typ. an antenna) with an Un-balanced device (typ. coax from your transmitter).
    • A Choke is an inductor used to block (by showing a high impedance) higher frequency AC signals, while passing DC or lower frequency AC. This happens because an inductor's impedance (reactance) increases with frequency.
      (per wiki) often wound in complex patterns (basket winding) to reduce self-capacitance and proximity effect losses.

  • Contradictions/Confusions:
    • One source (ref #4) says all current baluns are 1:1, but another source shows schematic for a "4:1 current balun"
    • One source says "Guanella's" are a type of voltage balun, but another source shows a "Guanella current balun"

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• Background:
  • Baluns and chokes are everywhere in ham radio.
    Broadly, they are devices composed of one or more coils of wire.
    They are often wound on a ferrite (or other magnetically permeable) core (either toroid or a rod).
    They can however be wound without a core, in which case they are "air-core" devices.

  • There are many Types/Categories of chokes and baluns.
    The actual configuration and wiring used in the construction determines the "category/name/type" of the device,
    though there is some cross-over in these categories.
    • Construction parameters:
      coreless (air-core), or one or two or more cores
      core type (toroid or rod) and size
      core material - magnetic permeability, frequency response
      size of wire and number of turns
      spacing, any crossing?, twisted, etc.
      connection schematic between the windings, the input, and the load
      (schematic shows start/end-points of all coils)
      • Types/Categories/Names:
        Best guess is that these labels/categories are not mutually-exclusive, and in fact a given balun may be described by more than one of these names:
        • transformer type
        • auto-transformer type
        • transmission line type:
          Often a coil of coax.
        • delay-line type
        • Single-core vs. double-core

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  • Function/Purpose in Ham Radio:
    • In ham radio, baluns are most commonly used to match the 50-ohm unbalanced coax source at your transmitter to the balanced load of eg. a dipole antenna.
      Chokes are used to eliminate common-mode current from flowing down the coax shield into the shack, and radiating like an antenna in the process.
    • Chokes and Baluns are used to combat two problems that can occur in ham radio installations:
      • High SWR (reflected power), occurs when there is an impedance mis-match between components in the signal flow from transmitter to antenna.
        • Such high SWR can often be tuned out with an antenna tuner
        • If not resolved, impedance mis-match causes poor efficiency of transmission:
      • Common-mode current:
    • Given the capabilities of modern antenna tuners, an operator might have no indication that their installation would benefit from a balun.
    • Even without a tuner, fact is that an antenna will radiate, however inefficiently, with even major impedance imbalance and common-mode current. And unless you can't make any contacts, or you have interference to radios or TV's or computers in your shack or vicinity, and you don't get an RF burn when you touch a metal part on your transmitter, and your transmitter doesn't over-heat, you may not even know that your installation is way out of balance and could benefit from a balun.

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  • Balanced vs. Un-balanced Devices:
    • "Balanced": means the two halves have the same impedance relative to "ground"
      Examples:
      dipole antennas (ideal)
      twin-lead (typ. 300 or 450-ohm)
      When balanced feedlines (like twin-lead) are driven with RF from your transmitter, each half will have an equal (but opposite polarity) current in it. These currents create electromagnetic fields around the conductors, but because they're equal and opposite, the fields cancel, so in net the feedline itself does not radiate, ie. does not act like an antenna, meaning you don't lose power (and efficiency) from the feedline as it carries your signal to your antenna.
      Note: even twin-lead can be unbalanced to 5% or more due to variations in manufacturing tolerances and poor quality-control in cheaper models.
    • unbalanced:
      vertical antennas
      dipole antennas (often in real-world with nearby trees & structures)
      coax

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  • Voltage vs. Current Baluns:
    • Voltage Balun: If changing the number of turns on any winding changes the voltage, then it is a Voltage Balun.
      • Guanella baluns are a type of Voltage balun. [have not yet found a description of features that categorize it as such...]
      • There are schematics on line of both single-core and dual-core versions. However DJ0IP (ref #6) states that single-core versions do not work to suppress common-mode current.
    • Current Balun: If changing the number of turns on any winding changes the current, then it is a Current Balun.
      • A current balun does nothing more than choke common-mode currents. There is no transformer action.
        All current baluns are 1:1 devices. (per ref #4: anyFlip)
        However: there are schematics listed online for "4:1 Current Baluns" ...
        VK6YSF ref #7 shows schematic composed of 2 current baluns cross-connected.

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  • Common-mode current:
    • Current-mode baluns are used to reduce/eliminate common-mode current in the feedline.
    • From ref #1: "... electrical length of the transmission line plays a significant role in the susceptibility of the transmission line to common mode currents. This is true for unbalanced (coax) and balanced transmission lines with or without a current balun."
    • From ref #2: " ... But when the dipole is not balanced, the currents accepted by each leg of the dipole can't be equal. But, all the current must go somewhere, so the difference in current between the dipole legs is reflected back down the coax, back at the transmitter. (Note, this is distinct from reflection from impedance mismatches that causes high SWR.) However, this current doesn't have an equal and opposite current on the center conductor, so it radiates off the coax, like an antenna."
    • From ref #2: " ... common mode currents ... can be carried just as easily on open-wire "balanced" transmission line. "
    • See ref #3: common-mode choke, also known as 1:1 current balun

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  • Toroid cores:
    • Toroid cores are ferro-magnetic material in the shape of a toroid (donut), used to wind wire onto to make a coil in the shape of the toroid core.
      Ferro-magnetic means they are permeable to magnetic field lines, however they do not conduct electrical current, making them ideal for transformers, chokes, etc.
    • toroids are specified in terms of their size and the characteristics of their core material.
    • Amidon (mfr) naming/numbering scheme:
      eg. FT-240-61 or FT-140-43
      • "240" = 2.4" OD
        "140" = 1.4" OD
        "114" = 1.14" OD
      • "61" and "43" are codes to specify the ferrite material type and properties:
        61 is good up to 25 MHz
        43 is designed for 20 - 250 MHz
    • Notes from forum:
      43 material is running out of gas on 160. 31 material is a far better
      choice for that band and possibly 75/80 as well. All I've built and
      evaluated with the VNA have shown 31 material superior on 160 and 75/80
      over 43 material. You may even consider 61 material for 15, 12, 10 and 6
      meters. Once again, there is absolutely no "one size fits all"!!

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