Master Balun Winding: Step-by-Step for a 1:1 Current Balun Using RG316 for 400W Radio

| January 4, 2026 | Amateur Radio (Ham Radio) | No Comments

Building a 1:1 current balun is one of the most effective ways to prevent “common-mode” current from flowing down the outside of your coax, to be exact, on the outer surface of the coaxial shield. This keeps RF out of your shack and ensures your antenna radiation pattern stays true to the design. Using RG316 on a proper ferrite core is an excellent choice for 400W, as it is high-temperature Teflon-insulated coax that can handle the heat. I have chosen 400W for the discussion as that is the legal limit for my class of license, though I do not have a 400W radio!

🛠️ Materials Required

  • Toroid Core: An FT240-31 or FT240-43 (Type 31 is generally better for the lower HF bands like 40-160m; Type 43 is a great all-rounder).
  • Coaxial Cable: Approximately 20–24 inches of RG316.
  • Enclosure: A weatherproof electrical box (e.g., a 4x4x2 PVC box).
  • Hardware: Two SO-239 connectors (or one SO-239 and two stainless steel binding posts for wire antennas).
  • Extras: Heavy-duty zip ties, 3M glass cloth tape (optional, to protect the core).

🪜 Step-by-Step Winding Instructions

1. Preparation

Before winding, it is helpful to wrap the ferrite core in a layer of fiberglass tape. Ferrite can be abrasive, and this prevents the core from nicking the jacket of your RG316 over time.

2. The “W1JR” Crossover Technique

For a 1:1 current balun, we use a crossover winding. This places the input and output on opposite sides of the toroid, providing better isolation.

  1. Start: Leave about 3 inches of lead and secure the coax to the toroid with a zip tie.
  2. First Half: Wind 6 turns tightly around the core. Each time the coax passes through the center hole, it counts as one turn.
  3. The Crossover: After the 6th turn, instead of continuing in the same direction, cross the coax across the face of the core to the opposite side.
  4. Second Half: Wind 6 more turns in the opposite direction.
  5. Finish: You should have a total of 12 turns (6 + 6). The “crossover” acts as a bridge, meaning your input and output leads are now 180° apart.

3. Securing the Windings

Use UV-rated zip ties at both ends of the coax to ensure the windings stay tight against the core. If the turns are loose, the impedance characteristics can shift.

⚡ Termination and Assembly

Once wound, you need to strip the ends of the RG316 to connect them to your hardware.

  1. Strip the Coax: Carefully remove the outer jacket, pull back the braid, and strip the inner dielectric to reveal the center conductor.
  2. Input Side: Solder the center conductor to the center pin of your SO-239 connector. Solder the braid to a ground lug attached to the connector body.
  3. Output Side:
    • For Coax Out: Repeat the input process to a second SO-239.
    • For Wire Out: Solder the center conductor to one stainless steel bolt (Antenna Leg A) and the braid to the other (Antenna Leg B).

🔍 Why this works for 400W

  • The Core: The FT240 (2.4-inch diameter) is large enough that it won’t easily saturate at 400W PEP.
  • The Wire: RG316 has a silver-plated copper-clad steel center and a PTFE (Teflon) dielectric. It is rated for significantly higher temperatures than standard RG58, making it safe for 400W even if the SWR is slightly high.

📝 Final Checklist

  • Check Continuity: Use a multimeter to ensure the center-to-center and braid-to-braid connections have zero ohms.
  • Check Isolation: Ensure there is infinite resistance between the center conductor and the braid.
  • Weatherproofing: If mounting outdoors, ensure your enclosure has a small “weep hole” at the bottom to allow condensation to escape.

To give you the most accurate picture of your balun’s performance, we need to look at the common-mode impedance (ZS). For a 1:1 current balun, we want this value to be as high as possible – ideally above 2000 Ω – to effectively “choke” off unwanted RF.

The total impedance is a combination of inductive reactance (XL) and resistive loss (RS), expressed as:

ZS = √[(RS)2 + (XL)2]

Comparison: Type 31 vs. Type 43 Ferrite

For 12 turns of RG316 on an FT240 size core, here is how the choking impedance generally stacks up across the bands:

FrequencyFT240-31 (12 Turns)FT240-43 (12 Turns)Recommendation
1.8 MHz (160m)~1200 Ω~400 ΩType 31 is much better for low end.
7.0 MHz (40m)~3500 Ω~2100 ΩType 31 is at its peak performance.
14 MHz (20m)~3000 Ω~4200 ΩType 43 starts to take the lead.
28 MHz (10m)~1100 Ω~2500 ΩType 43 maintains higher impedance.

Understanding the W1JR Crossover

The crossover isn’t just for neatness; it minimizes stray capacitance between the input and output turns. By crossing over to the other side of the toroid, you physically separate the high-voltage and low-voltage ends of the coil.

Thermal Considerations for 400W

At 400W, if your antenna is well-matched (SWR < 2:1), the balun will stay cool. However, if you are using this with a non-resonant doublet and an antenna tuner, the common-mode currents can be much higher.

  • RG316 Advantage: The Teflon dielectric won’t melt until it hits roughly 200°C, whereas standard RG58 would fail at 80°C.
  • The “Touch Test”: After a long transmission or a contest run, safely check the core temperature. If it’s too hot to hold, you may need to stack two cores (glue two FT240s together) to handle the flux density.