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Baluns & Ununs for Common Antenna Types

A Practical Guide for HF Antennas

Author: Joseph Vaughn, KF8ECP  •  kf8ecp.site

This guide explains which balun or unun to use with common amateur radio antennas, why each one is chosen, and how to build several proven designs. It is intended as a practical, no-nonsense reference that can live both on the web and as a printable handbook for field use.

How to use this page: Browse the sections online, or print to PDF from your browser (File → Print → Save as PDF) to create a portable manual.

Chapter 1 – Antenna Types & Recommended Baluns / Ununs

This chapter introduces common HF antenna types and gives a practical recommendation for which balun or unun to use with each. These are starting points that work well in typical installations.

1.1 Center-Fed Dipole

A classic two-leg wire antenna fed at the exact center. At resonance, the feedpoint impedance is typically 50–75 Ω and the structure is balanced.

  • Recommended device: 1:1 current balun (choke balun)
  • Reason: Keeps currents balanced and prevents RF on the coax shield.
Center-fed dipole 1:1 current balun Coax to radio Center-fed dipole photo (placeholder)
Figure 1 – Center-fed dipole with 1:1 current balun.
Replace the placeholder image with an actual dipole photo.

1.2 Off-Center-Fed Dipole (OCFD / Windom)

A dipole with the feedpoint moved off center, often around 33/67 %. This allows multiband operation using a single wire.

  • Typical feedpoint impedance: 150–300 Ω
  • Recommended device: 4:1 current balun
OCFD feedpoint 4:1 current balun Off-center-fed dipole photo (placeholder)
Figure 2 – Off-center-fed dipole with 4:1 current balun.

1.3 End-Fed Half-Wave (EFHW)

A wire approximately a half wavelength long on the lowest band of interest, fed at one end. The feedpoint impedance is very high (2.5–3 kΩ or more).

  • Recommended device: 49:1 or 64:1 transformer (EFHW unun)
Half-wave wire 49:1 Coax to radio End-fed half-wave antenna photo (placeholder)
Figure 3 – EFHW with 49:1 transformer at the feedpoint.

1.4 Random-Wire / Longwire

A non-resonant length of wire used with an antenna tuner. Impedance is typically a few hundred ohms.

  • Recommended device: 9:1 unun feeding a tuner
Random wire 9:1 Tuner Random wire with 9:1 unun photo (placeholder)
Figure 4 – Random-wire antenna with 9:1 unun and tuner.

1.5 Quarter-Wave Vertical with Radials

A vertical radiator about a quarter wavelength long with a radial or ground system at the base.

  • Typical feedpoint impedance: 30–60 Ω
  • Recommended device: 1:1 current balun / choke at the base
Vertical 1:1 choke Quarter-wave vertical antenna photo (placeholder)
Figure 5 – Quarter-wave vertical with radials and 1:1 choke.

1.6 Half-Wave Vertical (End-Fed)

A half-wave radiator oriented vertically and fed at or near the end. The feedpoint impedance is high, similar to a horizontal EFHW.

  • Recommended device: 49:1 or 64:1 transformer
49:1 1/2 λ vertical Half-wave vertical EFHW photo (placeholder)
Figure 6 – Half-wave vertical EFHW with transformer at the base.

1.7 Loop Antennas (Full-Wave Loops)

A closed loop of wire, usually one full wavelength around on the lowest band. Feedpoint impedance is typically 100–200 Ω.

  • Recommended device: 4:1 current balun (most cases)
4:1 Full-wave loop Full-wave loop antenna photo (placeholder)
Figure 7 – Full-wave loop fed with a 4:1 current balun.

1.8 Delta Loop

A triangular (delta) full-wave loop. Feedpoint impedance is often around 100–120 Ω when fed on a side or near a corner.

  • Recommended device: 4:1 current balun
4:1 Delta loop Delta loop antenna photo (placeholder)
Figure 8 – Delta loop with 4:1 current balun at the feedpoint.

1.9 Ground-Plane Vertical

A vertical radiator with sloping or horizontal radials from the feedpoint; common on VHF/UHF and some HF.

  • Recommended device: 1:1 current balun / choke
Ground-plane Ground-plane vertical antenna photo (placeholder)
Figure 9 – Ground-plane vertical with choke at feedpoint.

1.10 Fan Dipole / Multi-Dipole

Several dipoles sharing one feedpoint, each cut for a different band, forming a “fan” of wires.

  • Recommended device: 1:1 current balun
1:1 current balun Fan dipole antenna photo (placeholder)
Figure 10 – Fan dipole fed through a 1:1 current balun.

1.11 Inverted-V Dipole

A dipole with the center elevated and the legs sloping downward forming a “V” shape.

  • Recommended device: 1:1 current balun at the center
Inverted-V 1:1 balun & coax Inverted-V dipole photo (placeholder)
Figure 11 – Inverted-V dipole fed by 1:1 current balun.

1.12 Sloper

A wire sloping from a support down toward the ground. It may be resonant (half-wave) or random length.

  • If cut to half-wave: use a 49:1 EFHW transformer
  • If random length: use a 9:1 unun and tuner
49:1 Sloper antenna photo (placeholder)
Figure 12 – Sloper configuration with EFHW transformer (half-wave case).

Chapter 2 – Balun & Unun Basics

Baluns and ununs serve two main roles: keeping currents where they belong, and matching impedances. Understanding the difference will help you choose the right device for each antenna.

2.1 What is a Balun?

A balun (BALanced to UNbalanced) connects a balanced antenna, such as a dipole or loop, to an unbalanced feedline like coax. A good current balun forces equal and opposite currents in the balanced conductors and presents a high impedance to unwanted common-mode currents.

2.2 What is an Unun?

An unun (UNbalanced to UNbalanced) connects an unbalanced antenna, such as an end-fed or random wire referenced to ground, to an unbalanced feedline, while transforming the impedance. Examples include 9:1 ununs for random wires and 49:1 transformers for EFHW antennas.

2.3 Current vs. Voltage Baluns

Current baluns (chokes) control current and are more tolerant of load imbalance. Voltage baluns control voltage, but can allow significant common-mode current when the load is not perfectly balanced. For most HF applications, current baluns are preferred.

1:1 choke 49:1 EFHW Examples of balun and unun cores (placeholder)
Figure 13 – Conceptual view of a 1:1 choke and 49:1 EFHW transformer.

Chapter 3 – Construction Projects

This chapter walks through practical builds for four devices you will use again and again.

3A. 1:1 Current Balun / Common-Mode Choke

Typical uses: dipoles, inverted-Vs, fan dipoles, and verticals with radials (as a base choke).

Example materials (up to ~100 W HF):

  • 1 × FT240-43 ferrite toroid (or Mix 31 for strong chokes)
  • 4–6 ft of small coax (RG-58 or RG-316), or two insulated wires
  • Weather-resistant enclosure, SO-239 connector, hardware
1:1 choke on FT240 core 1:1 choke balun build photo (placeholder)
Figure 14 – 1:1 current balun using coax wrapped on an FT240 core.

Basic build steps:

  1. Cut 4–6 ft of RG-58 or similar small coax.
  2. Wind 8–12 turns through the core, tight and evenly spaced.
  3. Secure the coax with tape or zip ties.
  4. Mount the core inside an enclosure.
  5. Terminate one end to the antenna connector (SO-239).
  6. Terminate the other end to a short pigtail or connector for the feedline.
  7. Label power rating and intended bands.

3B. 4:1 Current Balun (OCFD, Loops, Delta)

Typical uses: off-center-fed dipoles, full-wave loops, delta loops.

4:1 current balun 4:1 balun build photo (placeholder)
Figure 15 – Conceptual 4:1 current balun with bifilar winding.

Basic build steps (bifilar design):

  1. Cut two equal lengths of insulated wire and twist into a bifilar pair.
  2. Wind 8–12 turns of the twisted pair around an FT240-43 core.
  3. Label wire ends (A1/A2 and B1/B2).
  4. Wire according to a 4:1 current balun schematic (series/parallel arrangement).
  5. Connect the 50 Ω side to the SO-239, and the balanced side to the antenna terminals.
  6. Mount in an enclosure and add strain relief.

3C. 9:1 Unun (Random Wires)

Typical uses: long non-resonant wires used with a tuner.

9:1 unun 9:1 unun build photo (placeholder)
Figure 16 – Conceptual 9:1 unun for random wire antennas.

Basic build steps:

  1. Cut three equal lengths of wire and twist into a trifilar bundle.
  2. Wind 5–7 turns of the bundle on an FT240-43 core.
  3. Identify the three separate windings and wire them according to a standard 9:1 unun diagram.
  4. Connect the low-impedance side to the SO-239, and the high-impedance side to a binding post for the long wire.
  5. Provide an optional counterpoise terminal.

3D. 49:1 EFHW Transformer

Typical uses: EFHW antennas (horizontal, vertical, or sloper) and half-wave slopers.

49:1 EFHW transformer 49:1 EFHW transformer build photo (placeholder)
Figure 17 – 49:1 EFHW transformer on a toroidal core (often two stacked).

Basic build steps:

  1. Stack two FT240-43 cores for higher power, or use one core for QRP.
  2. Wind a 2-turn primary between coax center and ground.
  3. Wind a 14-turn secondary from the antenna terminal to ground.
  4. Connect the primary to the SO-239, and the secondary to the antenna post.
  5. Mount the assembly in a weatherproof box with strain relief.
  6. Test with an analyzer and monitor core heating at your operating power.

Chapter 4 – Choosing Cores & Where to Buy Them

4.1 Selecting Core Mix & Size

Core material and size determine power handling and frequency range. For HF baluns and ununs:

  • Mix 31: excellent for common-mode chokes on 160–40 m and low HF.
  • Mix 43: general-purpose choice for many HF baluns and EFHW transformers.
  • Core size FT240: ~2.4" OD, 1.4" ID, 0.5" thick; very popular for 100 W class builds.

4.2 Example Part Numbers

Look for parts described as “FT240-43 ferrite toroid” or equivalent from reputable vendors. Always confirm dimensions and material.

4.3 Suggested Vendors

  • R&L Electronics – search for "FT240-43" or "ferrite toroid" and support a local ham dealer.
  • RF specialty vendors such as Palomar Engineers and HF Kits.
  • Major distributors (Mouser, Digi-Key, Newark) carrying Fair-Rite parts.
  • Online marketplaces (Amazon, etc.) – verify material and size carefully.

Appendix – Antenna / Balun Summary Table

Antenna Type Typical Feed Z Recommended Device Notes
Center-fed dipole 50–75 Ω 1:1 current balun Balanced antenna fed with coax.
OCFD / Windom 150–300 Ω 4:1 current balun Off-center feed for multiband use.
EFHW (end-fed half-wave) 2–3 kΩ 49:1 or 64:1 unun High impedance end feed.
Random wire / longwire 300–600 Ω 9:1 unun + tuner Non-resonant wire, tuner required.
¼-wave vertical 30–60 Ω 1:1 choke Base choke helps stop RF on coax.
½-wave vertical EFHW High (kΩ) 49:1 or 64:1 unun Same logic as EFHW.
Full-wave loop 100–200 Ω 4:1 current balun Balanced antenna, fed at low point.
Delta loop 100–120 Ω 4:1 current balun Triangular loop, side or corner feed.
Fan dipole ≈50 Ω 1:1 current balun Multiple dipoles at one feedpoint.
Inverted-V dipole 50–75 Ω 1:1 current balun Dipole with sloping legs.
Sloper (half-wave) High (kΩ) 49:1 EFHW transformer Half-wave sloping wire.
Sloper (random) 300–600 Ω 9:1 unun + tuner Random-length sloper.