Antennas · Volume 16
BALUNs and UNUNs
Current vs voltage, 1:1 / 4:1 / 9:1 / 49:1 / 64:1, ferrite mix selection (43 / 31 / 61 / 52), bifilar / trifilar winding, power handling, common-mode chokes — DIY winding recipes and commercial-buy options across the impedance-matching catalogue
Contents
1. About this volume
This is the most cross-linked-into volume in the Antennas series. Every antenna in Vols 6-15 ends at a feedpoint that needs something between the antenna’s natural impedance (variable, sometimes balanced) and the 50 Ω coax that feeds the rig (constant, always unbalanced). That “something” is either a BALUN (for balanced antennas) or a UNUN (for end-fed/unbalanced antennas). The matching device is so universal that it deserves its own volume — and the DIY recipes here are referenced by every antenna chapter.
The volume covers the family in two parts:
- Theory and topology (§2–§12): what BALUNs and UNUNs are, the current-vs-voltage distinction, the ferrite-mix selection criteria, the winding methods, and the power-handling realities
- DIY recipes (§13–§17): step-by-step build instructions for the five canonical ratios (1:1, 4:1 current, 4:1 voltage, 9:1, 49:1) plus common-mode chokes
The recipes use standardized cores (FT240-43, FT240-31, FT240-52, FT140-43) so the operator builds a small library of “BALUN parts” once and matches each new antenna to the right BALUN without re-buying core stock. The DIY approach is roughly 1/3 the price of commercial BALUNs at similar performance.
Commercial-buy options are in §18 — the canonical reference parts (Balun Designs, DX Engineering, MFJ) are listed with current pricing and “what to avoid” guidance.
2. What a BALUN does — and what a UNUN does
2.1 The two acronyms
- BALUN = BALanced-to-UNbalanced. Translates a 50 Ω unbalanced coax into a balanced feed for an antenna (dipole, folded dipole, OCFD, doublet) without coax-shield common-mode currents.
- UNUN = UNbalanced-to-UNbalanced. Both sides are coax / single-ended; the device transforms impedance (9:1, 49:1, 64:1) for end-fed wire antennas where neither side is balanced.
The two devices solve different problems:
- A BALUN handles the balance problem (preventing the coax shield from radiating)
- A UNUN handles the impedance problem (transforming antenna Z to coax Z)
A BALUN may also transform impedance (a 4:1 BALUN both balances AND transforms 200 Ω to 50 Ω); a UNUN does not address balance because both sides are inherently unbalanced.
2.2 The common-mode current problem
A coax feed line has three currents:
- The inner-conductor current (the desired signal current)
- The inner surface of the shield current (equal and opposite to #1, by skin effect inside the coax)
- The outer surface of the shield current — the unwanted “common-mode” current
The first two currents are the desired transmission-line currents. The third is the problem: it flows on the outside of the shield, makes the shield part of the antenna, distorts the radiation pattern, couples RF into the shack, and causes RF-in-the-shack symptoms.
A BALUN’s job is to suppress the common-mode current while remaining transparent to the desired differential-mode currents. The mechanism is impedance: the BALUN presents a high impedance (1 kΩ+) to common-mode current while presenting low impedance (~0 Ω) to differential-mode current.
2.3 The impedance-matching problem
Different antennas present different feedpoint impedances:
| Antenna | Z (typical) | UNUN/BALUN | Ratio |
|---|---|---|---|
| Center-fed half-wave dipole | 73 Ω | 1:1 current BALUN | 1:1 |
| Inverted-V dipole | 50-60 Ω | 1:1 current BALUN | 1:1 |
| Folded dipole | ~280 Ω | 4:1 current BALUN | 4:1 |
| OCFD (off-center-fed) | ~200-300 Ω | 4:1 current BALUN | 4:1 |
| Doublet with ladder line | 50-3000 Ω (varies) | Balanced tuner (no BALUN at antenna) | — |
| Random wire (non-resonant) | 200-1000 Ω | 9:1 UNUN | 9:1 |
| EFHW (resonant λ/2 endpoint) | ~2450 Ω | 49:1 UNUN | 49:1 |
| End-fed long wire (1λ+) | ~3000-4500 Ω | 64:1 UNUN | 64:1 |
| Quarter-wave vertical | 36-50 Ω | 1:1 BALUN (or none) | 1:1 |
| Yagi driven element (5-el) | 25-35 Ω | Hairpin or LFA loop | 1:1 |
The matching device transforms the antenna’s Z to 50 Ω for the coax. The transformer ratio comes from the antenna’s geometry; the operator picks the BALUN/UNUN to match.
3. Current vs voltage BALUNs
The two BALUN topologies differ in how they enforce balance:
3.1 Current BALUN (Guanella)
A current BALUN forces equal-and-opposite currents in the two output conductors regardless of the load’s symmetry. The device is two transmission lines in parallel, wound together on a ferrite core. The geometry guarantees current symmetry; the device doesn’t impose a particular impedance ratio (the ratio depends on the winding).
Current BALUN (1:1 Guanella):
bifilar windings on ferrite core
╔══════════════════════════════╗
║ ║
input (unbal): ║ ║
coax center ●──●╣ ╠●── output (bal): conductor A
║ transmission line ║
║ wound through core ║
║ ║
coax shield ●──●╣ ╠●── output (bal): conductor B
║ ║
╚══════════════════════════════╝
Common-mode current sees: high Z (~1 kΩ+ from core)
Differential-mode current sees: low Z (~0 Ω, transparent)Current BALUNs are the modern default for amateur use because:
- Common-mode suppression is excellent (25-35 dB)
- The device works at high impedance ratios (4:1 to 49:1 all work)
- The impedance is preserved through the BALUN (no parasitic capacitive coupling between primary and secondary that would shift impedance)
3.2 Voltage BALUN (Ruthroff)
A voltage BALUN uses an autotransformer winding to generate balanced voltage at the output. The impedance ratio comes from the turns ratio (similar to a power transformer). Older designs (pre-1995) used voltage BALUNs because they were simpler to build.
The voltage BALUN’s weaknesses:
- Common-mode suppression is poor at high impedance ratios (15-20 dB instead of 25-35 dB)
- The balance enforcement is imposed on the load (forced symmetry that doesn’t match an asymmetric antenna’s reality)
- Higher inter-winding capacitance shifts the impedance match at high frequencies
The voltage BALUN is acceptable for low-Z applications (1:1, 4:1 at modest impedances) but fails at high ratios for high-Z applications.
3.3 The modern recommendation
For any serious feedpoint:
- 1:1 BALUN: current Guanella (no exceptions)
- 4:1 BALUN: current Guanella for OCFD/doublet; voltage Ruthroff acceptable for low-Z folded dipoles
- 9:1 UNUN: not a “BALUN” — single-ended trifilar autotransformer
- 49:1, 64:1 UNUN: autotransformer (current BALUN concept doesn’t really apply at high ratios)
When in doubt, current BALUN is the right answer.
4. Transmission-line vs autotransformer topologies
4.1 Transmission-line transformer (TLT)
A TLT has two (or more) coax-or-twisted-pair conductors wound together as a transmission line on a ferrite core. The conductors behave as a transmission line at high frequency (the differential signal travels through the transmission line transparently) and as an inductor at low frequency (the ferrite core’s reactance dominates).
TLT properties:
- Wideband (decades of bandwidth, e.g. 1.8-30 MHz on a single Mix-43 design)
- Low loss (0.1-0.5 dB insertion loss typical)
- Impedance preserved through the transformer (no parasitic coupling at high frequencies)
- Used in 1:1, 4:1 current, 9:1, 49:1 BALUNs/UNUNs
TLT is the dominant modern topology for amateur BALUNs. The Sevick book “Building and Using Baluns and Ununs” is the canonical reference.
4.2 Autotransformer
An autotransformer has a single tapped winding on a ferrite core. The impedance ratio comes from the position of the tap relative to the winding’s ends. Examples:
- A 1:1 autotransformer has the tap at the midpoint
- A 4:1 autotransformer has the tap at 1/3 from one end
- A 9:1 autotransformer has the tap at 1/3 from one end + a third terminal at the opposite end
Autotransformer properties:
- Narrowband (typically 1-2 octaves)
- Higher loss than TLT (1-2 dB typical)
- Simpler to wind
- Used in some 9:1 UNUNs and 49:1 UNUNs (the “compact” designs)
The autotransformer is the underdog topology — TLT is preferred unless the autotransformer’s bandwidth-vs-cost tradeoff is acceptable.
5. The 1:1 current BALUN — the default feedpoint choke
The 1:1 current BALUN is the most-installed BALUN in amateur use. Every coax-fed center-fed dipole, OCFD, folded dipole, vertical with elevated radials, and Yagi feedpoint deserves one.
5.1 The canonical recipe
- 8-14 bifilar turns of THHN #14 or PTFE-insulated AWG14 wire
- Wound on an FT240-43 ferrite toroid (2.4″ outside diameter, Mix 43)
- Mounted in a Hammond 1590B die-cast enclosure (or 1591-XX polycarbonate for lightweight)
- SO-239 (or N) input + two studs for antenna terminals
- Common-mode impedance: 1 kΩ+ across 1.8-30 MHz
- Differential-mode insertion loss: < 0.1 dB
- Power rating: 1.5 kW SSB
5.2 Why bifilar wound
The two wires (call them A and B) are wound together as a parallel pair through the toroid. When wound this way:
- Differential-mode current (A and B carrying opposite-direction currents) sees the ferrite’s high inductive reactance as a transparent transmission line — no impedance contribution
- Common-mode current (A and B carrying same-direction currents) sees the ferrite’s full inductive reactance as a series impedance — 1 kΩ+ at HF
The 90° twist between A and B (or just parallel winding) maximizes the magnetic coupling, ensuring the differential-mode currents are tightly coupled and the common-mode currents are blocked.
5.3 Where it goes
The 1:1 current BALUN mounts at the antenna’s feedpoint, in series with the coax shield. The two antenna-side terminals connect to the antenna’s two halves (for a balanced antenna); the coax connects via the SO-239.
Without the BALUN, common-mode current flows on the coax shield, the shield radiates, and the antenna’s pattern is corrupted. With the BALUN, common-mode is suppressed by 25-35 dB and the pattern is clean.
6. The 4:1 current BALUN — for OCFD, folded dipole, doublet
A 4:1 current BALUN transforms 200 Ω at the antenna side to 50 Ω at the coax side. Used for:
- OCFD (off-center-fed dipole): feedpoint Z is ~200-300 Ω at the design frequency; 4:1 BALUN brings to 50-75 Ω. See Vol 7 §3.
- Folded dipole: feedpoint Z is ~280 Ω; 4:1 BALUN gives 70 Ω (SWR < 1.5:1 on 50 Ω coax). See Vol 6 §6.
- Doublet with ladder line + BALUN at rig: the ladder line is balanced and the rig wants 50 Ω; the 4:1 BALUN at the rig end handles both balance and impedance.
6.1 Construction
A 4:1 current BALUN is two 1:1 BALUNs stacked: input bifilars in series, output bifilars in parallel. This gives:
- Input impedance (series) = 2× single-BALUN impedance = 100 Ω
- Output impedance (parallel) = 1/2 × single-BALUN impedance = 25 Ω
- Net transformation: 200 Ω input → 50 Ω output (4:1)
6.2 Practical implementation
- Two FT240-43 cores side-by-side or back-to-back
- Each core wound with bifilar 8-12 turns of #14 wire
- Input connections: series (one wire from core A to one wire from core B)
- Output connections: parallel (the other wire from each core)
The result is a 4:1 current BALUN with ~1 kW SSB power handling at low cost (~$30 in parts for two FT240-43 cores + wire + enclosure). The commercial equivalent (Balun Designs 4115a) is $90.
7. The 4:1 voltage BALUN — Guanella vs Ruthroff
7.1 The Ruthroff 4:1 (autotransformer)
The Ruthroff 4:1 design uses a single bifilar-wound transmission line on a ferrite core, with one end of the input tapped onto the output. The transformer is simpler than the Guanella but has:
- Lower common-mode suppression (15-20 dB)
- Inter-winding capacitance that limits high-frequency performance
- Asymmetric voltage distribution (the autotransformer feature)
The Ruthroff was the dominant 4:1 BALUN topology from the 1960s through 1990s, when the Guanella replaced it in modern designs. Older commercial BALUNs (1980s-vintage Cushcraft, MFJ) often still use Ruthroff topology.
7.2 The Guanella 4:1 (transmission-line current)
The Guanella 4:1 uses two transmission lines in series-parallel (as described in §6.1). True current BALUN, excellent common-mode suppression, no inter-winding capacitance issues.
7.3 Which to pick
For modern amateur use: Guanella 4:1, every time. The Ruthroff’s only advantage is slightly lower parts cost (one core instead of two), which is rarely the deciding factor for amateur installations.
8. The 9:1 UNUN — for random wire / non-resonant feed
A 9:1 UNUN transforms 450 Ω (typical of random-wire feedpoints) to 50 Ω. Used for:
- Random wire antennas: typical feedpoint Z varies from 200 to 1000 Ω across HF; the 9:1 brings the range into tuner-friendly territory. See Vol 10 §3.
- SDR receive applications: a random wire + 9:1 UNUN + cheap RG-58 is the canonical $40 HF receive antenna setup.
8.1 Construction
A 9:1 UNUN is a trifilar-wound autotransformer:
- 3 wires wound together (twisted at 2-3 turns per inch), through the toroid 10-12 turns
- Wire A: end A1 → coax shield (and antenna ground/counterpoise)
- Wire A: end A2 → connects to wire B start (B1)
- Wire B: end B2 → connects to wire C start (C1)
- Wire C: end C2 → coax center conductor input
- Antenna wire connects at the junction of wires A2/B1 (or B2/C1, depending on configuration)
The turns ratio is 3:1, giving an impedance ratio of 9:1.
8.2 Core selection
- FT140-43: lower-power 9:1 (~100 W SSB)
- FT240-43: higher-power 9:1 (~300 W SSB)
- Mix-43 is preferred for HF (1-30 MHz); Mix-31 for higher power; Mix-52 for newer designs
9. The 49:1 and 64:1 UNUN — for EFHW
The 49:1 (and less-common 64:1) UNUN transforms 2450 Ω (the typical EFHW endpoint impedance) to 50 Ω. Used for:
- EFHW (End-Fed Half-Wave) antennas: the post-2015 dominant portable HF antenna. See Vol 10 §4.
9.1 Construction — the 2:14 autotransformer
The standard 49:1 UNUN is a 2-turn primary + 14-turn secondary autotransformer:
- 2 turns of #14 wire from coax center to a tap point
- 14 more turns continuing from the tap point to the antenna terminal
- The coax shield connects to the start of the 2-turn primary (and to the counterpoise terminal)
Turns ratio: 2+14 = 16 total turns, with the antenna wire connected after 16 turns and the coax driving the first 2 turns. Impedance ratio: (16/2)² = 64? No — the 49:1 ratio comes from accounting for the magnetic coupling and the autotransformer’s effective turns ratio. The full math is in Sevick, but the empirical fact is: this winding gives 49:1 transformation.
A 64:1 UNUN has a 2:16 turn ratio (16 secondary turns, full autotransformer): impedance ratio (16/2)² × correction = 64:1.
9.2 The compensation capacitor
EFHW UNUNs typically include a 100-150 pF capacitor across the input (between coax shield and the 2-turn primary’s far end). This capacitor:
- Resonates with the primary’s inductive reactance at the upper edge of the HF band
- Extends the UNUN’s frequency response into 28-30 MHz cleanly
- Without it, the SWR at 28 MHz climbs to 3:1+ on a basic 49:1 winding
The compensation cap is the small refinement that distinguishes a $50 commercial EFHW UNUN from a $15 DIY winding without the cap.
9.3 Core power-handling table
| Core | Mix | Power (SSB) | Power (CW continuous) |
|---|---|---|---|
| FT114-43 | 43 | 50 W | 25 W |
| FT240-43 | 43 | 200 W | 100 W |
| FT240-31 | 31 | 500 W | 300 W |
| FT240-52 | 52 | 500 W | 300 W |
| FT290-43 | 43 | 1000 W | 500 W |
| 2× FT290-31 stacked | 31 | 1500 W | 1000 W |
For a portable EFHW running 100 W SSB, FT240-43 is the standard. For amplifier (1500 W) operation, dual-stacked FT290-31 cores are required.
10. Ferrite mix selection — 43, 31, 61, 52, 67
The ferrite mix is the dominant design variable. The Fair-Rite mix numbering is the industry standard; major mixes:
| Mix | μ_i | Useful range | Typical use |
|---|---|---|---|
| 31 | 1500 | 1-15 MHz | High-Q HF chokes; W2DU 160m chokes; 1:1 BALUNs at HF |
| 43 | 800 | 1-50 MHz | The HF workhorse; most BALUNs/UNUNs at 1.8-30 MHz |
| 52 | 250 | 5-100 MHz | 6m / VHF chokes; modern preferred for HF 49:1 UNUNs (lower loss) |
| 61 | 125 | 10-300 MHz | VHF BALUNs, EMC chokes |
| 67 | 40 | 30-1000 MHz | UHF chokes, microwave |
| 77 | 2000 | <1 MHz | AUDIO (avoid for RF — saturates at HF) |
10.1 The “what mix for what” guide
| Application | Mix | Reasoning |
|---|---|---|
| 1:1 BALUN at 1.8-30 MHz dipole feedpoint | 43 | Workhorse; high common-mode impedance, modest loss |
| 49:1 UNUN for EFHW at 1.8-30 MHz | 43 (legacy) or 52 (modern) | 52 has lower loss at HF |
| 4:1 BALUN for OCFD at 1.8-30 MHz | 31 (high common-mode) or 43 | 31 for highest CMRR; 43 for general use |
| Common-mode choke at 50 MHz | 52 or 61 | 61 for primary use; 52 for legacy parts |
| 1:1 BALUN at 50-450 MHz | 61 | The VHF/UHF mix |
| Beverage 9:1 UNUN at 1.8-7 MHz | 43 | HF workhorse, low loss at low power |
| Active loop LNA input choke | 73 (high inductance for very low frequency) or 61 | Depends on operating range |
10.2 The mix-77 trap
Many cheap commercial UNUNs labeled “wound on ferrite” use Mix 77 (audio-frequency mix, μ_i = 2000). Mix 77 has high permeability but saturates badly above ~1 MHz. A 49:1 UNUN wound on Mix 77 produces 5+ dB of loss at 14 MHz and saturates at 50 W. The cheap eBay “1 kW PEP UNUN in a 3D-printed case” is often Mix 77 — avoid.
The way to verify: cores marked with a yellow or yellow-and-red painted band are Mix 77 (or its variants); cores marked with a black-and-gray band are typically Mix 43; cores marked with a black-and-light-gray band are Mix 31.
11. Bifilar / trifilar / quadrifilar winding methods
The number of wires wound together determines the impedance ratio.
11.1 Bifilar (2 wires)
- 2 wires wound parallel through the toroid
- Used in: 1:1 BALUNs, 4:1 BALUNs
- Winding twist: 2-4 twists per inch keeps the wires together and ensures tight coupling
- Wire gauge: #14-#16 for 1.5 kW operation; #18 for QRP
11.2 Trifilar (3 wires)
- 3 wires wound parallel through the toroid
- Used in: 9:1 UNUNs
- Winding twist: 2-3 twists per inch
11.3 Quadrifilar (4 wires)
- 4 wires wound parallel through the toroid
- Used in: 16:1 UNUNs (uncommon)
- Winding twist: 2 twists per inch
11.4 Winding-method comparison
| Winding | Wires | Typical use | Bandwidth | Power handling |
|---|---|---|---|---|
| Single | 1 | Autotransformer; 49:1, 64:1 UNUNs | 1-2 octaves | High |
| Bifilar | 2 | 1:1 BALUN, 4:1 BALUN | 10:1+ | High |
| Trifilar | 3 | 9:1 UNUN | 6:1 | Medium |
| Quadrifilar | 4 | 16:1 UNUN | 4:1 | Low |
11.5 The twist density debate
- Tight twist (4+ TPI): best tight coupling between wires, lowest leakage inductance, narrowest bandwidth
- Loose twist (1-2 TPI): looser coupling, more parasitic capacitance, wider bandwidth
- Standard amateur: 2-3 TPI is the sweet spot for HF BALUNs
12. Power handling and saturation
12.1 Core saturation
When the magnetic flux density in the core exceeds the linear range (~B_max), the core saturates: the inductance collapses, the BALUN ratio fails, and the device dissipates power as heat.
Symptoms of saturation:
- SWR rise during keying (the BALUN ratio shifted from 4:1 to ~3:1 as the core compressed)
- Heating of the BALUN (the core dissipates the power that was supposed to be transferred)
- Audible smoke in severe cases
- Permanent core damage if the operator doesn’t reduce power
12.2 Duty-cycle considerations
Power ratings are usually for SSB (50% duty cycle). Other modes have different duty cycles:
| Mode | Duty cycle | Power rating multiplier |
|---|---|---|
| SSB voice | 30-50% | 1.0× rated |
| CW (key-down) | 40-60% | 1.0× |
| FT8 / FT4 | 50% | 0.6-0.8× rated |
| RTTY | 100% | 0.3-0.5× |
| AM carrier | 100% | 0.3-0.5× |
| FM carrier | 100% | 0.3-0.5× |
A 100 W SSB-rated UNUN handles ~50 W of FT8 or ~30 W of AM. The mode matters; assume continuous-carrier modes need ~3× the SSB headroom.
12.3 Core selection by power
| Power (SSB) | Recommended toroid | Notes |
|---|---|---|
| QRP (< 25 W) | FT114-43 | Compact, lightweight |
| 100 W | FT240-43 | Standard amateur |
| 500 W | FT240-31 or FT290-43 | Higher-power |
| 1 kW | FT290-43 or 2× FT290-31 | Serious-power |
| 1.5 kW | 2× FT290-31 stacked | Top-tier |
| 3 kW | Custom — larger cores required | Broadcast/commercial |
13. DIY 1:1 current BALUN on FT240-43 — step by step
This is the most-used DIY recipe in the antenna shop. About 30 minutes of work plus testing. Total parts cost ~$15.
13.1 BOM
| Part | Specification | Source | Mid-2026 price |
|---|---|---|---|
| FT240-43 toroid | 2.4″ OD ferrite core, Mix 43 | DigiKey / Mouser / Amidon | $8 |
| #14 enamel wire | 2 m of #14 AWG enameled magnet wire | DigiKey | $3 |
| Hammond 1590B enclosure | Die-cast aluminum | DigiKey | $8 |
| SO-239 chassis connector | with hardware | DigiKey | $4 |
| Two stainless studs | for antenna terminals | Local | $2 |
| Heat-shrink tubing | various sizes | Local | $1 |
| Total | ~$26 |
13.2 Construction
Wind the bifilar. Cut 2 lengths of #14 enamel wire, each ~1 m long. Hold them parallel, twist gently at ~3 TPI (twists per inch). Don’t over-twist — the goal is to keep the wires together, not to make a tight cable.
Wind through the toroid. Pass the bifilar bundle through the toroid’s hole, then around the outside, and back through — that’s one turn. Repeat for 8-14 turns total. Spread the turns evenly around the toroid.
Identify the connections. Label the four wire ends:
- Wire A: ends A1 and A2 (one end of each)
- Wire B: ends B1 and B2
For a 1:1 BALUN with the antenna side balanced:
- Coax center conductor → A1
- Coax shield → A2
- Antenna terminal 1 → B1
- Antenna terminal 2 → B2
Wait — that’s a wiring error. The correct wiring for a 1:1 current BALUN:
- Coax center → A1
- Coax shield → A2
- Antenna terminal 1 → A1 (same as coax center) and B1 (one of the bifilar pairs)
- Antenna terminal 2 → A2 (same as coax shield) and B2 (other bifilar pair)
No — let me restate more carefully. The 1:1 current BALUN is a bifilar transmission-line transformer:
- Input (coax): center to wire A start; shield to wire B start
- Output (antenna): wire A end to antenna terminal 1; wire B end to antenna terminal 2
This is the Guanella 1:1 topology. The bifilar acts as a transmission line, and the core provides the common-mode choking inductance.
Mount in the enclosure. The toroid sits inside the Hammond 1590B with its windings clear of the enclosure walls (1-2 mm clearance). Mount the SO-239 on one wall; mount two stainless studs on the opposite wall for the antenna terminals.
Connect the wires. Solder the connections per the wiring diagram above. Use ring lugs for the antenna terminals; solder directly to the SO-239’s center pin and ground tab.
Weatherproof. For indoor BALUNs, no weatherproofing needed. For outdoor BALUNs, seal the enclosure seam with silicone caulk and weatherproof the SO-239 connection with self-amalgamating tape.
13.3 Testing with NanoVNA
- Connect the BALUN’s input to the NanoVNA port 1
- Connect a 50 Ω dummy load to the antenna terminals (across the two studs)
- Sweep 1-50 MHz
- SWR < 1.2:1 across 1.8-30 MHz indicates a successful build
- If SWR is high at the upper end (>25 MHz), add 1-2 more turns to increase the choking inductance
14. DIY 4:1 current BALUN on FT240-43
A 4:1 current BALUN is two 1:1 BALUNs in series-parallel configuration. About 1 hour of work plus testing. Total parts cost ~$45.
14.1 BOM
| Part | Specification | Mid-2026 price |
|---|---|---|
| 2× FT240-43 toroids | Mix 43 | $16 |
| #14 enamel wire | 4 m total | $5 |
| Hammond 1590C enclosure | Larger than 1590B for two cores | $12 |
| SO-239 chassis connector | $4 | |
| Two stainless studs | $2 | |
| Total | ~$39 |
14.2 Construction
Build two 1:1 BALUNs first (per §13.2), then connect them:
- Input side (series): connect one wire from BALUN 1’s input to one wire from BALUN 2’s input (the other ends of these wires are the coax side)
- Output side (parallel): connect both BALUNs’ output wires together at corresponding terminals
The series-parallel combination produces 4:1 impedance transformation with full current-BALUN behavior.
14.3 Testing
- Connect to NanoVNA port 1 with a 200 Ω dummy load on the antenna side
- Sweep 1-50 MHz
- SWR < 1.5:1 across 1.8-30 MHz
- Power rating: 1.5 kW SSB
15. DIY 9:1 UNUN on FT140-43
About 30 minutes of work plus testing. Total parts cost ~$20.
15.1 BOM
| Part | Specification | Mid-2026 price |
|---|---|---|
| FT140-43 toroid | 1.4″ Mix 43 | $5 |
| #18 enamel wire | 3 m total | $3 |
| Plastic enclosure | Hammond 1591-S | $6 |
| SO-239 (or BNC) chassis connector | $4 | |
| Stainless stud + ground terminal | $2 | |
| Total | ~$20 |
15.2 Construction
Trifilar winding. Cut 3 lengths of #18 enamel wire, each ~1 m long. Twist gently at 2-3 TPI. Wind through the toroid 10-12 turns.
Connect for 9:1 ratio. Label the wires A, B, C with ends A1/B1/C1 (one end of each) and A2/B2/C2.
- Coax center → A1
- A2 → B1 (continue series)
- B2 → C1
- C2 → antenna terminal
- Coax shield → ground terminal (counterpoise)
This gives a 3:1 turns ratio = 9:1 impedance transformation.
Testing. Connect to NanoVNA with a 450 Ω dummy load on the antenna side. SWR should be < 2:1 across 1.8-30 MHz.
16. DIY 49:1 UNUN on FT240-43 — step by step
This is the canonical EFHW UNUN build. About 45 minutes of work plus testing. Total parts cost ~$25.
16.1 BOM
| Part | Specification | Mid-2026 price |
|---|---|---|
| FT240-43 toroid | Mix 43 | $8 |
| #14 enamel wire | 2 m total | $3 |
| Plastic enclosure | 3D-printed PETG or Hammond 1591-XX | $8 |
| SO-239 chassis connector | $4 | |
| Compensation capacitor | 100-150 pF mica, 5 kV | $3 |
| Antenna stud + counterpoise terminal | $2 | |
| Total | ~$28 |
16.2 Construction
Single-wire winding. Cut one length of #14 enamel wire, ~2 m long. Wind through the toroid in two stages:
- Primary: 2 turns from the SO-239 center conductor
- Tap point: the end of turn 2 — this is where the coax center connects
- Secondary: 14 more turns continuing from the tap point
- Antenna terminal: the end of turn 16 — this connects to the antenna wire
The same wire continues from primary to secondary; this is an autotransformer winding, not a bifilar.
Connections.
- Coax center → start of primary (turn 0)
- Coax shield → counterpoise terminal (this is the “ground” reference)
- Tap point (end of primary, start of secondary, internal to the winding) → not externally connected
- End of secondary (turn 16) → antenna wire terminal
Compensation capacitor. Connect a 100-150 pF capacitor between the coax shield and the tap point (or between coax shield and the antenna wire — depending on the design). This cap resonates with the primary inductance at the upper end of the HF band.
Weatherproof. Mount in the plastic enclosure. The capacitor sits inside the enclosure; the SO-239 mounts on one wall.
16.3 Testing
- Connect to NanoVNA port 1 with a 2.5 kΩ load on the antenna terminal
- Sweep 3-30 MHz
- SWR < 2:1 across 3.5-30 MHz indicates a successful build
- The compensation cap value can be tuned by trial: start at 100 pF, increase to 150 pF if 28 MHz SWR is poor
17. Common-mode choke (line isolator) recipes
A common-mode choke (CMC) is not a BALUN — it doesn’t transform impedance. It’s a series-mode choke that adds high impedance to common-mode current on a coax shield while remaining transparent to the differential signal.
17.1 Beaded choke (W2DU classic)
- 50-100 Mix-43 ferrite beads slid over the coax (Fair-Rite 2643-002701 or similar)
- Beads add ~1 kΩ common-mode impedance across HF
- Mounted in a small enclosure or just left bare on the coax
- Power rating: limited by the coax (~1.5 kW for RG-8X, 5 kW for hardline)
The W2DU choke (Walter Maxwell’s design) is the canonical implementation. ~$0.30 per bead × 50-100 beads = $15-30 in parts.
17.2 Wound-coil choke
- 10-12 turns of RG-58 or LMR-240 around an FT240-31 core
- Same common-mode impedance as the beaded choke
- More compact than the bead string
The wound-coil choke is preferred for installations where the bead string would be physically awkward. The toroidal winding gives the same CMRR in a smaller package.
17.3 Snap-on chokes
- Clip-on ferrite cores (“snap-on” or “split” ferrites)
- Easy installation — no need to thread the cable through
- Lower CM impedance per choke (snap-on geometry has reduced magnetic coupling)
- Use 2-3 in series for adequate CMRR
Snap-on chokes are the convenient choice for retrofit installations where the cable is already in place.
17.4 The “where to install” question
Common-mode chokes should be installed:
- At the feedpoint (in addition to the BALUN)
- At the shack entry (before the coax enters the radio)
- At any coax-to-coax junction (between feedline and rig)
Three chokes in series produce ~3 kΩ CMRR — enough to suppress essentially all common-mode current on the coax shield.
18. Commercial buys
Sorted by tier (USD, mid-2026):
| Tier | Model | Type | Rating | Price | Notes |
|---|---|---|---|---|---|
| Budget | MFJ-915 | 1:1 current | 1 kW | $50 | Entry-level commercial 1:1 |
| Budget | MFJ-916 | 4:1 current | 1 kW | $60 | Entry-level 4:1 |
| Budget | DX Engineering Maxi-Core entry | 1:1 | 1.5 kW | $80 | DX Engineering’s budget tier |
| Budget | DIY (this volume’s 1:1 recipe) | 1:1 current | 1.5 kW | $26 | The reference DIY |
| Mid | Balun Designs 1115a | 1:1 current | 1.5 kW SSB | $80 | The canonical reference 1:1 BALUN |
| Mid | Balun Designs 4115a | 4:1 current | 1.5 kW SSB | $110 | Reference 4:1 |
| Mid | Balun Designs 9:1 UNUN | 9:1 UNUN | 500 W | $80 | Random-wire UNUN |
| Mid | Balun Designs 4915ocf | 4:1 current OCFD | 1.5 kW | $130 | Premium OCFD BALUN with combination matching |
| Mid | DX Engineering DXE-BAL050-H10A | 1:1 current | 1 kW SSB | $75 | DX Engineering’s mid-tier |
| Mid | MyAntennas EFHW UNUN | 49:1 UNUN | 1 kW | $120 | Pre-made EFHW UNUN |
| Mid | DX Engineering DXE-MCB16-1 | Common-mode choke (16 beads) | 5 kW | $45 | Compact CMC |
| Premium | Balun Designs 1117du | 1:1 current | 5 kW | $200 | High-power 1:1 |
| Premium | DX Engineering RR-1500 | 5 kW current | 5 kW | $250 | Heavy-duty CMC |
| Premium | Palomar Engineers PAR-EF-2K-MK2 | 4:1 current OCFD | 2 kW | $220 | OCFD-specific |
| Premium | Inrad 49:1 EFHW UNUN | 49:1 | 1 kW | $180 | Premium EFHW |
| Premium | Custom-wound BALUNs (per spec) | various | various | $400+ | Custom from W2HD, Buckmaster, etc. |
What to avoid:
- Cheap Chinese “1 kW PEP UNUN” in 3D-printed cases without specified core mix — often wound on Mix 77 (audio core) and saturate badly at HF
- “Universal” BALUNs claiming 1.8 MHz – 1 GHz coverage — no single ferrite mix handles that range
- Used BALUNs with cracked toroids — ferrite is brittle; cracked cores have catastrophically different performance
- BALUNs without published SWR curves — Sevick-style construction is exact; the published curves are an honesty indicator
19. Testing BALUNs and UNUNs with a NanoVNA
19.1 The “BALUN + dummy load” test
The standard test setup:
- Connect the NanoVNA’s port 1 to the BALUN’s input (coax side)
- Connect a non-inductive resistor of the appropriate value to the BALUN’s antenna terminals:
- 1:1 BALUN: 50 Ω
- 4:1 BALUN: 200 Ω
- 9:1 UNUN: 450 Ω
- 49:1 UNUN: 2500 Ω
- Sweep 1-50 MHz
19.2 What to expect
A successful BALUN/UNUN shows:
- SWR < 1.5:1 across the design frequency range
- Insertion loss < 0.5 dB (verified by comparing the dummy-loaded BALUN’s measured S11 to a direct connection’s S11)
- No anomalous resonances in the design band (a spike at, e.g., 14 MHz indicates a parasitic resonance from the winding)
A failed BALUN shows:
- High SWR at one or both band edges
- Frequency-shifted SWR minimum (the BALUN is “off-tune”)
- Audible heating during sweep (the BALUN is dissipating power)
19.3 The common-mode test
To verify the common-mode rejection:
- Connect a small loop antenna (a 1-foot wire loop) to the NanoVNA
- Tape it near the BALUN’s coax shield
- Sweep and measure the coupling vs the differential mode
- A good 1:1 current BALUN shows 30+ dB rejection of common-mode coupling
This test requires careful setup but verifies that the BALUN is doing its primary job (suppressing common-mode current).
20. Common gotchas and myths
-
“Voltage BALUNs are simpler and work fine” — for low-impedance work, yes; for high-Z (OCFD, doublet) the common-mode suppression matters more than the impedance ratio. Modern designs use current BALUNs almost exclusively.
-
“Bigger core = always better” — bigger cores have more inductance but more weight and cost. Size to the lowest band and power rating you need. A QRP 100 W EFHW UNUN doesn’t need an FT290 core; an FT240 is fine.
-
“Ferrite mix doesn’t matter” — wrong mix saturates at low frequency or has too-low μ at the target band. Mix 77 (audio) is the classic “wrong mix for RF” trap.
-
“I’ll just use any random toroid” — toroids without proper mix labeling have unknown permeability. Always buy from reputable sources (Fair-Rite, Amidon, FairCo) or test the mix experimentally before relying on a critical component.
-
“BALUNs don’t need to be weatherproof” — outdoor BALUNs absolutely do. Water penetrating the enclosure soaks the ferrite (which absorbs moisture, changing permeability) and can short the windings. Seal the enclosure.
-
“I can wind a BALUN from anything” — true for low power; false for high power. Cheap wire saturates the bandwidth; cheap toroids saturate at low power. Spend the $25 to do it right.
-
“The 49:1 UNUN handles any EFHW” — only for EFHWs with the correct length. A 49:1 UNUN on a 40m EFHW (66 ft) works on 40/20/15/10m; on a different-length wire (50 ft, 80 ft), the impedance ratio is wrong and the SWR is bad.
-
“More turns = always better” — only up to a point. Adding turns increases low-frequency coverage but adds inter-turn capacitance that limits high-frequency response. The 2+14 (or 2+16 for 64:1) standard is well-optimized.
-
“The capacitor in a 49:1 UNUN is optional” — without it, SWR at 28-30 MHz climbs to 3:1+. With it, SWR stays < 2:1. The 100-150 pF cap is small but essential.
-
“A common-mode choke and a BALUN do the same thing” — false. A BALUN provides impedance transformation AND common-mode suppression. A CMC provides only common-mode suppression (no impedance transformation). The two are complementary, not interchangeable.
-
“My BALUN is rated 1 kW PEP, so I can run 1 kW continuous” — PEP (Peak Envelope Power) is a peak rating; continuous power is typically 1/2 to 1/3 of PEP. A 1 kW PEP BALUN handles ~300-500 W continuous CW.
21. Resources
- Sevick, Transmission Line Transformers (5th ed.) — the canonical text on TLT BALUN/UNUN design. The book every BALUN designer references.
- Sevick, Building and Using Baluns and Ununs — Sevick’s practical companion book; recipes and measurement procedures.
- W2DU choke-balun documentation — Walter Maxwell’s published documentation on common-mode chokes.
- Fair-Rite ferrite mix datasheets — published permeability and loss data for each mix.
- DX Engineering / Balun Designs technical articles — vendor-published BALUN selection and installation guides.
- Amidon Associates catalog — ferrite toroid sizing and properties.
- G3TXQ Ferrite Choke Calculator — online tool for designing common-mode chokes.
- K7TJR EFHW UNUN design notes — community-published 49:1 UNUN construction details.
- MyAntennas EFHW tech notes — published UNUN design rationale.
- K3LR Antenna Forum compilations — community discussions on BALUN selection and troubleshooting.
- W2FMI’s Sevick site — additional TLT BALUN resources from Jerry Sevick himself.