Balanced
Equal and opposite currents flow in the two antenna conductors. A conventional dipole is the common example.
Baluns and Random-Wire Antennas
HFDXARC Practical RF Guide
Baluns, Ununs and Random-Wire Antennas
A practical guide to interpreting transformer ratios, selecting a sensible core size and choosing random-wire lengths that avoid the worst high-impedance trouble spots.
Start here
Balanced, unbalanced and common-mode current
Coaxial cable is an unbalanced transmission line. A centre-fed dipole is a balanced load. A balun connects these different systems and, when correctly designed, helps keep unwanted RF off the outside of the coax braid.
Unbalanced
One conductor is referenced to the shield, chassis or RF return path. Coax and end-fed wires are unbalanced systems.
Common mode
Current flowing on the outside of the coax braid can alter the radiation pattern, increase received noise and put RF in the shack.
Transformer comparison
Which device does what?
| Device | Connection | Typical use | Main job | Tuner? |
|---|---|---|---|---|
| 1:1 current choke | Balanced to unbalanced | Coax-fed dipole, feed-line isolation | Suppress common-mode current | Only if antenna requires it |
| 4:1 current balun | Balanced to unbalanced | Selected balanced loads and tuner interfaces | Approximately 200 Ω to 50 Ω transformation | Often |
| 9:1 unun | Unbalanced to unbalanced | Non-resonant end-fed or random wire | Move a wide impedance range nearer the tuner range | Normally yes |
| 49:1 unun | Unbalanced to unbalanced | End-fed half-wave antenna | Transform several thousand ohms towards 50 Ω | Often not after tuning |
1:1 current choke
Keep feeder current where it belongs
A current choke presents a high impedance to unwanted current on the outside of the coax while allowing the normal differential RF current inside the coax to pass. It is commonly placed at a balanced antenna feed point or at another point selected to control common-mode current.
- Useful on coax-fed dipoles and beams
- Can reduce RF feedback into station equipment
- Performance depends on choking impedance across the required bands
4:1 current balun
Impedance transformation plus balance
A 4:1 impedance ratio corresponds to an approximate 2:1 winding ratio. It is appropriate only where the expected balanced-load impedance and the balun's usable frequency range make sense.
9:1 unun
For a non-resonant end-fed wire
A 9:1 unun is not a universal matching solution. It provides an impedance transformation so that a suitable ATU may have a more manageable load. The installation needs a deliberate RF return path, such as a counterpoise or a controlled use of the feeder.
49:1 unun
For an end-fed half-wave
An EFHW is cut near a half wavelength on its fundamental band and presents a high feed-point impedance. A high-ratio transformer brings that impedance towards the range expected by 50 Ω coax. The wire still needs final trimming in its installed position.
Ferrite selection
Material matters more than colour
Core size, material, winding method, frequency coverage and required choking impedance must be considered together. A physically large core is not automatically suitable if its material is wrong for the intended frequency range.
Mix 31
Broad HF common-mode choking, especially useful into the lower HF range.
Mix 43
General HF use, commonly encountered in transformers and chokes.
Mix 61
Higher-frequency applications; typically more useful towards upper HF and VHF.
Mix 77
Lower-frequency material associated with LF and MF applications.
Historic workshop reference
4:1 core card transcription
This table preserves the figures visible on the supplied workshop card. It is a transcription of that design reference, not a universal winding schedule for every 4:1 balun.
| Core type | Printed power | Printed turns |
|---|---|---|
| T-80-2 | 60 W | 25 |
| T-106-2 | 100 W | 16 |
| T-130-2 | 150 W | 18 |
| T-157-2 | 250 W | 16 |
| T-200-2 | 400 W | 17 |
| T-200A-2 | 400 W | 13 |
| T-400-2 | 1000 W | 14 |
9:1 random-wire reference
Typical SWR by wire length
Every number in the matrix is an SWR ratio. For example, 1.4 means 1.4:1 SWR. The values are comparative results from the supplied chart and will vary with the actual installation.
What is SWR?
Standing Wave Ratio describes the impedance match seen by the transmission line. A lower ratio is generally easier for the transmitter or ATU to handle.
1.1:1–1.5:1
Good1.6:1–2.0:1
Usually usable2.1:1 and above
Higher mismatch
Good1.6:1–2.0:1
Usually usable2.1:1 and above
Higher mismatch
| Wire length feet | 1.8 MHz | 3.7 MHz | 5.3 MHz | 7.1 MHz | 10.1 MHz | 14.2 MHz | 18.1 MHz | 21.2 MHz | 24.9 MHz | 28.5 MHz | 50.1 MHz |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 175 | 1.2:1 | 1.6:1 | 1.1:1 | 1.1:1 | 1.1:1 | 1.8:1 | 1.3:1 | 1.6:1 | 1.7:1 | 1.2:1 | 1.5:1 |
| 169 | 1.4:1 | 1.2:1 | 1.2:1 | 1.2:1 | 1.2:1 | 2.1:1 | 1.4:1 | 1.4:1 | 1.5:1 | 1.2:1 | 1.1:1 |
| 162 | 1.4:1 | 1.5:1 | 1.7:1 | 1.3:1 | 1.6:1 | 1.8:1 | 1.9:1 | 1.1:1 | 1.5:1 | 1.7:1 | 1.5:1 |
| 146 | 1.7:1 | 1.5:1 | 1.4:1 | 1.4:1 | 2.4:1 | 1.5:1 | 1.3:1 | 1.2:1 | 1.4:1 | 1.5:1 | 1.5:1 |
| 135 | 2.0:1 | 1.4:1 | 1.3:1 | 1.8:1 | 1.6:1 | 2.0:1 | 2.0:1 | 1.7:1 | 1.5:1 | 1.6:1 | 1.3:1 |
| 124 | 1.3:1 | 1.3:1 | 1.2:1 | 1.3:1 | 1.7:1 | 1.6:1 | 1.8:1 | 1.6:1 | 1.4:1 | 1.1:1 | 1.4:1 |
| 96 | 1.8:1 | 1.7:1 | — | 1.7:1 | 2.3:1 | 1.9:1 | 1.4:1 | 1.2:1 | 1.7:1 | 1.2:1 | 1.2:1 |
| 88 | 1.8:1 | 2.2:1 | 1.7:1 | 2.3:1 | 1.9:1 | 1.3:1 | 2.0:1 | 1.8:1 | 1.4:1 | 1.5:1 | 1.5:1 |
| 72 | 2.0:1 | 2.0:1 | 1.4:1 | 1.2:1 | 1.2:1 | 1.9:1 | 1.9:1 | 1.5:1 | 1.1:1 | 1.5:1 | 1.1:1 |
| 59 | 1.6:1 | 1.6:1 | 1.3:1 | 1.5:1 | 2.0:1 | 1.5:1 | 2.0:1 | 1.1:1 | 1.7:1 | 1.2:1 | 1.5:1 |
| 52.5 | 1.6:1 | 1.4:1 | 1.2:1 | 1.1:1 | 1.5:1 | 1.1:1 | 1.9:1 | 1.2:1 | 1.1:1 | 1.7:1 | 1.1:1 |
| 49 | 1.5:1 | 1.3:1 | 1.4:1 | 2.4:1 | 2.4:1 | 1.3:1 | 1.6:1 | 1.6:1 | 1.4:1 | 1.7:1 | 1.5:1 |
| 44 | 3.0:1 | 1.2:1 | 1.5:1 | 2.1:1 | 2.1:1 | 1.7:1 | 1.3:1 | 1.7:1 | 1.6:1 | 1.1:1 | 1.2:1 |
| 35 | 2.2:1 | 1.2:1 | 1.3:1 | 1.3:1 | 1.3:1 | 2.0:1 | 1.6:1 | 1.2:1 | 1.7:1 | 1.6:1 | 1.5:1 |
| 28 | 3.0:1 | 1.3:1 | 1.3:1 | 1.2:1 | 1.2:1 | 2.1:1 | 2.0:1 | 1.3:1 | 1.2:1 | — | 1.3:1 |
| 24 | 3.2:1 | 1.7:1 | 1.6:1 | 1.6:1 | 1.6:1 | 1.4:1 | 2.1:1 | 1.8:1 | 1.3:1 | — | 1.4:1 |
The installed result depends on wire height, orientation, nearby structures, soil, counterpoise arrangement, feeder length and tuner range. Confirm the finished antenna with an analyser.
Interactive tools
Wire and wavelength calculators
Wavelength calculator
Quarter wave5.28 m
Half wave10.56 m
Full wave21.13 m
EFHW starting length
Free-space half wave21.11 m
95% starting length20.06 m
In feet65.81 ft
Feet and metres
Length53.34 m
Approx. quarter-wave frequency1.41 MHz
Quick decision guide
Which transformer should I investigate?
Coax-fed dipole or beamStart with a 1:1 current choke
Selected balanced load near 200 ΩInvestigate a 4:1 current balun
Non-resonant end-fed wire with ATUInvestigate a 9:1 unun and RF return path
Resonant end-fed half-waveInvestigate a 49:1 unun
Installation checks
Common mistakes
No planned RF return path
An end-fed system still needs current to return. Ignoring that path can make the coax and station wiring part of the antenna.
Expecting the transformer to tune the antenna
A transformer changes impedance. It does not make a non-resonant wire resonant and it does not replace an ATU where one is required.
Choosing ferrite by size alone
Core dimensions affect power handling, but material and frequency response determine whether the core is suitable.
Cutting to the exact calculated length
Nearby objects, insulation, height and end effects alter resonance. Start long and trim while measuring.
Testing at full power first
Commission a new antenna at low power. Check SWR, tuner behaviour, common-mode symptoms and transformer temperature.
Using the SWR table as a guarantee
The table is a useful comparison, but the installed geometry and RF return system will change the result.