I've done my best here to make sense of a very complicated subject. I've worn out my antenna books, argued with AI, googled until my eyes hurt and read 30 or more ferrite core datasheets, I feel like I know less now than I did five hours ago! go figure.. I dont care if I never see another balun again!! :-)

Antenna design, along with RF transformer and choke design, looks simple at first glance. The reality is very different. Part of the problem is that a lot of what you can buy has no markings at all, or is sealed in black resin, so you're left guessing what's inside. That's what this guide is for: working out what you've got using a NanoVNA and a few bits from the bench.

I'm sure I've got some things wrong or over-simplified them. If you have deep and meaningful knowledge of antennas or RF transformers and chokes, I'd love your input. What I've taken away from researching this is that "every day is a school day!"

Gaz

What you need

This guide identifies an unknown balun or unun using basic bench gear and a NanoVNA: the core, the turns, the ratio, whether it is a voltage or current type, and the wire.

• NanoVNA (or any VNA) with SMA cables and a 50 Ω calibration kit
• Digital calipers for the core size and wire diameter
• Oscilloscope and a signal source at about 1 MHz
• Non-inductive resistors: 50 Ω, 100 Ω, plus the load values in the ratio table further down
• A dummy load rated for the power you want to test at
• Multimeter with a continuity buzzer

Safety first

RF at power can burn skin and damage a NanoVNA, so test at low level first and only step up into a dummy load.

• Never touch the unit or the load while transmitting.
• Disconnect the NanoVNA before any transmitter is connected. RF power can destroy its input.
• A dummy load runs hot. Let it cool and keep it on a heatproof surface.
• The high side of a 49:1 or 64:1 carries high voltage: 100 W into 2450 Ω is about 500 V RMS, so keep fingers and loose leads away.
• Use a transmitter only for the power test (check 9), and only into the dummy load.

Test jig and NanoVNA calibration

Short leads and a proper calibration matter most at high impedance, so build a jig and calibrate before you test any unit.

1. Build a jig: an SMA or BNC connector with binding posts for the unit's terminals, with the leads as short as possible.
2. Calibrate at the end of the cable, using open, short and load (plus through for loss) over 1-30 MHz.
3. Use the same cable and jig for every unit, and recalibrate if you change the sweep range.
4. Check the calibration on a known 50 Ω resistor. It should read very close to 50 Ω.
5. Treat NanoVNA readings above a few hundred ohms as approximate and confirm them by counting turns.
6. Make load resistors from several low-inductance resistors in series or parallel, for example 4 × 51 Ω for about 200 Ω.

Plain-English glossary

A balun joins a balanced circuit (like a dipole) to an unbalanced one (like coax). An unun joins two unbalanced circuits. Either can also change impedance.

Term	Meaning
Balun	Balanced to unbalanced, e.g. dipole to coax
Unun	Unbalanced to unbalanced, e.g. end-fed wire to coax
Ratio	How far it steps impedance, e.g. 4:1 turns 200 Ω into 50 Ω
Voltage type (Ruthroff)	Forces equal voltage on both output terminals
Current type (Guanella)	Forces equal current in both output terminals; transmission-line windings
Common-mode choke	Blocks stray current flowing on the outside of the coax

In this guide the input is the coax side, with one signal wire and a shield, and the output is the side with two terminals that go to the antenna wires. I label the two output terminals A and B.

Ratio and turns table

Impedance ratio is the turns ratio squared, so a 2:1 turns ratio gives a 4:1 impedance ratio.

Impedance ratio	Turns ratio	High-side load for a 50 Ω input	Low-side reading with 50 Ω on the high side
1:1	1:1	50 Ω	50 Ω
4:1	2:1	200 Ω	12.5 Ω
9:1	3:1	450 Ω	5.6 Ω
16:1	4:1	800 Ω	3.1 Ω
49:1	7:1	2450 Ω	1 Ω
64:1	8:1	3200 Ω	0.8 Ω

The NanoVNA is least accurate at the extremes, so confirm 49:1 and 64:1 by counting turns. Some transmission-line 4:1 and 9:1 types get their ratio from how the wires are connected, not from the turns count.

What each ratio is used for

Ratio	Typical use
1:1	Feedline choke, or dipole to coax
4:1	Folded dipoles and loops (about 200 Ω)
9:1	Random-wire antennas (about 450 Ω)
49:1 and 64:1	End-fed half-wave wires (high-impedance feedpoint)

Winding types

Winding style is a clue, not proof. The same twisted pair can be wired as a voltage or a current type, so confirm it with the proof test further down.

What you see	Likely type	Notes
One coil with an extra tap wire	Voltage, autotransformer	Beep test shows a DC path
Two separate windings	Voltage, two-winding transformer	Beep test shows no DC path
Two wires wound side by side or twisted	Voltage or current	Depends on how the ends are connected
Thick coax cable through the core	Current, coax choke	Check the cable print or measure its diameter

The 10 lab checks

Run these in order and write each reading down in the results sheet at the end.

Check 1: Look at it

Note the core size and colour, the wire layout (twisted, side by side, coax) and the connectors.

Check 2: Count the turns

Count how many times each wire passes through the core hole. Each pass through the hole is one turn.

Check 3: Beep test

A DC path between input and output means an unun or autotransformer. No path means the sides are isolated.

Check 4: Find the ratio

Put a 50 Ω resistor on one side and read the impedance of the other side with the NanoVNA at 7 MHz, then swap sides and repeat. Compare the readings with this table.

You read	Ratio
50 Ω both ways	1:1
200 Ω or 12.5 Ω	4:1
450 Ω or 5.6 Ω	9:1
800 Ω or 3.1 Ω	16:1
2450 Ω or 1 Ω	49:1
3200 Ω or 0.8 Ω	64:1

Check 5: Check the ratio by voltage

Feed in about 1 MHz and compare the input and output voltages on the scope. Square the voltage ratio to get the impedance ratio.

Check 6: Measure insertion loss

Sweep 1.8-30 MHz through the unit into the matched load. Over 0.5 dB of loss is poor. One caveat: a 200 Ω load going into a 50 Ω NanoVNA port is not a perfect loss measurement, so treat the number as a good rough check rather than a lab figure.

Check 7: Measure SWR

Use the same sweep. Under 1.5:1 is good, and a sharp rise shows where the bandwidth runs out.

Check 8: Find the low-frequency limit

Leave the output open and read the input impedance. The higher it stays at low frequencies, the better the unit works down there. If you want the leakage inductance too, short the output and measure again.

Check 9: Test the heat

Run rated power into the dummy load for 5 minutes. The core should stay barely warm.

Check 10: Test the balance (baluns only)

Measure each output terminal to ground. The amplitudes should match and be opposite in phase.

Voltage or current: the proof test

Putting different resistors on the two output terminals shows whether the balun keeps the current equal or the voltage equal.

• Input side: where the coax connects. It has one signal wire plus the shield, so it is called unbalanced.
• Output side: the two terminals that go to the antenna wires. This side is called balanced. Call the two terminals A and B.
• Ground: the coax shield on the input side. Connect both resistors to it.
• Signal source: a signal generator, or the NanoVNA output, at about 1 MHz.

1. Connect a 50 Ω resistor from output terminal A to ground and a 100 Ω resistor from output terminal B to ground.
2. Connect the signal source to the input (the coax side) at about 1 MHz and a low level.
3. Measure the voltage from terminal A to ground, then from terminal B to ground, with the scope.
4. Work out each terminal's current as its voltage divided by its resistor value.

Type	Voltages at A and B	Currents at A and B
Current (Guanella)	Different; higher at B (100 Ω)	Equal
Voltage (Ruthroff)	Equal	Different; higher at A (50 Ω)

A common-mode impedance above 1 kΩ also points to a current type: tie both input terminals together, tie both output terminals together, and measure between the two pairs.

A low reading means the unit does little to block stray current on the coax.

Potted or unmarked units

A potted unit with no markings can still be identified from its terminals alone. Map them, then use measured inductance and the same electrical tests to find the turns, ratio and type.

1. Record its size and weight. The core is smaller than the resin block, so the block gives an upper limit.
2. Map the terminals. Beep every pair with the multimeter, mark the pairs that connect, and note each pair's resistance. A shared terminal means an autotransformer; separate pairs mean separate windings.
3. Measure the inductance of each winding at 1 MHz with the other windings open. Inductance rises with turns squared, so the turns ratio is the square root of the inductance ratio.
4. Short one winding and measure another. A large drop means the windings are tightly coupled; little change means they are not.
5. Confirm the ratio with resistors, as in check 4.
6. Sweep insertion loss and SWR from 1.8-30 MHz. A self-resonance dip marks the top of the usable range.
7. Run the proof test and the common-mode test to decide between a voltage and a current type.
8. Prove the power rating by test only: step up the power into a dummy load and check the heat at each step.

Cutting or stripping the resin destroys the unit, so treat it as a last resort.

Reading	What it tells you
DC path between terminals	Those terminals share a winding
Inductance ratio between two windings	Turns ratio is its square root
High open-circuit inductance at 1.8 MHz	Enough core and turns for the low bands
Loss rising steadily with frequency	A ferrite core is likely
Self-resonance dip	Upper limit of the usable band

Which type is it?

Three tests sort any unit: the beep test splits autotransformers from isolated windings, the proof test gives voltage or current type, and the terminated reading gives the ratio.

Wire gauges

Wire gauge follows power level and insulation needs. The figures below are rough guides, and the core size matters as much as the wire.

Use	Typical wire	Notes
QRP, up to 10 W	22-26 AWG (0.4-0.65 mm)	Enamelled copper is fine
Up to about 100 W	18-20 AWG (0.8-1.0 mm)	Most common for 1:1, 4:1, 9:1
200 W to 1 kW	14-16 AWG (1.3-1.6 mm)	PTFE insulated, large core such as FT-240
49:1 and 64:1 end-fed	18-22 AWG	Often on an FT-140 or FT-240 core for 100-200 W
Coax-wound chokes	RG-316 up to 100 W; RG-142 or RG-400 up to 1 kW	Use PTFE coax; RG-58 only for low power

On one unit, the low-impedance winding carries the most current and takes the thicker wire. The high-impedance winding carries little current but high voltage: 100 W into 2450 Ω is about 500 V RMS, so use PTFE or thick enamel insulation.

Identifying unknown wire

1. Scrape off a short section of enamel or strip the insulation.
2. Measure the bare copper with calipers or a micrometer.
3. Match it to the table below.

AWG	Diameter (mm)
14	1.63
16	1.29
18	1.02
20	0.81
22	0.64
24	0.51
26	0.40

For coax, read the print on the jacket. If it is blank, measure the outer diameter: RG-316 is about 2.5 mm, while RG-142 and RG-58 are both about 5 mm. Those two look alike, so the white PTFE insulation under the jacket is the giveaway for RG-142.

Core identification

Measure the core to get its size, then measure inductance to work out its mix. Take AL values from the manufacturer's datasheet for that size and mix.

Designation	Outer diameter
FT-50	12.7 mm (0.5 in)
FT-82	21 mm (0.82 in)
FT-114	29 mm (1.14 in)
FT-140	35.5 mm (1.4 in)
FT-240	61 mm (2.4 in)

The T-xx powdered iron cores follow the same inch-based sizing (T-50 is 0.5 in).

1. Colour: powdered iron cores are painted (red is mix 2, yellow is mix 6). Ferrite cores are usually dull grey or black with no code, so identify their mix by measurement.
2. Ten-turn inductance test: wind exactly 10 turns of insulated wire (about 0.8 mm, 20 AWG) on the bare core, spaced evenly round it. Measure the inductance with the NanoVNA (open the other end, use a short lead). AL = L in nH divided by 100, because 10 turns squared is 100. Compare AL with the manufacturer's datasheet (Fair-Rite for ferrite, Micrometals or Amidon for powdered iron). For example an FT-140-43 is roughly 885 nH per turn squared and an FT-140-61 roughly 140.
3. Frequency caveat: datasheet AL is measured at low frequency (about 10 kHz) and ferrite loses permeability as frequency rises, so at 1 MHz the NanoVNA will often read lower than the datasheet. Use the result to tell the mixes apart (43 reads far higher than 61), not to match the exact number.
4. Frequency sweep: measure Ls and Rs at 1, 3.5, 10 and 30 MHz. Powdered iron shows flat Ls and low Rs. Ferrite shows Ls falling and Rs rising, and the frequency where Rs climbs separates mix 43, 61 and 31.

Results record sheet

Fill in one row per check for each unit you test.

Check	What to record	Reading	Pass or fail
1. Visual	Core size, colour, winding layout
2. Turns	Turns on each winding
3. Beep test	DC path yes or no
4. Ratio (impedance)	Reading both ways at 7 MHz, ratio found
5. Ratio (voltage)	Vin, Vout, squared ratio
6. Insertion loss	dB at 1.8, 7, 14, 30 MHz
7. SWR	Worst SWR and where
8. Low-frequency limit	Open-circuit input impedance at 1.8 MHz
9. Heat	Core temperature after 5 min at rated power
10. Balance	Terminal amplitudes and phase difference
Proof test	Equal voltages or equal currents
Wire	Gauge and insulation
Core mix	AL and best-match mix

Pass or fail and troubleshooting

A good unit shows insertion loss under 0.5 dB, SWR under 1.5:1 across its bands, common-mode impedance above 1 kΩ (current types), and a core that stays barely warm.

Symptom	Likely cause	Fix
Hot core or high loss	Core saturating, wrong mix, too few turns	Larger core, more turns, lower-loss mix
SWR rises at high frequencies	Leakage inductance, long leads	Shorter leads, tighter winding, small compensating capacitor
Poor match at low frequencies	Too few turns or low AL	More turns or a higher-permeability core
Ratio reads wrong	Miscounted turns or wrong connections	Recount, check wiring against the ratio table
Unexpected DC path	Shorted turns or shared connection	Beep-test each winding separately
Poor balance	Low common-mode impedance	Measure common-mode impedance; add a choke