Testing a Balun

A balun (balanced-to-unbalanced) transforms between unbalanced systems like coax and balanced systems like dipole feedpoints or ladder line. An unun transforms impedance without changing balance. The NanoVNA-H can verify the three specs that determine whether a balun is working correctly: impedance transformation ratio, insertion loss, and frequency response.

Common Balun Types

Type	Ratio	Application
1:1 current balun	50 to 50 balanced	Dipole feed, common mode choke
4:1 voltage balun	50 to 200 balanced	Folded dipole, OCF dipole
9:1 unun	50 to 450 unbalanced	End-fed half-wave antenna
1:1 isolation transformer	50 to 50	Ground loop breaking

Equipment Needed

• NanoVNA-H or NanoVNA-H4, calibrated
• Precision termination resistors (non-inductive carbon film or metal film):
  • 50 ohms for 1:1 balun
  • 200 ohms for 4:1 balun (two 100-ohm in series, or four 50-ohm in series-parallel)
  • 450 ohms for 9:1 unun (use 470-ohm standard value)
• Short leads to connect resistor to balun output
• SMA adapter for balun input

Tip

Keep test leads between the balun output and the termination resistor as short as possible — under 2 cm. Long leads add inductance that distorts the measurement, especially above 10 MHz.

Test Principle

The basic test: connect the NanoVNA-H to the balun input (50-ohm side), terminate the output with the appropriate impedance, and measure S11. A properly working balun with correct termination should show low S11 (good match) across the design frequency range.

NanoVNA CH0 ---> Balun Input (50 ohm side) ---> Balun Output ---> Termination Resistor

If the balun is working correctly, it transforms the termination impedance back to 50 ohms, and the NanoVNA sees a good match. If S11 is poor, either the balun is not transforming correctly or the termination value is wrong.

Calibration

1. Attach the SMA adapter

   Use the same adapter that will connect to the balun input.

2. Calibrate at the adapter

   Perform OPEN, SHORT, LOAD calibration at the SMA connector. The calibration reference plane should be at the point where the balun connects.

   See Full Calibration for the complete procedure.

3. Set the sweep range

   Cover the balun’s rated frequency range with some margin on each side. For an HF balun rated 3.5-30 MHz, sweep from 1 to 60 MHz.

Impedance Transformation Verification (S11)

This is the primary test. It confirms the balun transforms impedance at the correct ratio.

1. Connect the termination resistor to the balun output

   Use the value that corresponds to the transformation ratio:

   • 1:1 balun: 50-ohm resistor
   • 4:1 balun: 200-ohm resistor
   • 9:1 unun: 450-ohm resistor (470-ohm standard value is close enough)

2. Connect the NanoVNA CH0 to the balun input

3. Configure traces

   Enable two traces:

   • Trace 1: S11 LogMag — shows return loss in dB
   • Trace 2: S11 Smith — shows impedance behavior

4. Read the results

   A good result is S11 below -15 dB across the operating band. This means the balun is presenting close to 50 ohms to the NanoVNA after transforming the termination impedance.

5. Place markers at band edges

   Use markers at the lower and upper limits of the balun’s rated range. Record the S11 value at each marker.

Frequency Response

Sweep S11 across a wide range to find the usable bandwidth. The usable bandwidth is where S11 stays below -10 dB (SWR below 2:1) or preferably below -15 dB (SWR below 1.5:1).

For an HF balun, sweep from 1 MHz to 60 MHz. You will typically see:

• A region in the middle with low S11 — this is the operating bandwidth
• Rising S11 at the low end — insufficient inductance
• Rising S11 at the high end — parasitic capacitance and core loss

The ratio of upper to lower -10 dB frequencies gives you the usable bandwidth ratio. A 3:1 ratio or better is typical for a well-designed broadband balun.

Open Circuit Test (No Termination)

Remove the termination resistor and sweep S11 again with the balun output open.

1. Disconnect the termination resistor from the balun output

2. Observe the Smith chart trace

   With no load, you are looking at the balun’s magnetizing impedance — the impedance of the windings themselves.

3. Interpret the result

   For a good broadband balun:

   • The trace should show high impedance (right side of Smith chart)
   • Higher magnetizing impedance means better low-frequency performance
   • If impedance is low, the balun will not work well at low frequencies

Note

The magnetizing impedance sets the low-frequency limit of the balun. As a rule of thumb, the magnetizing reactance should be at least four times the operating impedance at the lowest frequency of operation. For a 50-ohm balun, that means at least 200 ohms of reactance at the low end.

Insertion Loss (S21)

For S21 measurement, the signal must pass through the balun. This works cleanly only for 1:1 baluns.

1. Connect CH0 to the balun input

2. Connect the balun output to CH1

   For a 1:1 balun, connect directly — both sides are 50 ohms. Set the trace to S21 LogMag.

3. Read the insertion loss

   A good 1:1 balun shows less than 0.5 dB insertion loss across its operating range.

Caution

For ratio baluns (4:1, 9:1), direct S21 measurement is misleading because the NanoVNA expects 50 ohms on both ports. The impedance mismatch at the output port creates apparent loss that is not real insertion loss. Use the S11 termination method instead — it is more practical and more commonly used for ratio baluns.

Common Mode Rejection Test (Advanced)

Common mode rejection is the most important parameter for a current balun. It measures how well the balun suppresses current that flows equally on both conductors (common mode) versus the desired differential signal. This requires a slightly different setup.

1. Connect a 50-ohm termination to the balanced output normally

2. Short the two balanced terminals together

   This forces common mode only — any signal that passes through is common mode leakage.

3. Measure S11

   The NanoVNA should see high impedance (the choke effect of the balun).

4. Evaluate the result

   Good common mode impedance is above 500 ohms across the operating band. Higher is better. Below 500 ohms, the balun provides limited common mode suppression and feedline radiation may be a problem.

Interpreting Smith Chart Results

• 1:1 balun with 50-ohm load: The trace should cluster near the center of the Smith chart across the operating range.
• 4:1 balun with 200-ohm load: Same — the balun transforms 200 ohms down to 50 ohms, so the NanoVNA should see center.
• Trace spiraling outward: Frequency-dependent behavior, usually indicating the core material is not well suited to that frequency range.
• Trace jumping to edge of chart: The balun has lost its transformation properties at that frequency.

Pass/Fail Criteria

Parameter	Good	Acceptable	Poor
S11 (with correct load)	below -20 dB	-10 to -20 dB	above -10 dB
Usable bandwidth	above 3:1 freq ratio	2:1 to 3:1	below 2:1
Insertion loss (1:1 balun)	below 0.5 dB	0.5 to 1.0 dB	above 1.0 dB
Common mode impedance	above 1000 ohms	500 to 1000 ohms	below 500 ohms

Troubleshooting Common Issues

S11 Good at Low Frequencies, Bad at High Frequencies

The core material is running out of permeability. The ferrite mix is not suitable for the higher frequencies. This is common with Type 43 material above 30 MHz. Consider Type 61 material for VHF work, or accept the reduced high-frequency range.

S11 Bad at Low Frequencies, Good at High Frequencies

Insufficient turns or too-small core — not enough inductance for low-frequency operation. Wind more turns or use a larger core. Doubling the turns quadruples the inductance, which extends the low-frequency range by a factor of two.

S11 Bad Everywhere

Several possibilities, check in order:

• Wrong termination value — verify the resistor with an ohmmeter
• Wrong winding ratio — count the turns on each winding
• Damaged balun — shorted turn or cracked core
• Test lead inductance — leads too long, distorting the measurement

Resonance Dips in S11

Sharp dips at specific frequencies indicate parasitic resonances — inter-winding capacitance resonating with winding inductance. This can cause high loss at the resonant frequency. Spreading the windings apart or changing the winding style can shift or reduce these resonances.

Inconsistent Results When Touching the Balun

Common mode current is flowing on the outside of the coax. Your hand changes the common mode impedance. This indicates the balun’s common mode rejection is poor, or the coax shield is not properly grounded at the NanoVNA connection.

Tip

When testing homebrew baluns, measure before and after each modification. The NanoVNA-H shows results in real time, making it straightforward to iterate: wind a turn, sweep, check the result, decide whether to add another.

Next Steps

• Reading the Smith Chart — Deeper understanding of impedance plots
• Tuning an HF Antenna — Use the balun with an actual antenna
• L/C Matching Networks — Design matching networks for stubborn impedances