UnUn Loss Calculator

S21 measurement with L-pad divider on NanoVNA

Connection diagram

L-pad divider parameters

R₁ (series)

R₂ (shunt)

External R₂ shunt resistor
Disable if using only VNA Port 2 built-in 50 Ω termination

UnUn parameters

Impedance ratio (k)

: 1

Z₀ (VNA impedance)

Multi-frequency measurements & S2P import

📂

Drop .s2p file here or click to browse
Exported from NanoVNA Saver → File → Export as S2P

.s2p Touchstone (IEEE standard) — contains S11 + S21 at all frequencies

S11 return loss (mismatch analysis)
Enable to separate mismatch from dissipative loss

Freq	S21 dB	S11 dB	Total	Mismatch	Heat	SWR

📡 Does frequency matter?

Yes, significantly. UnUn losses change with frequency — they typically increase toward higher bands. Measure at multiple frequencies and compare.

Typical: 0.2 dB at 3.5 MHz, 0.5 dB at 14 MHz, 1+ dB at 28 MHz. Depends on core material (mix 43, 31, 61), turns, construction.

The resistive divider is broadband — provided you use non-inductive resistors.

⚠ Do not use wire-wound resistors! Their parasitic inductance corrupts measurements above ~10 MHz.

📊 S11 and mismatch analysis explained

S21 = total loss. But why are you losing power? Two mechanisms:

Mismatch (reflected) — power bouncing back. Not heat. On real TX + coax, most reflected power ends up radiated anyway.

Dissipative (heat) — core hysteresis, eddy currents, winding resistance. This is the real enemy — it degrades signal and overheats the transformer.

|Γ| = 10^(S11/20), Mismatch = −10·log₁₀(1−|Γ|²), Dissipative = Total − Mismatch

⚠ This separation is approximate but very useful for practical evaluation.

🔧 How to calibrate the NanoVNA?

Full 2-port calibration (SOLT + THRU):

1. STIMULUS → START 1 MHz, STOP 30 MHz

2. Connect the same cables you'll use

3. CH0: OPEN → SHORT → LOAD (50 Ω). CH1: OPEN → SHORT → LOAD. Both: THRU

4. CAL → SAVE → SAVE 0

5. Verify: THRU should show S21 ≈ 0 dB

⚠ Calibrate without UnUn and divider.

📋 Step-by-step procedure

1. Calibrate NanoVNA (2-port, 1–30 MHz)

2. DISPLAY → TRACE → CH1 → FORMAT → LOGMAG

3. Connect: P1 → UnUn → R₁ → junction → P2, R₂ to GND

4. Markers at 3.5, 7, 14, 21, 28 MHz

5. Read S21 (CH1) and optionally S11 (CH0) from each marker

6. Enter all readings in the table above

💡 Verify divider: connect without UnUn. S21 should match theoretical attenuation ±0.2 dB.

📂 Importing data from NanoVNA Saver

Instead of typing marker values manually, you can import a Touchstone .s2p file directly. This file contains S11 and S21 measurements at every sweep point — much faster and error-free.

How to export from NanoVNA Saver:

1. Connect NanoVNA to computer, open NanoVNA Saver

2. Set sweep: 1–30 MHz (or your range of interest)

3. Connect your DUT (UnUn + divider), run sweep

4. File → Export S2P → save as .s2p

5. Drag the file onto the import zone above, or click to browse

What happens: the calculator automatically reads S11 + S21 at all frequencies, filters to HF amateur bands (1.8–50 MHz), enables S11 mismatch analysis if available, and populates the table. You get instant results for all bands.

💡 Touchstone format (.s2p) is an IEEE standard supported by virtually all VNA software. DB, MA, and RI formats are all supported. S1P files (1-port, S11 only) can also be imported but will not contain S21 data.