LC Matching Network

This tutorial explains how to use the NanoVNA-H to design and verify LC matching networks for antennas and other RF loads.

What You Will Learn

• Understanding impedance matching principles
• Using the NanoVNA’s L/C matching calculator
• Building and testing matching networks
• Choosing between L-network topologies

Why Match Impedance?

When source impedance does not equal load impedance:

• Power is reflected back to the source
• Efficiency decreases
• Transmitter may be damaged (in radio applications)
• Signal quality degrades

A matching network transforms the load impedance to match the source (usually 50 ohms).

Understanding the Problem

Before designing a match, you need to know:

Parameter	How to Measure
Load impedance (Z)	S11 measurement with NanoVNA
Operating frequency	Your desired operating frequency
Source impedance	Usually 50 ohms (coax)

Measuring the Load

1. Connect the load to Port 1

   For an antenna, connect it to the NanoVNA’s Port 1 (CH0).

2. Set the frequency

   Either:

   • Single frequency: STIMULUS > CW FREQ and enter your frequency
   • Range: Set START and STOP around your operating frequency

3. Calibrate

   Perform at least OPEN, SHORT, LOAD calibration.

4. Enable Smith chart display

   Go to DISPLAY > FORMAT S11 (REFL) > SMITH

5. Place marker at operating frequency

6. Read the impedance

   The marker shows R + jX (resistance + reactance).

   Example: 35 + j25 ohms

Using the L/C Matching Calculator

The NanoVNA includes a built-in matching network calculator.

1. Enable L/C Match display

   Go to MARKER > MEASURE > L/C MATCH

2. Position the marker

   Move the marker to your operating frequency.

3. Read the calculated values

   The display shows component values for matching networks.

   The calculator suggests two solutions:

   • Network topology 1 (often series-then-shunt)
   • Network topology 2 (often shunt-then-series)

4. Record the values

   Note both the topology and the component values.

Note

The L/C matching calculator assumes you want to match to 50 ohms. If your source impedance is different, you will need to calculate manually or use external software.

L-Network Topologies

An L-network uses two reactive components to transform impedance.

Series L, Shunt C

Configuration:

Source ---[L]--- Load
            |
           [C]
            |
           GND

Use when:

• Load resistance is LOWER than source (R < 50 ohms)
• Load is capacitive (negative reactance)

This topology moves impedance clockwise then down on Smith chart.

Shunt L, Series C

Configuration:

Source ---[C]--- Load
      |
     [L]
      |
     GND

Use when:

• Load resistance is LOWER than source (R < 50 ohms)
• Load is inductive (positive reactance)

This topology moves impedance up then clockwise on Smith chart.

Series C, Shunt L

Configuration:

Source ---[C]--- Load
            |
           [L]
            |
           GND

Use when:

• Load resistance is HIGHER than source (R > 50 ohms)
• Load is inductive (positive reactance)

This topology moves impedance counter-clockwise then down.

Shunt C, Series L

Configuration:

Source ---[L]--- Load
      |
     [C]
      |
     GND

Use when:

• Load resistance is HIGHER than source (R > 50 ohms)
• Load is capacitive (negative reactance)

This topology moves impedance down then counter-clockwise.

Example: Matching a 35 + j25 Ohm Antenna

1. Analyze the load

   • R = 35 ohms (less than 50)
   • X = +25 ohms (inductive)
   • Need to increase R and cancel the inductance

2. Check the L/C Match display

   The NanoVNA suggests component values.

   Example output:

   • Solution 1: Series C = 180 pF, Shunt L = 82 nH
   • Solution 2: Shunt C = 56 pF, Series L = 120 nH

3. Choose practical components

   Consider:

   • Available component values
   • Component Q (quality factor)
   • Power handling
   • Frequency stability

4. Build the network

   Assemble on a small PCB or “ugly construction” style.

   Tip

   For HF frequencies, use air-wound inductors for best Q. For VHF/UHF, consider microstrip or transmission line elements.

5. Test the network

   Connect the network between the NanoVNA and the load.

   Measure S11 - it should show:

   • Return loss better than -10 dB (preferably -15 dB or better)
   • Smith chart should be close to center (50 ohms)

Building Practical Components

Inductors

HF (1-30 MHz)

Air-wound coil:

L (uH) = (d^2 * n^2) / (18d + 40l)

Where:

• d = diameter in inches
• n = number of turns
• l = length in inches

Toroid:

• Use iron powder cores (T-50-2 red, T-50-6 yellow)
• L (uH) = AL * n^2 / 1000000
• AL in nH/turn^2 (from core datasheet)

VHF (30-300 MHz)

Air-wound coil:

• Larger diameter, fewer turns
• Use heavy gauge wire for low loss

Chip inductors:

• SMD inductors available with good Q
• Check self-resonant frequency (SRF)

UHF (300+ MHz)

Transmission line:

• Short sections of coax as inductors
• Microstrip lines on PCB

Chip inductors:

• Must have SRF well above operating frequency
• Q decreases at high frequency

Capacitors

Frequency	Capacitor Type
HF	Silver mica, NP0/C0G ceramic
VHF	NP0/C0G ceramic, porcelain
UHF	NP0/C0G chip capacitors

Caution

Avoid X7R, Y5V, and other temperature-compensated ceramics for matching networks. Their capacitance varies with temperature, voltage, and frequency.

Verifying the Match

1. Connect the matching network

   Source (NanoVNA Port 1) -> Matching Network -> Load (antenna)

2. Measure S11

   Display LOGMAG and SMITH chart traces.

3. Check return loss

   • Better than -10 dB: Acceptable (SWR < 2:1)
   • Better than -15 dB: Good (SWR < 1.4:1)
   • Better than -20 dB: Excellent (SWR < 1.2:1)

4. Check bandwidth

   Move the marker across your desired frequency range. Ensure return loss stays acceptable across the full range.

5. Adjust if needed

   If the match is not centered on the correct frequency:

   • Increase inductance: Match moves to lower frequency
   • Decrease inductance: Match moves to higher frequency
   • Similar for capacitance

Bandwidth Considerations

L-networks have limited bandwidth. For wider bandwidth:

Wideband Matching Options

Approach	Bandwidth	Complexity
Single L-network	Narrow	Simple
Pi or T network	Moderate	Medium
Multi-section match	Wide	Complex
Transformer	Wide	Simple (limited ratios)

Estimating Bandwidth

For an L-network, the bandwidth depends on the Q of the match:

Q = sqrt(Rhigh/Rlow - 1)

Bandwidth approximately = Fc / Q

Example:

• Matching 25 ohms to 50 ohms
• Q = sqrt(50/25 - 1) = 1
• At 7 MHz: Bandwidth approximately = 7 MHz / 1 = 7 MHz (very wide)

Matching 10 ohms to 50 ohms:

• Q = sqrt(50/10 - 1) = 2
• At 7 MHz: Bandwidth approximately = 3.5 MHz

Common Matching Scenarios

End-Fed Half-Wave

Load: 2000-5000 ohms

Solution: 49:1 or 64:1 transformer, not L-network

L-networks for such high ratios have extremely narrow bandwidth.

Low Dipole

Load: 20-40 ohms, slightly reactive

Solution: Simple L-network works well

Calculate for the measured impedance at your operating frequency.

Vertical with Few Radials

Load: 30-40 ohms

Solution: L-network or 4:1 transformer

Adding more radials often improves match more than matching network.

Mismatched Feedline

Load: Could be anything

Solution: Measure at the point where you will install the match

If matching at the shack end, measure there (not at the antenna).

Troubleshooting

Match works but on wrong frequency

• Component values are off
• Stray inductance or capacitance in layout
• Measure actual component values and adjust

Match is not deep enough

• Component Q may be too low
• Layout issues causing loss
• Try higher-Q components

Match is very narrow

• High impedance transformation ratio
• Consider different network topology
• May need multi-section matching

Next Steps

• Reading the Smith Chart - Understand impedance visualization
• Tuning HF Antennas - Practical antenna work
• Your First S11 Measurement - Basic reflection measurements