Resonance Finder

The Resonance measurement mode automatically searches for and displays multiple resonant frequencies from S11 measurements. This is useful for characterizing antennas, tuned circuits, and transmission line stubs.

Enable Resonance Mode

1. Connect the device under test to CH0
2. Set sweep range to cover expected resonances
3. Go to DISPLAY > MEASURE
4. Tap RESONANCE
5. Found resonances appear on screen

Display Output

S11 RESONANCE
10.125 MHz, 48.5 + j2.3 ohm
20.250 MHz, 52.1 - j1.8 ohm
30.375 MHz, 49.2 + j5.6 ohm
40.500 MHz, 55.3 - j12.4 ohm
50.625 MHz, 45.8 + j28.2 ohm
60.750 MHz, 62.4 - j45.1 ohm

Each line shows:

• Resonant frequency
• Impedance at resonance (R + jX)

Note

The analyzer finds up to 6 resonant frequencies within the sweep range.

What is Resonance?

A resonance occurs when the reactive part (X) of the impedance equals zero:

• Series resonance: Minimum impedance magnitude
• Parallel resonance: Maximum impedance magnitude
• Zero crossing: Reactance changes from capacitive to inductive (or vice versa)

The resonance finder detects zero-crossings of the imaginary part of S11.

Shell Commands

# Enable resonance analysis mode
measure resonance

# Disable measurement mode
measure none

Practical Applications

Multi-band Antenna Analysis

1. Connect antenna to CH0
2. Set sweep to cover all bands (e.g., 1-500 MHz)
3. Enable RESONANCE mode
4. See all resonant frequencies
5. Check impedance at each band

Transmission Line Stub Characterization

1. Connect stub to CH0
2. Set sweep to cover multiple harmonics
3. Enable RESONANCE mode
4. Resonances appear at 1/4, 3/4, 5/4 wavelength, etc.

Trap Antenna Tuning

1. Connect trap antenna to CH0
2. Set wide sweep
3. Enable RESONANCE mode
4. Verify resonance at each trap frequency

Understanding the Results

Good Resonance (well-matched)

145.000 MHz, 50.2 + j1.5 ohm

Near 50 ohms with low reactance - well-matched at this frequency.

Poor Resonance (needs matching)

145.000 MHz, 25.3 + j0.5 ohm

Resonance exists but impedance far from 50 ohms - needs matching network.

No Pure Resonance Found

If the search doesn’t find zero-crossings, it falls back to finding the minimum reactance point:

145.000 MHz, 35.2 + j8.5 ohm

Tip

When no perfect resonance exists, the display shows the point of minimum reactance. Consider adjusting the antenna or circuit to achieve true resonance.

Harmonic Analysis

Antennas often show resonances at odd multiples of the fundamental:

Harmonic	Example for 7 MHz antenna
1st (fundamental)	7 MHz
3rd	21 MHz
5th	35 MHz
7th	49 MHz

Use resonance mode to quickly verify harmonic behavior.

Series vs Parallel Resonance

At a series resonance:

• Impedance is minimum
• Current is maximum
• Good for series-fed antennas

At a parallel resonance:

• Impedance is maximum
• Voltage is maximum
• Good for parallel-fed circuits

The zero-crossing search finds both types.

Tips for Accurate Results

Tip

• Wide sweep first: Find approximate resonances
• Narrow span: Zoom in for accurate reading
• More points: Increase sweep points for sharp resonances
• Calibration: Ensure good calibration at test frequency

Marker Placement

When resonances are found, markers are automatically placed at each resonant frequency. This allows you to:

• Read precise frequency from marker display
• See impedance in Smith chart format
• Use marker search to fine-tune position

Limitations

Caution

• Maximum 6 resonances displayed
• Very close resonances may not be resolved
• Weak resonances (small zero-crossing slope) may be missed
• Noisy measurements can cause false detections

Combining with Other Tools

After finding resonances, use other tools for detailed analysis:

1. Smith Chart: View impedance locus around resonance
2. L/C Match: Calculate matching network at resonance
3. Markers: Fine-tune position and read exact values
4. Time Domain: See physical location of resonant elements