Vector Network Analyzers: Introduction
Of all the instruments in the advanced test bench, the vector network analyzer (VNA) is the most powerful and the most versatile. It answers a question that no other instrument can answer as completely: how does RF energy interact with this device? When you connect a VNA to an antenna, it tells you not just whether the antenna is resonant, but the exact complex impedance (resistance and reactance) across the entire frequency range — simultaneously. When you connect it to a filter, it measures insertion loss, return loss, phase response, and group delay in a single sweep. When you connect it to a coaxial cable, it tells you the cable's characteristic impedance and, combined with time-domain transformation, the location of any fault.
Until 2018, VNAs cost thousands of dollars and were found only in professional laboratories and well-equipped amateur clubs. The emergence of the NanoVNA changed this entirely. A device smaller than a deck of cards, costing under $80, the NanoVNA measures S11 and S21 from 50 kHz to 900 MHz (or 50 kHz to 3 GHz for the V2 version) with accuracy sufficient for all but the most demanding professional applications. Understanding how to use this instrument — what it is actually measuring, why calibration matters, how to interpret the displays — has become an essential skill for every serious amateur.
What a VNA Measures
A network analyzer measures the response of a circuit to a known stimulus. The VNA generates a test signal at a specific frequency, applies it to the device under test (DUT), and measures what comes back (reflected) and what comes out the other side (transmitted). It does this at each frequency in a sweep, building up a complete picture of the device's behavior across the frequency range.
The key word "vector" means the VNA measures both the magnitude (amplitude) and the phase of the signals. This is the fundamental difference from a scalar network analyzer, which measures only amplitude. Phase information is essential for characterizing reactive components (capacitors, inductors), for understanding filter phase response, for measuring group delay, and for navigating the Smith chart to determine complex impedance.
Think of it this way. You shine a torch at a window. Some light reflects back (reflection), and some light passes through (transmission). If you measured only the brightness of each — that is scalar analysis. If you also measured the angle and color (phase and frequency) of the reflected and transmitted light — that is vector analysis. The vector measurement tells you far more about the glass's properties.
The Concept of Port
A VNA has one or more ports — physical connectors where signals enter and leave. A one-port VNA (like the NanoVNA in its simplest use case) measures reflection only: it applies a signal at Port 1 and measures what reflects back into Port 1. A two-port VNA measures both reflection and transmission: Port 1 applies the signal, Port 2 measures what comes out the other side, and both ports also measure their own reflection. The NanoVNA has two ports and measures both S11 and S21.
VNA Block Diagram
Understanding the block diagram makes calibration and measurement much clearer. A basic two-port VNA contains these key functional blocks:
RF source (synthesizer): A voltage-controlled oscillator or direct digital synthesizer generates the test signal. It sweeps through the frequency range under software control. Frequency accuracy and stability determine the frequency accuracy of all measurements.
Power splitter: Divides the source signal — one path goes to the DUT, another to a reference receiver. The reference receiver measures what the source is actually generating, so that any source-level variations can be compensated in the measurement.
Directional coupler: A two-port device that samples the forward signal going toward the DUT and the reflected signal coming back from the DUT separately. This separation is imperfect (finite directivity), which is one of the sources of measurement error corrected by calibration.
Measurement receivers (A and B): Downconvert the RF signals to a lower IF frequency for accurate amplitude and phase measurement. The ratio of receiver B (reflected or transmitted) to receiver A (reference) gives the S-parameter.
Digital signal processor: Computes S-parameters, applies calibration corrections, formats the data for display in various formats (log magnitude, Smith chart, polar, phase, group delay, etc.), and stores calibration data.
Block diagram of a two-port VNA. The synthesized RF source sweeps through the measurement frequency range. The directional coupler separates incident and reflected waves at Port 1. Measurement receivers A (reference), B (reflected), and C (transmitted) capture the complex signals. The ratio B/A gives S11; C/A gives S21.
View LargerScalar vs Vector Network Analysis
A scalar network analyzer measures only the magnitude of S-parameters — the amplitude ratio of transmitted or reflected signal to incident signal, expressed in dB. It cannot measure phase. Instruments such as the tracking-generator-plus-spectrum-analyzer combination act as scalar analyzers: they show the shape of a filter's frequency response but cannot measure phase or group delay.
A vector network analyzer measures both magnitude and phase. This additional information enables:
- Complex impedance: Both resistance and reactance can be determined from the phase of S11, enabling the Smith chart display and precise impedance matching
- Group delay: The rate of change of phase with frequency, which reveals filter phase distortion and is critical for digital signal transmission
- Time domain transformation: Inverse Fourier transformation of the frequency-domain S21 data produces a time-domain response — the basis for TDR measurements from a VNA
- Full error correction: The 12-term error model used for full VNA calibration requires both magnitude and phase measurements at each frequency to solve for the six complex error terms
For most amateur radio measurements — antenna impedance, filter response, SWR across a frequency range — vector measurements are worth the modest extra cost of a VNA over a scalar analyzer. The NanoVNA provides full vector capability at scalar-analyzer prices.
Key Specifications
Frequency Range
The VNA's usable frequency range. The NanoVNA covers 50 kHz to 900 MHz; the NanoVNA V2 covers 50 kHz to 3 GHz. For HF and VHF amateur work (1.8–148 MHz), either version is adequate. For UHF and microwave measurements above 3 GHz, laboratory-grade instruments are needed.
Dynamic Range
The ratio between the maximum signal the VNA can measure and its noise floor. Expressed in dB, dynamic range determines how accurately you can measure high-loss devices. If a filter has 80 dB of insertion loss at the stopband frequency, you need at least 80–90 dB of dynamic range to measure that attenuation accurately. The NanoVNA achieves about 50–70 dB of dynamic range at HF, sufficient for most amateur filter and antenna measurements.
Return Loss / Directivity
Directivity is the key specification for S11 (reflection) measurements. It represents how well the directional coupler inside the VNA separates the incident signal from the reflected signal. Low directivity causes measurement error — a perfect (zero-reflection) load appears to have a small reflection because the directional coupler "leaks" incident power into the reflected measurement path. After calibration, directivity error is corrected, but pre-calibration directivity limits the uncorrected accuracy.
Port Impedance
Almost universally 50 Ω. All measurements are normalized to 50 Ω. If your circuit uses 75 Ω impedance (as in cable TV systems), you either use 75 Ω calibration standards or apply an impedance normalization in software.
Trace Noise
The noise level visible on the measurement traces, typically visible as a "fuzz" on the magnitude and phase curves. Averaging multiple sweeps reduces trace noise at the cost of slower updates. On the NanoVNA, averaging 4–8 sweeps noticeably improves the smoothness of the display without significantly slowing down measurement.
Ham Radio Applications
The VNA is useful for a remarkable range of amateur radio measurements.
Antenna Resonance and Impedance
Connect the VNA to a dipole or vertical antenna (with the coax feedline disconnected to eliminate cable effects, or with calibration at the antenna end of the feedline). The S11 display shows the antenna's input impedance as a function of frequency. The Smith chart display shows whether the impedance is purely resistive (on the real axis), inductive (upper half), or capacitive (lower half). The resonant frequency is where the impedance is purely resistive.
Coaxial Cable Characterization
By measuring S11 with the far end of the cable open or short-circuited, a VNA can determine the cable's velocity factor, characteristic impedance, and insertion loss. The time-domain transformation (if available) shows the cable length as a distance.
Filter Measurement
A two-port measurement with the filter between Port 1 and Port 2 gives S21 (insertion loss and phase response) and S11 (input return loss). This fully characterizes the filter's behavior — passband insertion loss, ripple, stopband attenuation, cutoff frequency, and bandwidth — all in a single sweep.
Component Characterization
Individual inductors, capacitors, transformers, and ferrite cores can be measured. Placing a component across the VNA port (or in series with a test fixture) and measuring S11 reveals the component's impedance as a function of frequency. This is invaluable for verifying the self-resonant frequency of inductors and the loss characteristics of ferrite cores at specific operating frequencies.
Matching Network Design and Verification
After designing a matching network on paper or in simulation, a VNA verifies that the actual circuit achieves the intended impedance transformation at the target frequency. The Smith chart display shows whether you are hitting the intended 50 Ω match or whether component tolerances have shifted the result.
Affordable VNA Options
| Instrument | Frequency Range | Typical Price | Notable Features |
|---|---|---|---|
| NanoVNA (original) | 50 kHz – 900 MHz | $50–$80 | 2-port, Smith chart display, open-source firmware |
| NanoVNA V2 (SAA-2) | 50 kHz – 3 GHz | $80–$120 | Improved dynamic range, covers 2m/70cm/1.2 GHz/2.4 GHz |
| LiteVNA | 50 kHz – 6.3 GHz | $100–$150 | Extends into microwave range |
| miniVNA Pro | 100 kHz – 200 MHz | $150–$250 | USB-connected, well-supported software |
| Keysight E5063A | 100 kHz – 18 GHz | $20,000+ | Professional lab instrument, traceable calibration |
Return Loss Calculator
Return loss is the S11 measurement expressed in positive dB — the ratio of incident power to reflected power. It is used interchangeably with S11 in VNA readouts. A return loss of 20 dB means 1% of the incident power reflects. The calculator below converts between S11 (as a negative dBm ratio), SWR, reflection coefficient magnitude (|Γ|), and return loss.
A VNA shows S11 = −14 dB at the operating frequency of a 40m dipole. What does this mean?
Return loss = 14 dB (positive value; 14 dB of return loss is equivalent to S11 = −14 dB)
|Γ| = 10^(−14/20) = 10^(−0.7) = 0.200
SWR = (1 + 0.200) / (1 − 0.200) = 1.200 / 0.800 = 1.50
Power reflected = |Γ|² = 0.200² = 0.040 = 4% of incident power
The antenna has an SWR of 1.5:1 and reflects 4% of the transmitter power. This is an acceptable match for most HF work.
Return Loss / SWR / Reflection Coefficient Converter
Enter one value and convert to all others. Return loss in dB (positive), S11 magnitude in dB (negative), SWR, or |Γ| (reflection coefficient magnitude, 0 to 1).
Frequently Asked Questions
Can a NanoVNA replace a professional lab VNA for ham radio work?
For most amateur radio measurements — antenna resonance and SWR, HF filter characterization, coaxial cable testing — the NanoVNA is entirely adequate. Its dynamic range of 50–70 dB covers the majority of amateur applications. Where it falls short is in measurements requiring very wide dynamic range (measuring filters with 100 dB stopband attenuation), very high frequency coverage above 3 GHz, or traceable calibration for regulatory compliance testing. For the amateur radio shack, the NanoVNA V2 is excellent value.
Do I need to calibrate the VNA before every measurement?
You need to calibrate whenever the measurement conditions change: when you change the frequency range, when you change cables or adapters connected between the VNA and the DUT, or when the temperature changes significantly. A valid calibration from the previous day is usually still good enough for most amateur measurements as long as the cables and adapters are the same and the environment is stable. Calibration is covered in detail in Module 17G — understanding why calibration is essential is more important than how often you do it.
Test Your Knowledge
Answer the questions below to check your understanding. Every answer can be found in the lesson above.