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SDRstore.eu • 06/18/2026

How to Test RF Filters with a NanoVNA: Band-Pass, Low-Pass, High-Pass, and Notch Filters

A NanoVNA is one of the most useful low-cost RF tools for testing filters. With a proper calibration and a simple two-port setup, it can show whether a band-pass filter passes the correct frequency range, whether a low-pass filter rolls off where expected, whether a high-pass filter blocks low frequencies, and whether a notch filter rejects the target frequency deeply enough.

The most important measurement for RF filter testing is usually S21. S21 shows how much signal passes through the filter from port 1 to port 2. It is used to check insertion loss, passband shape, cutoff frequency, roll-off, stop-band rejection, and notch depth. S11 is also useful because it shows how well the filter input is matched, especially inside the passband.

This guide explains how to test RF filters with a NanoVNA, including band-pass, low-pass, high-pass, and notch filters. It covers calibration, S21 and S11 traces, marker placement, insertion loss, return loss, cutoff points, stop-band rejection, common mistakes, and recommended RF accessories.

Browse the NanoVNA-H4 portable vector network analyzer, spectrum analyzers and RF analysis tools, RF dummy loads, RF power meters, RF test and measurement equipment, and request a formal quote from SDRstore.eu.

Quick Answer: How Do You Test an RF Filter with a NanoVNA?

To test an RF filter with a NanoVNA, connect the filter between Port 1 and Port 2, calibrate across the frequency range you want to measure, display S21 LOGMAG to see the filter transmission response, and use markers to read insertion loss, cutoff frequency, passband ripple, and stop-band rejection. Add S11 LOGMAG if you also want to check input return loss.

Filter type	Main NanoVNA trace	What to check
Band-pass filter	S21 LOGMAG	Passband, insertion loss, center frequency, bandwidth, roll-off, stop-band rejection.
Low-pass filter	S21 LOGMAG	Low-frequency passband, cutoff frequency, high-frequency attenuation, roll-off.
High-pass filter	S21 LOGMAG	Low-frequency rejection, 3 dB cutoff point, high-frequency passband, insertion loss.
Notch filter	S21 LOGMAG	Notch frequency, notch depth, notch width, passband loss outside the notch.
Any filter input match	S11 LOGMAG	Return loss, impedance match, passband matching, reflections.

The simple rule: S21 tells you what gets through the filter. S11 tells you how well the filter input is matched.

What NanoVNA Measurements Matter for Filters?

S21: Transmission and insertion loss

S21 is the most important filter trace. It shows how much signal leaves Port 2 when the NanoVNA sends a signal into Port 1.

Use S21 to measure:

Insertion loss
Passband shape
Cutoff frequency
3 dB point
Bandwidth
Stop-band rejection
Notch depth
Filter roll-off
Passband ripple

For most passive filters, the S21 trace will be near 0 dB in the passband and much lower in the stopband. For example, a good receive filter may show -1 dB to -3 dB insertion loss in the passband and -30 dB, -40 dB, or more rejection in the stopband depending on design.

S11: Return loss and input match

S11 shows how much energy is reflected back from the filter input. A well-matched filter should usually have better return loss in the passband than in the stopband.

Use S11 to check:

Input matching
Return loss
VSWR
Impedance behavior
Whether the filter looks close to 50 ohms in the passband

For simple SDR receiving, S21 is usually the first trace to inspect. For RF design, lab reports, or product testing, inspect both S21 and S11.

What You Need Before Testing

NanoVNA or NanoVNA-H4
Open, short, load, and through calibration kit
Two short SMA cables
Known-good SMA adapters if needed
Filter under test
Optional attenuators if testing active filters or amplified devices
Optional NanoVNA Saver or PC software for larger plots and screenshots
Optional TinySA Ultra or spectrum analyzer for confirming real-world signal behavior

Keep cables short, use good connectors, and avoid moving the cables after calibration. Cable movement can change the measurement, especially at higher frequencies.

Step-by-Step: NanoVNA Filter Test Setup

Step 1: Choose the frequency range

Set the NanoVNA start and stop frequency around the filter you want to test. Do not always scan the entire NanoVNA range. A narrower sweep gives better detail around the filter response.

Filter example	Suggested sweep	Why
433 MHz band-pass filter	350–500 MHz	Shows passband, roll-off, and nearby rejection.
868 MHz band-pass filter	750–950 MHz	Shows LoRa/ISM passband and surrounding rejection.
1090 MHz ADS-B filter	900–1300 MHz	Shows ADS-B passband and out-of-band rejection.
FM broadcast notch or block filter	70–130 MHz	Shows rejection around 88–108 MHz and loss outside the notch.
HF high-pass filter	100 kHz–30 MHz	Shows AM broadcast rejection and HF passband behavior.

Step 2: Set traces

For a clear filter measurement, use these traces:

Trace 1: S21 LOGMAG
Trace 2: S11 LOGMAG
Trace 3: S11 SWR or Smith Chart if you want impedance view
Trace 4: S21 PHASE or delay if needed for advanced work

For most users, S21 LOGMAG alone is enough to see whether the filter passes or blocks the correct frequencies.

Step 3: Calibrate over the exact sweep range

Calibration is the most important step. Calibrate after setting the exact start and stop frequency you will use for the measurement.

For a two-port filter measurement, perform:

Open on Port 1
Short on Port 1
Load on Port 1
Isolation step if available
Through connection between Port 1 and Port 2
Save calibration if you will reuse the same sweep and setup

After calibration, connect the filter between Port 1 and Port 2 without changing the cables more than necessary.

Step 4: Connect the filter correctly

Most passive filters have an input and output. Some are symmetrical, but not all. If the filter has labels, connect NanoVNA Port 1 to the filter input and Port 2 to the filter output.

Basic connection:

NanoVNA Port 1 → short SMA cable → filter input
Filter output → short SMA cable → NanoVNA Port 2

If the filter has N-type, BNC, U.FL, MCX, or other connectors, use the minimum number of adapters possible. Extra adapters add loss and mismatch.

Step 5: Read S21 with markers

Place markers at important points:

Center of passband
Lower 3 dB point
Upper 3 dB point
Deepest notch point
Stopband target frequency
Known interference frequency
Wanted receiver frequency

Use the marker values to compare the filter against its datasheet or expected behavior.

How to Test a Band-Pass Filter with a NanoVNA

A band-pass filter passes a chosen frequency range and rejects frequencies below and above it. This is common for ADS-B, AIS, LoRa, GNSS, ham radio, satellite reception, and SDR front-end protection.

What to check

Center frequency
Passband insertion loss
Lower cutoff frequency
Upper cutoff frequency
3 dB bandwidth
Passband ripple
Lower stopband rejection
Upper stopband rejection
S11 return loss inside the passband

Marker example

Marker	Where to place it	What it tells you
Marker 1	Center frequency	Insertion loss at the wanted frequency.
Marker 2	Lower 3 dB point	Lower edge of useful passband.
Marker 3	Upper 3 dB point	Upper edge of useful passband.
Marker 4	Known strong interferer below passband	How much the filter rejects that signal.
Marker 5	Known strong interferer above passband	How much the filter rejects that signal.

Good result

A good band-pass filter should show low insertion loss in the wanted band and strong rejection outside it. For SDR receiving, a filter with slightly higher insertion loss may still be useful if it prevents receiver overload from strong out-of-band signals.

How to Test a Low-Pass Filter with a NanoVNA

A low-pass filter passes lower frequencies and rejects higher frequencies. It is often used to reduce harmonics or block unwanted high-frequency signals.

What to check

Passband insertion loss below cutoff
Cutoff frequency
3 dB point
Roll-off slope
High-frequency stopband rejection
Passband return loss

Test workflow

Set start frequency below the expected passband.
Set stop frequency well above the expected cutoff.
Calibrate across that range.
Display S21 LOGMAG.
Place one marker in the passband.
Place one marker at the 3 dB cutoff.
Place one or more markers in the stopband.

For example, if you are checking a low-pass filter designed to pass up to 30 MHz, sweep from 1 MHz to 100 MHz or wider if your NanoVNA and filter specification allow it.

How to Test a High-Pass Filter with a NanoVNA

A high-pass filter rejects lower frequencies and passes higher frequencies. SDR users often use high-pass filters to reduce strong AM broadcast signals, especially when using direct-sampling HF modes.

A practical example is an AM broadcast reject high-pass filter. The expected result is strong rejection through the AM broadcast band and much lower loss above the filter cutoff.

What to check

Low-frequency stopband rejection
3 dB cutoff point
Passband insertion loss above cutoff
Roll-off slope
Input match in the passband

Marker example for an AM reject high-pass filter

Marker	Example placement	What it tells you
Marker 1	1 MHz	Rejection in the AM broadcast area.
Marker 2	2.6 MHz	Approximate cutoff area for some AM reject high-pass filters.
Marker 3	7 MHz	Passband insertion loss in the HF range.
Marker 4	14 MHz	Higher HF passband behavior.

If the high-pass filter is designed to reject 500 kHz to 1.7 MHz, the S21 trace should be far below the passband level in that region and rise after the cutoff point.

How to Test a Notch Filter with a NanoVNA

A notch filter rejects one narrow frequency range while passing frequencies below and above it. This is useful for reducing strong local interference such as FM broadcast, paging transmitters, or a known unwanted carrier.

What to check

Notch center frequency
Notch depth
Notch width
Insertion loss outside the notch
Symmetry around the notch
Input match near the passband

Test workflow

Set the sweep around the expected notch area.
Calibrate across that sweep.
Display S21 LOGMAG.
Place a marker at the deepest point of the notch.
Place markers just below and just above the notch.
Compare notch depth against the expected specification.

For example, an FM broadcast notch filter should show strong rejection between 88 and 108 MHz while keeping lower and higher frequencies less affected. The exact trace depends on filter design.

How to Interpret Insertion Loss

Insertion loss is the loss added by the filter in the passband. On a NanoVNA S21 LOGMAG trace, it appears as a negative dB value.

S21 value	Meaning	Typical interpretation
0 dB	No loss	Ideal value, rarely perfect in real passive filters.
-0.5 dB	Very low loss	Excellent for many receive filters.
-1 dB to -3 dB	Moderate loss	Often acceptable if rejection is useful.
-6 dB	Significant loss	May still be useful in some cases but reduces signal noticeably.
-20 dB or lower	Strong attenuation	Usually stopband or notch region.
-40 dB or lower	Very strong rejection	Good stopband result, if measurement noise floor allows confidence.

Do not judge a filter only by insertion loss. A filter with 2 dB passband loss may still improve real SDR reception if it removes a much stronger out-of-band signal that was overloading the receiver.

How to Interpret Return Loss with S11

S11 LOGMAG shows how much signal is reflected from the filter input. More negative is generally better for return loss.

S11 value	Meaning	Interpretation
-6 dB	Poor match	May still work, but mismatch is visible.
-10 dB	Usable match	Common minimum target in many practical RF setups.
-15 dB	Good match	Generally good for many receive filter setups.
-20 dB or lower	Very good match	Strong result if measured correctly.

For passive filters, S11 may look worse in the stopband. That can be normal because the filter is rejecting energy instead of passing it into a matched load.

3 dB Cutoff, Bandwidth, and Stop-Band Rejection

3 dB cutoff point

The 3 dB point is where the S21 trace has dropped by about 3 dB from the passband level. It is commonly used to define cutoff frequency or passband edge.

For example, if a filter passband is around -1 dB and the response drops to -4 dB, that point is approximately 3 dB below the passband level.

Bandwidth

For a band-pass filter, bandwidth is usually measured between the lower and upper 3 dB points. Place markers at both points and subtract the lower frequency from the upper frequency.

Stop-band rejection

Stop-band rejection is how much the filter blocks unwanted frequencies. On the S21 trace, compare the passband value to the stopband value.

Example:

Passband marker: -1 dB
Stopband marker: -41 dB
Approximate rejection relative to passband: 40 dB

Be careful when measuring very deep rejection. A NanoVNA has a limited noise floor and dynamic range, so very high rejection values may require a better VNA or spectrum analyzer setup for confirmation.

NanoVNA Filter Testing Examples

Example 1: 1090 MHz ADS-B band-pass filter

Set sweep: 900–1300 MHz
Trace: S21 LOGMAG
Marker 1: 1090 MHz
Marker 2: nearby lower stopband
Marker 3: nearby upper stopband
Check: insertion loss at 1090 MHz and rejection outside the ADS-B band

Use this measurement to see whether the filter passes ADS-B while reducing nearby mobile-network or other out-of-band energy.

Example 2: 433 MHz band-pass filter

Set sweep: 350–500 MHz
Trace: S21 LOGMAG
Marker 1: 433.92 MHz
Marker 2: lower 3 dB point
Marker 3: upper 3 dB point
Check: center frequency, bandwidth, and rejection

This is useful for Sub-GHz sensors, remotes, telemetry devices, and receive-side monitoring around 433 MHz.

Example 3: 868 MHz LoRa/ISM filter

Set sweep: 800–930 MHz
Trace: S21 LOGMAG
Marker 1: wanted 868 MHz area
Marker 2: lower stopband
Marker 3: upper stopband
Check: whether the filter improves 868 MHz receiver selectivity

This is useful for LoRaWAN EU868, industrial sensors, and Sub-GHz monitoring.

Example 4: FM broadcast notch filter

Set sweep: 50–150 MHz
Trace: S21 LOGMAG
Marker 1: 88 MHz
Marker 2: 98 MHz
Marker 3: 108 MHz
Marker 4: frequency outside the notch
Check: notch depth and insertion loss outside the FM band

This helps determine whether an FM filter can reduce strong FM broadcast overload without hurting nearby wanted signals too much.

Example 5: AM broadcast high-pass filter

Set sweep: 100 kHz–30 MHz
Trace: S21 LOGMAG
Marker 1: 1 MHz
Marker 2: 2.6 MHz
Marker 3: 7 MHz
Marker 4: 14 MHz
Check: AM broadcast rejection and HF passband loss

This is useful when strong AM broadcast stations overload an SDR receiver during HF direct-sampling work.

NanoVNA vs TinySA for Filter Testing

NanoVNA and TinySA Ultra are both useful RF tools, but they answer different questions.

Question	Best tool	Why
What is the filter insertion loss?	NanoVNA	S21 shows how much signal passes through the filter.
Where is the 3 dB cutoff?	NanoVNA	Markers on S21 show cutoff points clearly.
Is the filter input matched?	NanoVNA	S11 shows return loss and impedance behavior.
What signals are currently in the air?	TinySA Ultra or spectrum analyzer	A spectrum analyzer shows live RF energy.
Does the filter reduce real interference?	Both	NanoVNA tests the filter response; TinySA shows real spectrum before and after.
Does the antenna match the filter and cable setup?	NanoVNA	NanoVNA can check antenna match and cable behavior.

For the best workflow, measure the filter with NanoVNA first, then use TinySA Ultra or an SDR to confirm how it affects real-world reception.

Common NanoVNA Filter Testing Mistakes

Calibrating across the wrong frequency range

Calibration is valid for the frequency range used during calibration. If you change the sweep significantly, calibrate again.

Using long or moving cables after calibration

Long cables add loss and mismatch. Moving cables after calibration can change the result. Use short, stable cables and do not move them unnecessarily.

Forgetting the through calibration

For S21 filter testing, through calibration is important. Without it, cable and adapter loss may be included in the filter measurement.

Testing active filters like passive filters

Active filters, powered filters, LNAs with filtering, and amplified modules can overload the NanoVNA or create misleading results. Check power, bias, input limits, and test setup first.

Expecting a low-cost NanoVNA to measure very deep rejection perfectly

NanoVNA is excellent for learning and practical RF work, but very deep stopband rejection may require a higher-dynamic-range VNA or professional analyzer.

Ignoring filter direction

Some filters are directional. If the filter has input and output labels, test it in the correct direction and optionally repeat the measurement reversed.

Using bad adapters

Cheap or damaged adapters can add ripple, loss, or mismatch. Use good adapters and keep the adapter chain short.

Not checking S11

A filter may have a good-looking S21 passband but poor input match. For serious work, check both S21 and S11.

How to Document a Filter Measurement

A good filter test report should include enough information for someone else to repeat the measurement.

Filter model and serial number if available
Filter type: band-pass, low-pass, high-pass, notch
Expected passband or cutoff
NanoVNA model
Firmware version if known
Calibration range
Calibration method
Cables and adapters used
S21 screenshot
S11 screenshot if measured
Insertion loss at key frequencies
3 dB cutoff points
Stopband rejection markers
Notch depth if relevant
Test date and operator

Recommended SDRstore.eu Hardware Packages

Package 1: Beginner NanoVNA filter testing kit

NanoVNA-H4
Open, short, load, through calibration kit
Short SMA cables
SMA adapter set
Known example filters

Best for: beginners, SDR users, ham radio operators, students, and first RF filter measurements.

Package 2: SDR filter validation kit

NanoVNA-H4
TinySA Ultra
RTL-SDR receiver
Band-pass, high-pass, and notch filters for the target bands
Attenuators, adapters, and short cables
SDR++ or SDRangel

Best for: SDR users who want to test filters on the bench and then confirm real reception improvements.

Package 3: RF cybersecurity and facility monitoring kit

NanoVNA-H4
TinySA Ultra
HackRF Pro
RTL-SDR monitoring node
Sub-GHz, 2.4 GHz, 5.8 GHz, and GNSS antennas
Band-pass, high-pass, and notch filters
RF log template and marker checklist

Best for: defensive RF monitoring, rogue wireless detection, Sub-GHz audits, drone RF awareness, and facility interference troubleshooting.

Package 4: Product-development filter testing bench

NanoVNA-H4 or higher-grade VNA
TinySA Ultra or professional spectrum analyzer
RF power meter
Attenuators and dummy loads
Short phase-stable cables where possible
Filters, adapters, DC blocks, and test fixtures
Documented calibration workflow

Best for: IoT product developers, RF labs, universities, and product-security teams testing filters before integration.

Purchase-Order Justification Examples

NanoVNA filter testing justification

NanoVNA-H4 is required for RF filter testing, including S21 insertion-loss measurement, S11 return-loss measurement, cutoff frequency checks, passband and stopband validation, antenna matching, and RF lab training.

TinySA Ultra filter validation justification

TinySA Ultra is required to confirm real RF spectrum behavior before and after filtering, investigate interference, check band occupancy, and support field troubleshooting alongside NanoVNA measurements.

RF cable and adapter justification

Short SMA cables, adapters, calibration standards, attenuators, and RF accessories are required to create repeatable NanoVNA filter measurements and reduce errors caused by poor cabling or connector mismatch.

Professional VNA justification

A higher-grade vector network analyzer is required when filter measurements need higher dynamic range, wider frequency coverage, better calibration confidence, group delay analysis, production validation, or customer-facing reports.

Request a Quote for NanoVNA and RF Filter Testing Equipment

Universities, RF labs, SDR users, ham radio clubs, IoT developers, cybersecurity teams, telecom labs, and product-testing teams can request a formal quotation directly from SDRstore.eu.

Use the Add to Quote button on product pages or the document icon on product cards. Add NanoVNA-H4, TinySA Ultra, filters, attenuators, dummy loads, RF power meters, antennas, cables, adapters, SDR receivers, and project notes to one quote request.

A quote request is useful when you need:

NanoVNA filter testing kits
Band-pass filter measurement hardware
Low-pass and high-pass filter test setups
Notch filter testing accessories
SDR filter validation bundles
RF product-development test benches
University RF teaching kits
Formal pricing for company, university, or public-sector procurement

Read the SDRstore.eu quote-request guide.

Related SDRstore.eu Guides

Official and Technical Resources

Final Recommendation

Use NanoVNA to test RF filters before installing them in an SDR, receiver, transmitter chain, or RF product. Start with S21 LOGMAG to see insertion loss, cutoff, passband, stopband, and notch depth. Add S11 LOGMAG when you also need to check return loss and impedance matching.

For basic SDR use, NanoVNA-H4, short SMA cables, a calibration kit, and a few known filters are enough to learn filter testing. For better real-world troubleshooting, add TinySA Ultra to compare the RF spectrum before and after filtering.

The best filter test is not just a screenshot. It is a calibrated measurement with known cables, correct markers, documented settings, and a clear answer: does this filter pass the signal we want and reject the signal we do not want?

FAQ

Can NanoVNA test RF filters?

Yes. NanoVNA can test RF filters by measuring S21 transmission and S11 reflection. S21 shows insertion loss, passband, cutoff, stopband rejection, and notch depth. S11 shows input match and return loss.

What NanoVNA trace should I use for filter testing?

Use S21 LOGMAG as the main trace for filter testing. Add S11 LOGMAG if you also want to check return loss and input matching.

How do I test a band-pass filter with NanoVNA?

Connect the band-pass filter between Port 1 and Port 2, calibrate across the target range, display S21 LOGMAG, and place markers at the center frequency, lower 3 dB point, upper 3 dB point, and stopband frequencies.

How do I test a low-pass filter with NanoVNA?

Set a sweep from below the passband to well above the cutoff, calibrate, connect the filter, and inspect S21 LOGMAG. Check passband insertion loss, 3 dB cutoff, roll-off, and high-frequency rejection.

How do I test a high-pass filter with NanoVNA?

Set a sweep from below the rejected range to above the wanted passband, calibrate, connect the filter, and inspect S21 LOGMAG. Check low-frequency rejection, cutoff point, and passband insertion loss.

How do I test a notch filter with NanoVNA?

Set the sweep around the expected notch, calibrate, connect the filter, and inspect S21 LOGMAG. Place a marker at the deepest point of the notch and markers outside the notch to check normal passband loss.

What is insertion loss on NanoVNA?

Insertion loss is the loss through the filter in the passband. On a NanoVNA, it appears as a negative S21 LOGMAG value such as -0.8 dB, -1.5 dB, or -3 dB.

What is return loss on NanoVNA?

Return loss is related to how much signal is reflected from the filter input. On NanoVNA, S11 LOGMAG shows this behavior. More negative values usually mean a better match.

Why does my NanoVNA filter measurement look wrong?

Common causes include poor calibration, wrong sweep range, missing through calibration, bad cables, too many adapters, moved cables after calibration, wrong filter direction, receiver dynamic range limits, or testing an active device like a passive filter.

Can SDRstore.eu quote a NanoVNA filter testing kit?

Yes. Use the Add to Quote button on product pages or the document icon on product cards. Add NanoVNA-H4, TinySA Ultra, filters, cables, adapters, attenuators, dummy loads, RF power meters, and project notes so the full filter testing setup can be quoted together.

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