A practical guide to testing coax cables with the NanoVNA-H and viewing results on your computer.
Equipment used: AURSINC NanoVNA-H • MFJ-261 50Ω Dummy Load • NanoVNA-Saver Software
NanoVNA TOPICS COVERED:
Page One — firmware updates; calibration; antenna testing
Page Two — coax testing and coax testing tips; connecting nanoVNA to your computer.
1 Testing Coax Cable with the NanoVNA
The NanoVNA can measure how much signal loss your coax cables have, helping you identify damaged cables, bad connectors, and water ingress before they become a problem on the air.
What You Will Need
- NanoVNA with the supplied CH0-to-CH1 jumper cable
- The coax cable(s) you want to test
- A connector with the center pin shorted to the shield (for the SHORT calibration step — a PL-259 or BNC with the center pin soldered to the outer shell works perfectly)
- MFJ-261 50Ω dummy load (for the LOAD calibration step)
Step 1 — Set Your Frequency Range
Set a wide sweep that covers all the HF bands you use. Do this before calibrating — calibration is only valid for the range set when it was performed. Do not change the range after calibrating.
- Go to STIMULUS → START → enter 1 → tap M (sets 1 MHz)
- Go to STIMULUS → STOP → enter 30 → tap M (sets 30 MHz)
Tip: If you also use VHF, set STOP to 150 MHz or 300 MHz depending on your NanoVNA's upper frequency limit.
Step 2 — Calibrate
Attach your jumper cable to CH0 first, then calibrate. This zeros out the jumper so it doesn't affect your readings. Go to CAL → CALIBRATE and perform each step in order:
| Cal Step |
CH0 |
CH1 |
Action |
| OPEN |
Nothing connected |
Nothing |
Tap OPEN → wait for sweep |
| SHORT |
Shorted connector |
Nothing |
Tap SHORT → wait for sweep |
| LOAD |
MFJ-261 dummy load |
Nothing |
Tap LOAD → wait for sweep |
ISOLN |
Skip — requires two dummy loads |
| THRU |
Jumper to CH1 |
Jumper to CH0 |
Tap THRU → wait for sweep |
When complete: tap DONE → then CAL → SAVE → SAVE 0
Pro tip: Save different frequency ranges to different slots — for example SAVE 0 for 1–30 MHz and SAVE 1 for a VHF range. Use CAL → RECALL to load whichever you need without recalibrating every time.
Step 3 — Set the Display for Easy Reading
Switch away from the Smith Chart to a simple loss graph:
- Go to DISPLAY → TRACE → TRACE 1 (S21)
- Go to FORMAT → LOGMAG
- Go to DISPLAY → SCALE → set SCALE/DIV to 1 dB
- Set REFERENCE POSITION to the top line of the grid
- Set REFERENCE VALUE to 0 dB
The top of the grid is now 0 dB. Each grid line down = 1 dB of loss. A good coax will show a flat line near the top of the screen.
Note: The circular Smith Chart display is useful for antenna impedance work but tells you nothing useful during a coax loss test. Always switch to LOGMAG for coax testing.
Step 4 — Verify Your Setup is Working
Before testing any coax, confirm the NanoVNA and calibration are working correctly:
- Connect CH0 directly to CH1 using only the short supplied jumper — no coax under test yet
- The trace should sit near the top of the screen, close to 0 dB
- If it does — your calibration is good and you are ready to test ✓
- If it shows large negative numbers (e.g. −20 dB or worse) — recalibrate from Step 2
Important: If your coax readings later look terrible, always come back to this step first. A calibration issue produces the same symptoms as a bad cable. Confirm the jumper-only reading before condemning any coax.
Step 5 — Test Each Coax Cable
- Connect one end of the coax to CH0
- Connect the other end to CH1
- Observe the trace and read the loss value
| LOGMAG Reading |
Meaning |
| 0 to −1 dB |
Excellent coax ✓ |
| −1 to −3 dB |
Acceptable ✓ |
| −3 dB |
Losing half your power — inspect connectors ⚠ |
| Worse than −3 dB |
Replace cable or repair connectors ✗ |
Note on short vs. long runs: A 6-foot jumper will always read better than a 100-foot tower run. Don't compare them directly. What matters is whether a cable reads significantly worse than expected for its length and type.
Step 6 — Use a Marker for Exact Readings
Rather than trying to estimate values by eyeballing the grid lines, use a marker to get an exact number:
- Go to MARKER → MARKER 1
- Move the marker to any point on the trace
- The exact dB value displays as a number on screen — no guesswork needed
✓ Tips for Coax Testing
Check connectors before condemning the cable. A bad solder joint on a PL-259 is the single most common cause of high loss readings. Before cutting out a cable, inspect both connectors. A cold joint or loose center pin is a cheap fix.
Test at temperature. A connector that reads fine on the bench may fail outdoors when it expands or contracts. If you suspect an intermittent problem, wiggle the coax at each connector while watching the trace — a flickering reading points directly to the bad connection.
Water is the enemy. Loss that increases steeply as frequency goes up — a trace that slopes dramatically downward from left to right — is a classic sign of water inside the coax. Replace it. Water-damaged coax does not recover.
Test new coax too. Don't assume a brand new cable or connector is good. A bad crimp or factory solder joint is more common than you would think. Test before you run it up a tower.
Save your results. If your NanoVNA has a micro-SD card slot, save a screenshot of each cable's trace and label them. Comparing a cable's reading this year to last year is the best way to catch a slowly deteriorating connection before it becomes a problem.
Mark your cables as you go. Put a piece of colored tape or a cable tie on each end of every cable you test — one color for good, another for bad. Saves a lot of confusion when you're up on a ladder or digging through a pile of coax.
Label everything permanently. Once you've confirmed a cable is good, mark both ends with a permanent label showing the cable type, length, and date tested. This pays dividends for years.
Fault location with TDR. If your NanoVNA firmware supports it, the TDR (Time Domain Reflectometry) function under the Transform menu can show you approximately where in a cable a fault is — very useful for long buried runs or tower feedlines. Set your cable's velocity factor first: RG-8/RG-213 = 0.66, RG-8X/RG-58 = 0.78–0.82, LMR-400 = 0.85.
2 Connecting the NanoVNA to Your Computer
The NanoVNA can be connected to your computer via USB, giving you a much larger screen to view traces, save data, and analyze results. The most popular free software for this is NanoVNA-Saver, available for Windows, Mac, and Linux.
What You Will Need
- USB-C cable (the same one used to charge the NanoVNA)
- A Windows, Mac, or Linux computer
- NanoVNA-Saver software (free — see Step 1)
Step 1 — Download and install NanoVNA-Saver
Step 2 — Connect the NanoVNA
- Power on your NanoVNA first
- Connect the USB-C cable from the NanoVNA to your computer
- Wait a few seconds for Linux Mint to recognize the device
Windoze may install a driver automatically — give it a moment to complete
Step 3 — Connect in NanoVNA-Saver
- Launch NanoVNA-Saver
- In the left column, find the Serial port control dropdown
- Click Rescan to find your device
- Select the port that appears:
- Linux: will show as /dev/ttyUSB0 or similar
- Windoze: will show as COM3 or similar
- Click Connect to device
- The software will begin pulling live data from the NanoVNA — you should see the traces appear on screen
Linux users: If you get a permissions error connecting to the serial port, run the following command in a terminal, then log out and back in:
sudo usermod -aG dialout $USER
Step 4 — Set Your Frequency Range in NanoVNA-Saver
- In the Sweep settings panel on the left, enter your Start and Stop frequencies
- Set the number of segments — more segments means more measurement points across the sweep. For 1–30 MHz, 3 to 4 segments gives a good balance of speed and detail
- Click Sweep to run a scan
Calibration note: NanoVNA-Saver will use the calibration already stored on the NanoVNA. Always calibrate on the device first (see Section 1, Step 2) before connecting to the computer.
Step 5 — Reading Coax Loss in NanoVNA-Saver
With your coax connected CH0 to CH1 as described in Section 1:
- Look at the S21 panel in NanoVNA-Saver
- Click the format dropdown and select Gain (dB) — this is the equivalent of LOGMAG on the device itself
- Use the same loss reference table from Section 1, Step 5 to interpret your readings
- The larger computer screen makes it much easier to spot a subtle downward slope in a lossy or water-damaged cable that can be hard to see on the small NanoVNA display
Step 6 — TDR: Locating a Fault in the Cable
NanoVNA-Saver includes a TDR (Time Domain Reflectometry) tab that can show you approximately where in a cable a fault is located — very useful for long buried runs or tower feedlines.
- Click the TDR tab in NanoVNA-Saver
- Enter your cable's velocity factor:
| Cable Type | Velocity Factor |
|---|
| RG-8, RG-213 | 0.66 |
| RG-8X, RG-58 | 0.78 – 0.82 |
| LMR-400 | 0.85 |
The TDR display will show a spike at the point of reflection. The distance on the horizontal axis tells you how far the fault is from your end of the cable in feet or meters.
Step 7 — Saving Your Results
- Go to File → Save to save sweep data as a .s1p or .s2p Touchstone file for later comparison
- To save a screenshot of the NanoVNA-Saver window, use your computer's built-in tool:
- Windows: Win + Shift + S
- Mac: Cmd + Shift + 4
- Linux: use your desktop's screenshot tool or scrot
- Name files descriptively — e.g. coax_tower_run_2026.png — so you can compare them over time
SD Card shortcut: If your NanoVNA has a micro-SD card slot, format a card as FAT32 and insert it. You can save screenshots directly from the device without needing a computer — handy when tuning antennas in the field.
AA4TE • NanoVNA-H Coax Testing & PC Display Guide • Last updated 2026
Device: AURSINC NanoVNA-H • Software: NanoVNA-Saver • Written with the assistance of Claude by Anthropic
