Published on 10/13/2026
Two cables of the same type, both cut to 500.0 mm, are connected to the two inputs of a phase detector. The reading says 14 degrees of phase difference at 10 GHz. The cables are the same length, so the question is how.
The answer is that physical length and electrical length are different things. Dielectric constant varies from lot to lot, connectors add small delays, and solder or crimp positions differ by a fraction of a millimeter. At microwave frequencies a fraction of a millimeter is many degrees.
Phase-matched cable assemblies are built so that two or more cables have the same electrical length, within a stated tolerance, at a stated frequency. This guide explains what that means, how to read the numbers, what affects stability, and when you actually need them. For general cable selection see our RF coaxial cable selection guide.
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Phase matching sets the transmission phase (the S21 phase angle on a VNA) of each cable in a set to be equal within a specified tolerance, for example plus or minus 2 degrees at 6 GHz. The cables may differ slightly in physical length to achieve that.
| Term | Meaning |
|---|---|
| Matched length | Equal physical length, no phase guarantee |
| Phase matched pair or set | Equal electrical length within a stated tolerance at a frequency or band |
| Relative phase | Difference between cables in the set; usually what matters |
| Absolute phase | Phase against a defined reference length; rarely specified |
| Phase stability | How much phase changes with temperature, flexing, and aging |
Electrical length in degrees equals 360 times the physical length divided by the wavelength in the cable. For PTFE-dielectric cable with a velocity factor of about 0.695, the wavelength in the cable is the free-space wavelength multiplied by 0.695.
That is why you cannot cut cables with a ruler and expect them to match. A 0.5 mm error is 5 degrees at 6 GHz and almost 9 degrees at 10 GHz.
A matched set is only useful if it stays matched. Four effects matter:
Phased array antennasBeamformingMIMO and diversity antennasDirection findingRadar and EW testInterferometersCalibration of multi-channel receiversLO distribution
If your system combines signals coherently or measures angle of arrival from phase difference, you need matching. If it only needs power at one antenna port, you do not. Typical Wi-Fi and cellular antenna jumpers do not need phase matching, though diversity systems with correlation measurements sometimes do.
| Application | Typical tolerance | Cable choice |
|---|---|---|
| 2x2 MIMO bench test | Plus or minus 5 deg to 10 deg | Flexible RG316 or similar |
| Direction finding to 6 GHz | Plus or minus 2 deg | Phase-stable flexible or semi-rigid |
| Phased array to 18 GHz | Plus or minus 1 deg or better | Semi-rigid or phase-stable low-loss |
Step 01
Define the band and tolerance
State the frequency (or range) and the tolerance in degrees, for example plus or minus 2 deg at 6 GHz.
Step 02
Choose the cable family
Pick flexible, semi-flexible or semi-rigid for your temperature and flex needs. Semi-rigid gives the best stability.
Step 03
State the set size
A pair is straightforward. Sets of 4 to 16 need a common reference length.
Step 04
Ask for the VNA data
Request the measured S21 phase and VSWR report for each cable in the set.
Step 05
Mark and keep the set together
Use serial labels and keep matched cables as a set. Swapping one invalidates the match.
What are phase-matched cables?
Cables built so their electrical length, measured as S21 phase on a VNA, is equal within a stated tolerance at a stated frequency.
What is the difference between matched length and matched phase?
Matched length means the physical lengths are equal. Matched phase means the measured phase is equal, which usually needs small length trims.
How accurate is phase matching?
Plus or minus 1 to 5 degrees is typical in the microwave range. The tolerance achievable depends on frequency, cable type and measurement repeatability.
Do phase-matched cables stay matched with temperature?
Only if they are the same cable type from the same lot and see the same temperature. PTFE cables shift near 19 C. Phase-stable constructions reduce this.
When do I need phase-matched cables?
For phased arrays, beamforming, direction finding, interferometers and multi-channel test setups where phase differences carry information.
How do you measure cable phase?
Connect the cable to a VNA and read the S21 phase at the target frequency, using the same calibration and torque for every cable in the set.
What is the velocity factor of PTFE coax?
About 0.69 to 0.70 for solid PTFE cables such as RG316, RG142 and semi-rigid with PTFE dielectric.
Can I phase match cables myself?
You can trim and measure on a VNA, but connector repeatability and phase stability make professional matching more reliable for tolerances under 2 degrees.
Phase matching is a measurement-and-trimming process, not a length spec. Decide whether your system needs it, set the tolerance by frequency, choose a stable cable type, and keep the set together.
At SigmaRF every assembly is VNA-swept for VSWR and insertion loss before shipment from 100 MHz to 40 GHz, and matched sets are built with the same measurement system. Our India (Vadodara) and USA (Cleveland area) facilities handle matched pairs and sets, with lead times as short as 2 days for standard builds.
Need a matched pair or set?
Send us the frequency, tolerance in degrees, connector types and quantity.
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