Published on 10/13/2026
Cable length in wavelengths equals physical length divided by the wavelength inside the cable: N = Lf / (c x VF). Multiply N by 360 to obtain electrical length in degrees, using the cable velocity factor VF rather than free-space wavelength.
Two 300 mm antenna feeds can look identical and still deliver different phases. Different dielectric constructions, connector reference planes or small length differences change electrical length, particularly at several GHz. That matters in coherent arrays, matching sections and calibrated test setups.
The worked numbers here use an assumed VF of 0.70. They demonstrate the calculation, not a guaranteed value for every PTFE cable or a measured SigmaRF assembly. Read the exact cable datasheet and specify phase at defined reference planes.
On This Page
Physical length is a distance in mm or m. Electrical length expresses that distance as a fraction of the guided wavelength at one frequency. Delay is a time in ns. They describe the same simple propagation model in different units, but electrical length changes with frequency while delay can remain approximately constant in a low-dispersion band.
| Quantity | Formula | Units |
|---|---|---|
| Guided wavelength | lambda = c x VF / f | m with f in Hz |
| Electrical length | N = L / lambda | wavelengths |
| Phase length | theta = 360 x N | degrees |
| One-way delay | t = L / (c x VF) | s |
Key idea: Use c = 299,792,458 m/s and frequency in Hz. Convert 1 GHz to 1,000,000,000 Hz and 300 mm to 0.300 m before calculating.
Velocity factor is propagation speed divided by the speed of light in vacuum. A VF of 0.70 means the ideal guided wavelength is 70% of the free-space wavelength. Using free-space wavelength would underestimate electrical length by a factor of 1/0.70, about 1.43.
Dielectric construction sets the first estimate; the actual cable specification is the purchasing reference. Do not apply one generic VF to all solid, foamed and air-spaced cable constructions. For precision work, measure the completed assembly over the required band.
Step 01
Choose the actual frequency
Electrical length needs a frequency. A cable that is 90 degrees at one frequency is not 90 degrees across a broad band.
Step 02
Choose the correct length reference
State whether length is jacket-to-jacket, connector-face-to-face or the propagation length between calibrated electrical reference planes. Connectors contribute delay too.
Step 03
Use the specified VF
Enter the datasheet VF as a fraction, such as 0.70, not the percentage 70. Keep the same physical units throughout.
For an ideal 300 mm line with VF = 0.70, electrical length rises in proportion to frequency. The table gives positive phase-length magnitude; measured S21 propagation phase commonly appears as a negative phase progression and wraps unless unwrapped.
| Frequency | Guided wavelength | Length in wavelengths | Phase-length magnitude |
|---|---|---|---|
| 100 MHz | 2.0985 m | 0.1430 | 51.46 degrees |
| 1 GHz | 209.85 mm | 1.4296 | 514.64 degrees |
| 2.4 GHz | 87.439 mm | 3.4310 | 1235.14 degrees |
| 5.8 GHz | 36.182 mm | 8.2915 | 2984.92 degrees |
A phase display wrapped to one 360 degree cycle discards the number of full cycles. Two cables with the same displayed phase at one frequency can have different delays. Use unwrapped phase and appropriate delay measurements when absolute timing or broadband matching matters.
Specify the loss you can afford, not only the cable name
Send SigmaRF your frequency band, connector interfaces, installed length and insertion-loss target. Every assembly is VNA-swept for VSWR and insertion loss before shipment.
Specify a Custom AssemblyPhase matching requires a tolerance in degrees, a frequency or band, reference planes and a test configuration. A ruler tolerance alone does not cover dielectric variation, connector delay, bending or temperature effects. Specify the operating conditions that matter instead of requesting only two cables of the same length.
| RFQ item | Example request | Why it matters |
|---|---|---|
| Frequency | Match at 2.4 GHz | Degrees are frequency dependent |
| Phase tolerance | Pair difference within 5 degrees | Defines the allowable electrical mismatch |
| Reference planes | Connector mating faces | Includes connector contributions consistently |
| Routing | Measured in the agreed bend configuration | Movement can change measured phase |
| Temperature | State the required operating range | Phase stability is a separate requirement |
The values above are example requirements, not SigmaRF guarantees. Read SigmaRF's Phase-Matched Cable Assemblies guide for matched-set procurement and its Velocity Factor Table for cable-family context. This guide focuses on the calculation and its limits.
Assume 300 mm of propagation length at 1 GHz with VF = 0.70. Use an ideal uniform line and exclude additional connector delay for this calculation.
The 1.21 mm result is an equivalent ideal length difference, not a sufficient manufacturing tolerance for a real matched pair. Connectors and cable VF variation still contribute. Confirm the completed assemblies electrically.
How do I calculate wavelength inside coaxial cable?
Multiply the speed of light by the cable velocity factor, then divide by frequency in Hz. The resulting wavelength is in meters when c is expressed in m/s.
How many wavelengths is my coax cable?
Divide propagation length by guided wavelength, or use N = Lf/(c x VF). State the frequency and VF, because physical length alone cannot determine electrical length.
What is a quarter-wave coax cable length?
In the ideal line model it is c x VF/(4f). Real matching sections need connector and termination effects included; do not assume a nominal cut length alone gives the desired impedance transformation.
Does one extra millimeter of coax matter at GHz frequencies?
It can. At 2.4 GHz with VF = 0.70, 1 mm represents about 4.12 degrees of phase length. Whether that matters depends on the phase tolerance and system design.
Are equal-length coax cables phase matched?
Not necessarily. VF variation and connector delay can produce phase differences. A phase-matched set needs electrical verification at the specified reference planes, frequency and configuration.
Is cable delay the same as phase shift?
Delay is a time; phase shift is a fraction of a cycle at a particular frequency. In a simple nondispersive line, phase magnitude equals 360 x frequency x delay, but broadband delay and phase need careful measurement.
Calculate with guided wavelength, not free-space wavelength. Keep the units explicit and distinguish full electrical length from a wrapped phase reading. When phase matters to performance, make it a measured assembly requirement.
SigmaRF VNA-sweeps every assembly for VSWR and insertion loss before shipment. State any additional phase-match or phase-stability requirement separately. Its assembly range covers 100 MHz to 40 GHz, with Vadodara volume and Cleveland-area precision facilities and lead times as short as 2 days for suitable builds.
Specify the loss you can afford, not only the cable name
Send SigmaRF your frequency band, connector interfaces, installed length and insertion-loss target. Every assembly is VNA-swept for VSWR and insertion loss before shipment.
Standard AssembliesCustom AssembliesTalk to an Engineer