Phase & Electrical Length

Phase depends on frequency and delay

Two RF paths can have the same physical length but different phase responses because their propagation velocities differ. For a fixed, non-dispersive signal delay, the phase lag in degrees is 360 multiplied by frequency in hertz and delay in seconds. As frequency changes, that phase offset changes too.

A cable, transmission-line section, or mechanical phase shifter may therefore show one phase offset at one frequency and a different offset elsewhere in its operating band. The actual response must be checked against the component’s measured or specified characteristics.

Electrical length versus physical length

Electrical length measures how much phase a signal accumulates while propagating along a path. It depends on propagation speed and frequency, not simply the number of centimeters of material. Connectors, bends, dielectric materials and mechanical transitions can affect the overall phase response.

At a single frequency a full 360-degree phase rotation represents one cycle; however, an instrument measuring phase modulo 360 degrees cannot distinguish phase changes separated by whole cycles without additional information.

Important specifications

• Frequency range over which the stated phase performance applies
• Phase adjustment span, resolution and repeatability
• Insertion loss and how loss changes across phase settings
• Return loss or VSWR across the selected band
• Connectors, power handling, temperature and mechanical mounting

Measurement and calibration

Electrical phase measurements depend on where the reference plane is established. Cables and fixtures between that plane and the component add their own phase delay. Calibrate and document the test arrangement before comparing two parts or two phase settings.

See What Is an RF Phase Shifter? or browse Impulse phase shifters for applicable component options.