מהם כבלי RF יציבי פאזה?
A phase stable RF cable is one that maintains consistent electrical length — and therefore consistent phase delay — when the cable is flexed, repositioned, or subjected to temperature changes. For most signal transmission applications, small phase shifts are irrelevant. But in precision measurement and coherent RF systems, phase instability is a source of error that cannot be calibrated out.
Why Phase Stability Matters
In vector network analyzer (VNA) measurements, the reference plane is established by calibration. Any change in cable electrical length after calibration — from repositioning the cable, changing the test fixture, or ambient temperature shifting — moves that reference plane and introduces phase error in every subsequent measurement. This error is indistinguishable from the device under test's actual phase response. For phased array radar and satellite systems, matched phase across antenna feed lines determines beam pointing accuracy. A 1-degree phase mismatch across a 64-element array introduces beam steering error. For these applications, phase stability is not an optional specification — it is a system requirement.
What Causes Phase Shift in Cables
Phase shift in RF cables results from changes in the cable's electrical length — the product of physical length and velocity of propagation. Three mechanisms drive phase instability: dielectric compression when the cable bends changes the relative permittivity of the dielectric material; conductor displacement relative to the shield alters the geometry of the transmission line; and thermal expansion of the dielectric changes the physical length as temperature varies. In conventional PTFE-dielectric cables, bending compresses the solid dielectric directly against the center conductor, causing measurable changes in permittivity and phase. The effect is more pronounced at higher frequencies where even small dimensional changes represent significant fractions of a wavelength.
ePTFE Dielectric: The Engineering Solution
Junkosha's solution to flex-induced phase instability is the ePTFE (expanded PTFE) tape-wrapped dielectric construction used across the EMF series. Unlike solid PTFE, ePTFE is a porous, low-density form of PTFE with a dielectric constant significantly lower than solid PTFE (approximately 1.4–1.6 vs. 2.1). The tape-wrapped construction creates a mechanical decoupling between the center conductor and the outer jacket — when the cable bends, the ePTFE tape accommodates the strain without transmitting it to the conductor geometry. The result is dramatically reduced flex-induced phase shift compared to solid PTFE cables, and improved thermal phase stability due to the lower thermal expansion coefficient of ePTFE versus PTFE foam or air-spaced designs.
How Phase Stability Is Specified
Phase stability is typically specified as maximum phase deviation (in degrees) under a defined test condition — such as bending the cable through a 360-degree loop of a specified diameter, or cycling temperature through a defined range. A well-specified phase-stable cable will publish separate specifications for mechanical phase stability (flex) and thermal phase stability. Typical values for precision VNA cables are 1–5 degrees of phase change over a defined bend at the rated maximum frequency. Cables marketed as 'phase stable' without published numerical specifications for these conditions should be treated skeptically. Keysight and Rohde & Schwarz specify maximum phase deviation of port cables used with their VNA systems — Junkosha's EMF series is designed to meet these requirements.
Phase Matched Cable Pairs
For differential signal applications — balanced measurement ports, two-port coherent radar, MIMO antenna feed systems — matched phase between a cable pair matters as much as absolute phase stability. A phase-matched pair specifies the maximum difference in electrical length between two assemblies at a given frequency. Junkosha's EMF-06 is designed specifically for skew matching, with pairs matched to sub-picosecond differential delay at frequencies up to 120 GHz. Phase-matched pairs are ordered as sets; field-replacing one cable of a matched pair with an unmatched spare will degrade differential performance. When ordering matched pairs, specify the required skew tolerance and the measurement frequency.
Junkosha Phase Stable Cables
The Junkosha EMF series provides phase-stable options across the full microwave and millimeter wave frequency range. EMF-00 is the flagship precision cable for 26.5–145 GHz metrology work, used by calibration labs and VNA manufacturers for reference measurements. EMF-03 is optimized for coherent applications including radar and satellite test. EMF-07 is the general-purpose phase-stable cable for 5G testing and VNA port applications, balancing phase stability with flex durability for production environments. All EMF series cables use ePTFE tape-wrapped dielectric and are available through Koto Electronics in Israel.