Why does bend radius matter for RF cables?

Bending a coaxial cable tighter than its MBR causes: (1) Impedance change: the cable geometry (D/d ratio) is distorted at the bend, changing the characteristic impedance from the nominal 50 ohms. This creates a reflection point (VSWR bump) at the bend location. (2) Increased insertion loss: the distorted geometry increases conductor losses and dielectric losses at the bend. (3) Dielectric damage: foam dielectrics can crush, permanently altering the impedance. Solid PTFE can crack. Both are irreversible. (4) Conductor damage: the center conductor may shift off-center, or the outer conductor may kink (especially copper tube in semi-rigid cable). (5) Shield damage: braid shields can open up at the outside of a tight bend, reducing shielding effectiveness. (6) Phase instability: bending a cable changes its electrical length. For phase-sensitive applications (antenna arrays, test cables), cables with phase stability specifications are required.

What are the MBR rules for different cable types?

Solid dielectric (PE, PTFE): MBR = 10× cable OD. Example: RG-316 (0.098 inch OD): MBR = 1 inch. Foam dielectric (low-loss cables): MBR = 15× OD. Foam is softer and compresses more easily. Example: LMR-400 (0.405 inch OD): MBR = 1.0 inch (single bend), 4.0 inches (repeated). Semi-rigid (solid copper outer): MBR = 5× OD for a single formed bend. NEVER re-bend once formed. Example: 0.141 inch semi-rigid: MBR = 0.71 inch. Corrugated hardline: MBR = 10× OD per manufacturer. Example: 7/8 inch hardline: MBR = 10 inches. Flex cables (conformable): designed for repeated flexing. MBR = 3-5× OD. Rated for 100,000+ flex cycles. More expensive. Installation vs storage: MBR during installation (pulling through conduit, routing around corners) is typically stricter than static MBR because the cable is also under tension.

How do you detect bend damage?

(1) DTF (Distance-to-Fault) measurement: use a VNA or cable analyzer in DTF mode to look for return loss peaks along the cable length. A damaged bend shows up as a localized reflection at the bend location. (2) VSWR sweep: measure VSWR across the operating frequency range. A damaged cable shows elevated VSWR compared to its original sweep test data. (3) Visual inspection: look for kinks (visible deformation), flat spots (crushed dielectric), or outer conductor wrinkles (corrugated cable). (4) Insertion loss comparison: compare current insertion loss against the cable's factory test data. Excessive loss indicates damage. (5) Phase measurement: for phase-sensitive applications, measure phase vs frequency and compare against baseline. Phase deviation indicates impedance changes from bend damage.

Cable Installation

Cable Bending

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Minimum bend radius (MBR): solid dielectric 10×OD, foam 15×OD, semi-rigid 5×OD (single bend, no re-bend), corrugated 10×OD, flex 3-5×OD (100K+ cycles). Below MBR: impedance change, VSWR bump, dielectric crush, conductor shift, shield opening. Detect: DTF, VSWR sweep, visual inspection.

Solid: 10×OD

Foam: 15×OD

Semi-rigid: No re-bend

Understanding Cable Bending

Every RF cable has a minimum bend radius that must be respected during installation. Violating it is the number one cause of field-installed cable failures. The damage is often invisible from the outside but shows up clearly on a cable sweep as a VSWR bump at the bend location. Once a cable is kinked, the damage is permanent and the cable must be replaced. Proper routing, bend support, and cable management prevent these costly failures.

MBR by Cable Type

Cable Bending:
Minimum bend radius (MBR): solid dielectric 10×OD, foam 15×OD, semi-rigid 5×OD (single bend, no re-bend), corrugated 10×OD, flex 3-5×OD (100K+ cycles). Below MBR: impedance change,...

Key specifications:
100 K | 10.3 mm | 1 a | 2.5 mm

Power: P(dBm) = 10log(P_mW), 0dBm = 1mW

Cable Type Bend Radius

Cable	OD	MBR (single)	MBR (flex)	Dielectric
RG-316	2.5mm	25mm	50mm	PTFE solid
RG-58	5.0mm	50mm	100mm	PE solid
LMR-400	10.3mm	25mm	100mm	Foam PE
0.141 semi	3.6mm	18mm	N/A	PTFE solid
7/8" hardline	22mm	250mm	N/A	Air/foam

Key Equations

Decibel conversion:
Power: dB = 10log(P₂/P₁)
Voltage: dB = 20log(V₂/V₁)

dBm to watts:
P(W) = 10^{(dBm−30)/10}
0 dBm = 1 mW, +30 dBm = 1 W

Wavelength:
λ = c/f = 300/f(MHz) meters

Comparison

Connector	Freq Max	Impedance	Power	Interface
SMA	18 GHz	50 Ω	0.5 W	Threaded
N-Type	11 GHz	50 Ω	5 W	Threaded
2.92mm (K)	40 GHz	50 Ω	0.3 W	Threaded
1.85mm (V)	67 GHz	50 Ω	0.2 W	Threaded
1.0mm (W)	110 GHz	50 Ω	0.1 W	Threaded

Common Questions

Frequently Asked Questions

Why it matters?

Below MBR: impedance distortion (VSWR bump), dielectric crush (permanent), conductor shift, shield opening, increased loss. Foam is most fragile. Semi-rigid: never re-bend. Most common field cable failure cause.

MBR rules?

Solid PE/PTFE: 10×OD. Foam: 15×OD. Semi-rigid: 5×OD single bend only. Corrugated: 10×OD. Flex/conformable: 3-5×OD, rated 100K+ cycles. Installation MBR stricter than static (tension during pull).

Detect damage?

DTF: localized RL peak at bend. VSWR sweep: elevated vs factory data. Visual: kinks, flat spots, wrinkles. IL comparison vs baseline. Phase deviation for phase-sensitive systems. Damage is permanent; replace cable.

Related Terms

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