What Are RF Coaxial Cables?
RF coaxial cables are transmission-line cables used to carry radio-frequency signals between antennas, RF modules, test instruments, filters, amplifiers, wireless devices and PCB assemblies. A coaxial cable has a center conductor, dielectric insulation, an outer shield and a jacket. This circular structure helps the signal travel with controlled impedance while reducing radiation and outside interference.
For PCB engineers, RF coaxial cables matter because the cable is only one part of the signal path. The signal also passes through the connector, the PCB launch, the ground return, the trace, the matching network and sometimes the antenna feed. A good RF cable cannot fix a poor connector footprint, weak ground stitching or uncontrolled PCB impedance. The whole path should be reviewed as one RF interconnect.
This article explains how to choose and use RF coaxial cables from a PCB design and assembly point of view: impedance, loss, shielding, connector type, bend radius, PCB launch layout, inspection and test.
Why Coaxial Cable Is Used for RF Signals
At low frequency, many wires can carry a signal without careful impedance control. At RF frequencies, the cable behaves as a transmission line. The physical geometry of the conductor, dielectric and shield determines the characteristic impedance and affects reflection, insertion loss, phase stability and noise pickup.
RF coaxial cable is widely used because it provides:
- a defined signal path and return path;
- stable impedance such as 50 ohms or 75 ohms;
- shielding against external noise and unwanted radiation;
- mechanical flexibility for board-to-board or board-to-antenna connections;
- a practical interface for test equipment such as spectrum analyzers, VNAs and RF signal generators.
In many RF products, the PCB is designed around a coaxial interface. The antenna, enclosure, cable, connector and PCB stackup should be considered together.
50 Ohm vs 75 Ohm RF Coaxial Cables
The two most common coaxial cable impedances are 50 ohms and 75 ohms. They are not interchangeable without consequences.
| Impedance | Common use | PCB design note |
|---|---|---|
| 50 ohm | RF modules, wireless products, antennas, RF test equipment, transmitters and receivers | Most RF PCB traces, SMA connectors and lab test setups are designed around 50 ohms. |
| 75 ohm | Video, broadcast, CATV, satellite distribution and some measurement systems | Do not connect to a 50 ohm PCB path unless the mismatch is acceptable or a matching method is used. |
| Other impedances | Specialized RF, instrumentation, pulsed systems or custom assemblies | Confirm the complete interconnect impedance before routing the PCB or selecting connectors. |
If a 75 ohm cable is connected into a 50 ohm RF path, part of the signal reflects at the mismatch. At low power or short cable length, the system may still work. At higher frequency, higher power or tighter RF performance requirements, that mismatch can reduce power transfer, create ripple and make test results confusing.
Key Cable Parameters Engineers Should Check
RF coaxial cables are often selected too late in the project. The cable should be chosen early enough to affect connector placement, enclosure clearance, bend radius and test planning.
Important parameters include:
- Characteristic impedance: usually 50 ohms for RF PCB work, but confirm the system requirement.
- Frequency range: the cable, connector and assembly should support the highest operating frequency with margin.
- Insertion loss: longer cables and higher frequencies increase loss.
- Return loss or VSWR: poor impedance matching creates reflections.
- Shielding effectiveness: important when the product is close to noisy power circuits, radios or digital clocks.
- Power handling: depends on cable size, dielectric, temperature, frequency and connector.
- Bend radius: sharp bends can damage the cable geometry and change impedance.
- Phase stability: important in test, radar, phased-array, timing or multi-channel RF systems.
- Connector type: SMA, SMB, MMCX, U.FL/IPEX, BNC, N-type and others have different size, frequency and mechanical limits.
For mass production, also check cable assembly tolerance, strain relief, locking method, cable routing, pull force and whether the assembly process can repeat the same routing path.
RF Coaxial Cable Types and Typical Tradeoffs
There is no universal best coaxial cable. The right choice depends on frequency, loss budget, enclosure space, flexibility and mechanical reliability.
| Cable style | Strength | Design caution |
|---|---|---|
| Flexible coax | Easy routing inside products, useful for antenna pigtails and board-to-board RF links | Loss and phase can change with cable movement, bend radius and assembly handling. |
| Semi-rigid coax | Stable geometry, good shielding and repeatable RF performance | Needs careful forming and mechanical planning; not suitable for repeated flexing. |
| Low-loss coax | Lower attenuation for longer runs or higher-frequency applications | Usually larger, less flexible and more expensive than thin pigtail cables. |
| Micro coax | Small size for compact wireless, IoT, wearable or embedded modules | Connector handling, mating cycles and cable strain relief become critical. |
How Cable Loss Affects RF PCB Performance
Cable loss is not just a catalog number. It affects the link budget, output power, receiver sensitivity and test result. Loss increases with frequency and cable length. A cable that looks acceptable at 100 MHz may be unsuitable at several GHz.
For example, if an RF transmitter output is measured through a lossy test cable, the measured power at the instrument may be lower than the actual board output. If the loss is not calibrated or documented, engineers may tune the board incorrectly. The same problem can appear during antenna matching: the cable becomes part of the measured system.
Good practice:
- define the allowed cable loss at the working frequency;
- keep RF cables as short as practical;
- avoid unnecessary adapters;
- use a known cable for lab measurements;
- document whether the test setup includes cable loss compensation;
- replace damaged or unstable test cables before blaming the PCB.
PCB Connector Launch: Where Many RF Problems Start
The transition from coaxial cable to PCB trace is called the connector launch. This area often causes more trouble than the cable itself. A connector may be rated for the frequency, but the PCB footprint and ground structure must also support that frequency.
The connector launch should be reviewed for:
- correct connector footprint and land pattern;
- controlled 50 ohm trace geometry from the connector pad;
- short and smooth transition into the RF trace;
- ground vias close to the connector ground pads;
- no unnecessary stubs, test pads or copper gaps;
- enough clearance from noisy digital, switching power or high-current circuits;
- solid RF return path under or beside the launch;
- mechanical reinforcement if the cable can pull on the connector.
If the PCB uses a multilayer stackup, the connector launch should be matched to the actual dielectric thickness, copper thickness and solder mask condition. Copying a connector footprint without checking the stackup can create impedance discontinuity.
SMA, U.FL, MMCX and Other RF Connectors
The cable connector and PCB connector must be chosen together. A large SMA connector may be robust and easy to test, but it may be too large for a compact product. A U.FL-style micro connector saves space, but it has limited mating cycles and needs careful assembly handling.
| Connector | Typical use | PCB/assembly note |
|---|---|---|
| SMA | RF modules, test ports, antennas, lab-accessible products | Check launch geometry, edge clearance, ground vias and mechanical stress from repeated cable mating. |
| MMCX/MCX | Compact RF products and internal cable assemblies | Confirm retention force, cable routing and connector height inside the enclosure. |
| U.FL / IPEX style | Wi-Fi, Bluetooth, GPS, LTE, IoT modules and small antennas | Use the exact footprint, protect against cable pull and avoid excessive re-mating during production test. |
| BNC / N-type | Test equipment, industrial RF systems and higher-power external interfaces | Board edge, panel cutout, enclosure grounding and mechanical load should be planned early. |
RF PCB Layout Checks Around Coaxial Cable Connections
When a coaxial cable connects to the PCB, the PCB layout should protect impedance continuity and return current. The following checks are useful before fabrication.
Keep the RF Path Short and Direct
The connector pad should transition into the RF trace without a long neck-down, unnecessary jog or unused copper stub. Every discontinuity can add reflection. At higher frequencies, even a small extra pad or via stub may affect performance.
Stitch Ground Near the Connector
The coax shield connects to ground. The PCB must provide a low-inductance return path. Place ground vias near connector ground pads and along coplanar waveguide edges where required by the design. Avoid splitting the ground plane under the launch.
Match the Trace to the Stackup
A 50 ohm microstrip or coplanar waveguide depends on dielectric thickness, dielectric constant, copper thickness, solder mask and trace width. Do not use a generic trace width without confirming the stackup. If impedance matters, ask for an impedance-controlled stackup before routing is finalized.
Separate RF from Noise Sources
Keep the coax connector and RF trace away from switching regulators, fast digital buses, clock lines, relays and high-current loops. A shielded cable helps, but poor PCB placement can still inject noise into the RF path.
Plan Test Access Without Creating Stubs
RF test points are useful, but they can damage impedance if added carelessly. If a test port is needed, design it as part of the RF path rather than adding a random pad to the trace.
Assembly Risks with RF Coaxial Cables
RF cable problems are often mechanical first and electrical second. A cable that is bent too sharply, pulled during assembly or routed across a hot component can become unstable.
For PCB assembly, check:
- cable length tolerance and routing path;
- minimum bend radius;
- connector mating direction and assembly tool access;
- whether the cable blocks screws, shields, heat sinks or enclosure clips;
- strain relief and adhesive requirements;
- risk of connector lift-off during handling;
- production test method and allowed re-mating cycles;
- whether the final product needs vibration, drop or thermal cycling review.
For micro coax connectors, operators should use proper insertion/removal tools when required. Pulling the cable instead of the connector body can damage the connector or change RF performance.
Common RF Coaxial Cable Problems
Many RF failures appear as poor range, unstable output power, weak receiver sensitivity or inconsistent test data. The cable is not always the cause, but it should be checked early.
| Symptom | Possible cause | What to check |
|---|---|---|
| High return loss or poor VSWR | Impedance mismatch, poor connector launch, damaged cable or wrong cable impedance | Cable type, connector footprint, ground vias, trace width and VNA calibration |
| Unexpected low RF power | Cable loss, adapter loss, bad connector mating or board output issue | Known-good cable, calibrated loss, connector torque and direct board measurement point |
| Test result changes when cable moves | Poor cable stability, loose connector, damaged shield or weak strain relief | Cable bend, connector retention, fixture repeatability and cable replacement history |
| Poor antenna range | Mismatch, cable loss, antenna placement, enclosure detuning or noisy PCB environment | Antenna matching, coax route, ground clearance, enclosure material and RF noise sources |
What Files Should You Share for PCB Review?
If the PCB includes RF coaxial cable connections, QFPCB can review the fabrication and assembly risks more effectively when the RF interface information is included with the PCB files.
Useful files and notes:
- Gerber files and drill files;
- PCB stackup, board thickness and copper thickness;
- RF connector part number and footprint source;
- target impedance and frequency range;
- antenna or cable assembly drawing;
- enclosure drawing or cable routing requirement;
- BOM and assembly drawing if PCBA is required;
- any VNA, RF power or antenna test requirement.
If the board needs impedance control, include the impedance target and reference layer. If the cable connects to an antenna, include the antenna location and keepout requirement so the PCB layout can be reviewed in context.
How QFPCB Supports RF Cable-to-Board PCB Projects
QFPCB does not need to treat the coaxial cable as a separate afterthought. For RF PCB projects, we can review the PCB data around the cable-to-board connection and help identify manufacturing or assembly risks before production.
Typical support includes:
- RF connector footprint and PCB launch review;
- stackup and impedance-control discussion;
- DFM checks for RF trace width, spacing, ground via placement and solder mask;
- PCB fabrication for RF, wireless and antenna-related boards;
- PCB assembly review for connector orientation, soldering and cable handling;
- test planning support when the customer defines RF acceptance requirements.
For related RF design checks, see What Is an Unun in RF Antenna Design? and PCB Design Services. If the RF board will be assembled, review PCB Assembly early so connector handling and test access are planned before production.
RF Coaxial Cable FAQ
Are RF coaxial cables always 50 ohms?
No. Many RF systems use 50 ohm coaxial cable, but 75 ohm cable is common in video, broadcast and CATV applications. Specialized systems may use other impedances. The cable, connector and PCB trace should match the system requirement.
Can I use a longer RF coaxial cable if the connector fits?
Mechanically yes, but electrically it may reduce performance. Longer cable increases insertion loss, especially at higher frequencies. It can also affect phase, test calibration and antenna matching.
Why does the PCB connector launch matter?
The cable may have controlled impedance, but the signal must pass through the PCB connector footprint and trace. Poor grounding, wrong trace width, extra stubs or an unsuitable stackup can create impedance discontinuity even when the cable is good.
Should the RF cable be chosen before PCB layout?
Yes, at least the connector type, cable direction, bend radius and impedance should be known before the PCB is finalized. Late cable selection can force poor connector placement or unsafe cable routing.
What is the difference between cable loss and return loss?
Cable loss usually means signal power lost as the signal travels through the cable. Return loss describes how much signal is reflected by impedance mismatch. Both can reduce RF performance, but they point to different problems.
Final Design Advice
RF coaxial cables are simple in appearance, but they are part of a sensitive RF transmission path. Choose the cable impedance, loss, connector and routing method early. Then review the PCB connector launch, ground return, stackup, assembly handling and test setup as one system.
If your RF PCB uses coaxial cable, antenna pigtails, SMA test ports or cable-to-board RF connectors, send the Gerber files, stackup, connector part number and frequency requirement to QFPCB for review before production.
