RG178 is the better starting point when a cable close to 2.5 mm in diameter cannot pass through the available routing path. RG316 requires more space, but its lower cable loss makes it more suitable when the route is longer or the RF loss budget is limited. The decision also depends on the connector rear body, cable exit direction, bend radius, heat shrink, and termination dimensions, because the finished cable assembly can occupy more space than the bare coaxial cable.
RG316 vs RG178 at a Glance
The table compares typical RG178 and RG316 specifications. Confirm the datasheet for the selected cable before approving the finished assembly.
| Comparison Point | RG178 Example | RG316 Example | Selection Meaning |
|---|---|---|---|
| Characteristic impedance | 50 ohms | 50 ohms | Both suit standard 50 ohm RF assemblies |
| Nominal cable diameter | 1.8 to 1.85 mm | Approximately 2.5 mm | RG178 needs less routing width |
| Attenuation at 1 GHz | Approximately 1.49 dB/m | Approximately 0.91 dB/m | RG316 leaves more cable loss margin |
| Static bend radius | At least 10 mm | At least 15 mm | RG178 can follow a smaller fixed bend path |
| Repeated bend radius | Approximately 18 mm | Approximately 25 mm | Repeated movement still requires controlled routing |
| Common dielectric and jacket | PTFE and FEP | PTFE and FEP | Temperature may not separate the two cable types |
| Example maximum operating temperature | Up to approximately 200°C | Up to approximately 200°C | Check the specific cable version before approval |
These figures are examples from specific RG178 and RG316 datasheets. Cable diameter, attenuation, bend radius, temperature rating, and connector compatibility can vary by manufacturer and cable construction.
How Much Routing Space Does Each Cable Need
RG178 is commonly about 1.8 to 1.85 mm in outside diameter, while RG316 is commonly about 2.5 mm. A difference of approximately 0.7 mm can determine whether the cable passes through a narrow slot, between PCB components, behind a display, or beside a shielded module.
Cable diameter is still only the first check. A finished RF cable assembly includes the connector rear body, crimp sleeve, center contact, heat shrink, boot, label, and sometimes a panel or bulkhead structure. The cable may fit through the path, while the assembled connector end may not have enough exit space.
Measure four dimensions on the enclosure drawing: the narrowest routing width, the available straight distance behind the connector, the location of the first bend, and the maximum diameter after heat shrink or a protective boot is installed. These dimensions show whether the cable, connector, and strain relief can fit as one assembly.
Choose RG178 When Cable Diameter Is the Hard Limit
RG178 is suitable when the route cannot accept cable close to 2.5 mm in diameter. It is commonly considered for short internal jumpers between a PCB, antenna, radio module, or internal RF port where the cable is installed once and remains fixed inside the enclosure.
The size advantage is most useful when the complete connector termination is also designed for RG178. A small cable connected to an oversized rear body, long heat shrink section, or unsuitable ferrule may not reduce the total routing envelope.
Cable length changes the loss tradeoff. Using a representative attenuation of approximately 1.49 dB/m at 1 GHz, a 100 mm RG178 cable contributes about 0.15 dB of cable loss. At 500 mm, the cable loss rises to about 0.75 dB before the connectors and termination transitions are included.
For other frequencies and cable lengths, the coaxial cable loss chart guide explains how to convert datasheet attenuation into estimated cable loss.
Choose RG316 When Loss Margin Matters More Than Diameter
RG316 is suitable when the routing path can accept cable close to 2.5 mm in diameter and lower attenuation is more important than saving approximately 0.7 mm of cable width. A representative RG316 value at 1 GHz is approximately 0.91 dB/m, compared with approximately 1.49 dB/m for RG178.
The difference is about 0.06 dB over 100 mm, 0.29 dB over 500 mm, and 0.58 dB over 1 m. The difference may have little influence on a very short internal connection, but it becomes more relevant as cable length increases or the available insertion loss budget becomes smaller.
RG316 is available with many cable-specific SMA, SMB, MCX, and MMCX connector versions. Mechanical performance still depends on the ferrule dimensions, crimp quality, heat shrink length, cable exit support, and handling conditions.
RG316 does not automatically produce better VSWR or return loss. Those results belong to the finished cable assembly and depend on the cable, connector, center contact, strip dimensions, termination process, bend condition, and test frequency.
Finished assembly insertion loss also includes connector and termination transitions, so it should be evaluated separately from the cable attenuation value alone.
Check Bend Radius and Cable Exit Space Separately
A cable that feels flexible in hand can still fail in a tight assembly if the bend starts too close to the connector. The most sensitive area is often the cable exit point behind the connector. If the cable is bent sharply right after the crimp sleeve or heat shrink, stress can concentrate at the termination area.
A bend below the specified cable or assembly limit can deform the shield and dielectric, creating a local impedance change. The resulting discontinuity may increase insertion loss or worsen return loss.
In one representative cable family, RG178 has a static bend radius of at least 10 mm and a repeated bend radius of approximately 18 mm. The corresponding RG316 values are at least 15 mm for a static bend and approximately 25 mm for repeated bending. These values describe the cable and do not automatically define the bend limit of the terminated assembly.
Heat shrink and protective boots can change the bend behavior. A short RF jumper may use a very thin cable, but the heat shrink at the connector end can make the first part of the cable stiffer. When the layout requires the cable to turn immediately, the connector direction and rear body length may matter more than the cable type.
When the cable must turn 90 degrees immediately behind the port, compare a right angle connector with the straight version. A right angle connector changes the cable exit direction within the connector body and reduces the need for an immediate sharp bend.
Connector Fit: Same Interface, Different Cable Termination
RG178 and RG316 can both be terminated with interface families such as SMA, MCX, MMCX, SMB, or BNC. The mating interface may be the same, but the cable entry structure usually needs to match the selected cable.
The cable side of the connector includes the center contact, dielectric support, ferrule or crimp sleeve, cable entry bore, strip dimensions, and termination method. A connector prepared for RG316 may not correctly grip the smaller RG178 jacket and braid. A connector prepared for RG178 may not provide enough space for RG316.
For module-side micro coax connections, cable diameter and connector compatibility should be evaluated separately from the RG178 vs RG316 decision. Smaller micro coax options may be more suitable when the connector itself sets the cable diameter limit.
FAQ
Conclusion
Choose RG178 when the route cannot accommodate cable close to 2.5 mm in diameter and the assembly is a short, fixed internal connection with enough RF loss margin. Choose RG316 when the enclosure has enough space and lower cable attenuation carries more weight than saving approximately 0.7 mm of cable diameter. Before confirming either option, check the operating frequency, cable length, connector part number, cable preparation dimensions, first bend point, heat shrink dimensions, insertion loss, and VSWR requirements. If you are comparing RG178 and RG316 for a custom RF cable assembly, you can share these specifications with the Bafitop team for review.