Cable Assembly Buying Guide: How to Specify Custom RF Cable Assemblies
Most custom cable assembly problems are not manufacturing problems. They are specification problems. A buyer orders “an SMA cable, one metre,” and the factory has to guess the cable type, the connector gender at each end, the length tolerance, and what “good” means when the assembly is tested. Guesses get built in — and then the assembly fails in the field, or gets remade.
A complete specification has seven parts. Get them into the RFQ before you ask for a price, and most of the back-and-forth disappears.
1. Both connectors, both ends, stated in full
State the series, gender, and mounting style at each end — not just one end. “SMA to SMA” is not a specification: it could be plug-to-plug, plug-to-jack, straight-to-right-angle, or bulkhead-to-panel. Each combination is a different part.
Also state the mating side. If one end plugs into a board connector, a module, or an antenna, say what it mates with. A connector that is correct on paper and wrong against the mating part is still the wrong part.
2. The cable type, not just the diameter
Cable diameter alone does not define a cable. Two cables of the same diameter can differ in loss, flexibility, shielding, and power handling. Name the cable type — for example, a flexible RG-type, a low-loss LMR-type, a semi-rigid, or a micro-coax for board-to-board use — or state the properties the cable must meet: impedance, outer diameter limit, minimum bend radius, and shielding construction.
If the assembly runs outdoors, add the jacket requirement in the same line: UV resistance, temperature range, and whether the jacket must survive oil, abrasion, or salt air. A cable that is fine on a bench can fail in a year on a rooftop.
3. Length, and how much tolerance you allow
State the length as measured between defined points — connector tip to connector tip, or mating face to mating face — and state the tolerance. A one-metre assembly with a ±10 mm tolerance and one with a ±50 mm tolerance are different jobs, and the price and the fit in the enclosure both depend on which one you meant.
If the assembly must fit a fixed route inside a box, say so, and give the route length rather than the straight-line distance. Cable that is too long coils and stresses connectors; cable that is too short pulls on them.
4. Impedance and frequency, stated together
Impedance without frequency is half a specification. State both: 50 ohm (or 75 ohm where the system calls for it) and the operating band, including the top frequency the assembly must actually pass. Loss, VSWR, and connector choice all scale with frequency, so a cable that is fine at 2 GHz can be the wrong cable at 6 GHz.
If the system has a VSWR limit, put the number and the band in the RFQ, and say whether it applies to the finished assembly or to the connector alone. The two are not the same test.
5. Loss budget and shielding
For longer runs and higher frequencies, ask for the loss budget, not just “low loss.” State the maximum acceptable insertion loss at the top frequency, or ask the supplier to quote loss per metre at that frequency so you can compare like with like. A cable chosen on diameter alone often loses the budget at the top of the band.
Shielding matters wherever interference is present: near transmitters, motors, or dense board layouts. State the shielding requirement — single braid, double braid, or foil plus braid — or state the environment and let the datasheet decide. Do not assume the cheapest cable shields enough.
6. Bend radius and mechanical life
State the smallest bend the assembly must survive in service, and whether it will be bent once during installation or flexed repeatedly. A semi-rigid cable holds its shape and shields well but does not like repeated flexing; a flexible cable tolerates movement but gives up some loss and shielding performance. Choosing between them is a mechanical decision first, an electrical decision second.
Also state any strain relief, heat-shrink, labelling, or connector boot requirement. These look like details, but they are what keeps an assembly working after the tenth mating cycle instead of the first.
7. What testing you want, and what report comes with it
State the tests before the order, not after delivery. The usual set is continuity, and VSWR or return loss across the stated band; for higher-frequency or low-PIM work, add the specific test and its limit. Then say what you want to receive: a test report per batch, per assembly, or a sample report with the first shipment.
A test without a stated limit cannot be failed, and a limit without a stated test method cannot be verified. Both belong in the RFQ.
RFQ checklist: copy this into your inquiry
| Item | State this |
|---|---|
| Connector, end A | Series, gender, straight or right-angle, panel or cable mount |
| Connector, end B | Same, plus what it mates with |
| Cable | Type, or impedance, diameter limit, bend radius, jacket requirement |
| Length | Measured between which points, plus tolerance |
| Impedance and frequency | 50 or 75 ohm, operating band and top frequency |
| Loss and shielding | Maximum loss at top frequency, shielding construction or environment |
| Mechanical | Minimum bend radius, flexing or fixed, boots and labelling |
| Testing | Tests, limits, band, and what report you want |
Mistakes we see in RFQs
- Ordering by connector name only. “SMA cable” says nothing about gender, angle, cable, or length. Always state both ends.
- Stating length without tolerance or measuring points. The assembly arrives the wrong length for the enclosure, and nobody can say who was wrong.
- Stating frequency but not loss, or loss but not frequency. Both are needed; either one alone leaves the cable choice to guesswork.
- Forgetting the environment. Indoor cable on a rooftop, or a jacket that cannot take the temperature range, fails slowly and expensively.
- Asking for testing after delivery. Test method, limit, and report format belong in the RFQ, not in a complaint.
Conclusion
A custom cable assembly is only as good as its specification. Connectors at both ends, cable type, length with tolerance, impedance with frequency, loss and shielding, bend and environment, and testing with limits — seven items, written once, save a remake later. Send those seven with your drawing, and the quote you get back will be for the assembly you actually need.
Need custom RF cable assemblies built to your drawing? Send us your drawing and your specification — including the connector at both ends, cable type, length and tolerance, frequency band, and any test requirement — and we will confirm feasibility and quote against your specification. MOQ: 500 pcs | Samples: 15 days. Custom RF connector and cable assembly factory in Dongguan, China.