Published on 10/13/2026
A coax datasheet describes one cable construction under stated conditions. Read attenuation at the operating frequency and convert its length units; read velocity of propagation as a fraction of light speed; read power with its thermal and mismatch conditions. None of those cable-only lines automatically rates a finished assembly.
A spreadsheet can be wrong by a factor of 30.48 before a single connector is fitted: 12.4 dB/100 ft copied as 12.4 dB/m is a common unit failure. More subtle errors come from mixing nominal cable loss, maximum assembly loss and a voltage-withstand test.
This guide makes a coax datasheet explained in practical engineering terms, using published cable examples and labeled calculations. SigmaRF can use your selected cable and interfaces to agree a build and acceptance sweep, but the drawing must identify what the datasheet does and does not cover.
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Start with the part number, revision or current document, construction and intended environment. A family such as RG58 or RG316 can contain different conductors, shields and jackets. Similar diameter does not prove similar RF or mechanical behavior.
| Datasheet field | What to identify | Common trap |
|---|---|---|
| Conductor | Material, plating, solid/stranded and diameter | Calling copper-clad steel solid copper |
| Dielectric | Material and construction, such as solid PTFE or foam PE | Assuming all 50 Ω cables share velocity |
| Shield | Foil/braid layers and any measured screening data | Equating braid coverage with dB shielding |
| Jacket | Material, diameter and environment/fire approvals | Treating a material name as a building approval |
| Impedance | Nominal value and tolerance | Matching only connector size |
| Qualification | Exact source and evidence where needed | Family name treated as QPL proof |
For example, commercial reference RG316 83284 explicitly says non-QPL even though it cites M17/113-RG316. Its PTFE/FEP construction is different from PE/PVC reference RG58 8259. Keep source and qualification requirements on the purchase specification rather than assuming them from a cable name.
Key idea: A cable datasheet is input to the assembly design. The complete product needs its own interfaces, routing and acceptance limits.
A cable attenuation table gives loss per unit length at selected frequencies. Confirm whether the values are nominal, typical or maximum and note the stated temperature and impedance conditions. Compare different cables only after matching frequency and length units.
| Published value | Equivalent dB/m | Use |
|---|---|---|
| 12.4 dB/100 ft | 12.4 / 30.48 = 0.407 | Reference RG58 8259 at 400 MHz |
| 47.2 dB/100 ft | 47.2 / 30.48 = 1.549 | Reference RG316 83284 at 2.4 GHz |
| 12.8 dB/100 m | 12.8 / 100 = 0.128 | Standard LMR-400 typical at 900 MHz |
Loss for a uniform cable is approximately attenuation in dB/m multiplied by length in m. Connector and mismatch effects require separate treatment. If you add hypothetical connector allowances, label them as assumptions and validate the completed assembly. Do not call the result a measured insertion-loss guarantee.
Table endpoints are not automatically the cable's maximum operating frequency. A selected product may separately state a rated band; otherwise obtain the needed performance evidence. Avoid extrapolating a nominal attenuation line into an RF acceptance claim. SigmaRF's signal-loss and VNA measurement guides cover the measurement side.
Velocity of propagation, VP or velocity factor, is the signal speed as a fraction of the speed of light. A listed 66% means approximately 0.66c, not 66 m/s. It relates physical length to electrical delay, but nominal VP is not a tight phase-matching tolerance.
| Nominal VP | Approximate delay | Engineering use |
|---|---|---|
| 66% | 5.05 ns/m | Common solid-PE reference example |
| 70% | 4.77 ns/m | Common PTFE reference example |
| 84% | 3.97 ns/m calculated from VP | Foam-dielectric reference example |
Calculate delay as length divided by c times VP. Nominal VP and a separately tabulated delay can differ slightly through rounding or characterization; use the supplier's specified value for the approved design. For phase-sensitive work, ask for electrical-length tolerance and measurement rather than cutting two cables to equal mechanical length.
Phase changes with frequency, physical length and dielectric behavior. A datasheet VP line does not prove stability after movement or temperature cycling. SigmaRF's semi-rigid guides discuss fixed-routing alternatives; choose the actual construction and test conditions for the phase requirement.
RF power handling is both an electrical and thermal problem. Average CW power, peak power, operating voltage and dielectric-withstand voltage describe different failure limits. Read every footnote, not only the largest number.
| Rating | Meaning | What else to check |
|---|---|---|
| Average/CW power | Sustained thermal capability under stated conditions | Frequency, ambient, altitude, VSWR, duty and cooling |
| Peak power | Short-duration electric-field limit under stated assumptions | Pulse width, duty and connector limits |
| Operating voltage | Permitted voltage under a defined service condition | AC/DC/RMS definition and altitude |
| Dielectric test voltage | Test stress for specified duration/method | Not a continuous service limit |
| Jacket spark voltage | Manufacturing jacket test | Not RF power or connector withstand |
One low-loss cable datasheet bases its power figures on matched VSWR, +40 °C ambient, a specified inner-conductor temperature, sea level, dry air and no solar loading. Those are conditions, not decoration. A hot roof, mismatch or restrictive enclosure can change the safe result.
⚠️ Do not derive assembly RF watts from a hipot line or assume a connector can carry the cable's listed power. Obtain a cable-and-connector derating basis for the application.
Specify the complete RF assembly
Send the frequency band, connector interfaces, finished length and routing drawing. SigmaRF can match the cable and terminations to your application.
Static, installation and repeated bend limits are different. A minimum radius is not proof of unlimited flex cycles. Jacket operating temperature may differ from installation temperature, and another component can narrow the assembled product range.
The reference RG316 examples illustrate the problem: one lists a 25 mm installation radius, another lists 15 mm static and 25 mm repeated bending. You cannot combine the smallest radius with another part's preferred rating. Name the approved construction and use its actual instructions.
Fire class, UV suitability, burial, water blocking and sealing are separate installation questions. A PVC, FEP or PE material name does not answer all of them. Cable suitability also does not make exposed connector interfaces weatherproof.
Step 01
Freeze the approved components
Record exact cable and connector parts, material requirements, source qualification and compatible cable group. Resolve contradictions in supplier documents before releasing the drawing.
Step 02
Define the geometry and service
Give finished length, datums, tolerance, route, bend limits, strain relief, environment and motion. Add electrical-length limits when phase or delay matters.
Step 03
Set the acceptance criteria
Specify VSWR and insertion loss across the operating band, calibration/reference planes and report requirements. Add insulation, power or environmental validation separately when needed.
SigmaRF VNA-sweeps every assembly for VSWR and insertion loss before shipment. Its overall RF assembly capability spans 100 MHz to 40 GHz across suitable cable and connector systems, not every product discussed here. Volume work is supported in Vadodara, India, with precision work in the Cleveland area, USA. Lead times can be as short as 2 days for suitable builds, subject to parts, quantity and the agreed specification.
Company-wide capability is not a substitute for cable/connector performance evidence. SigmaRF's RF assembly design guide helps turn the physical route into a repeatable specification. Its loss-measurement guide helps keep fixture and calibration differences from being mistaken for cable defects.
Assume a 2 m cable with nominal 12.4 dB/100 ft attenuation at 400 MHz and nominal VP of 66%. For an illustrative budget only, assume 0.05 dB per connector. Use c = 299,792,458 m/s.
Attenuation = 12.4 / 30.48 = 0.407 dB/m
Cable loss = 0.407 x 2 = 0.814 dB
Estimated matched loss = 0.814 + 0.05 + 0.05 = 0.914 dB
Delay = 2 / (299792458 x 0.66) = 10.11 ns
Matched power ratio = 10^(-0.914/10) = 0.810, about 81.0%
That calculation produces a nominal design estimate, not a maximum acceptance result. A real assembly still needs variation allowance and a defined measurement. Do not assume that a nominal VP of 66% specifies delay to the displayed 0.01 ns precision.
Avoid these datasheet shortcuts
Useful reading order
How do I read a cable attenuation table?
Match frequency, confirm nominal/typical/maximum status and convert length units. Multiply dB per length by cable length, then account separately for connector and mismatch effects.
What does velocity of propagation mean in coax?
It is signal speed relative to light speed. A 66% velocity factor means about 0.66c. It helps estimate delay, but does not by itself specify precision phase matching.
How do I convert dB/100 ft to dB/m?
Divide by 30.48 because 100 ft is 30.48 m. For example, 12.4 dB/100 ft is about 0.407 dB/m.
Is cable test voltage the operating voltage?
No. Test/withstand voltage applies a specified stress for a defined method and duration. Continuous operating voltage and RF power are separate limits.
Does a maximum cable frequency rate my assembly?
Not alone. The connectors, termination, routing and agreed VSWR/insertion-loss acceptance band also matter.
What should I send SigmaRF for a custom assembly?
Provide approved cable and connector parts, finished length, routing, environment, operating band and electrical acceptance limits. Add power, phase or qualification evidence when required.
Reading a coax datasheet correctly is mostly about definitions and conditions. Units, nominal-versus-maximum status and the difference between cable and assembly limits decide whether the numbers can be used.
Turn those numbers into an approved drawing and test specification. That produces a repeatable RF interconnect rather than a collection of plausible cable claims.
Build an RF assembly around the real requirements
Send the frequency band, connector interfaces, finished length and routing drawing. SigmaRF can match the cable and terminations to your application.