RF Coaxial Cable: Impedance First, Then Loss
Quick answer / TL;DR: Short answer: match the impedance to the system, then choose the cable by loss at your highest frequency and by the bend radius of the route. 50 ohm systems use RG58, RG213 or LMR types; 75 ohm systems use RG6, RG11 or hardline. Connectors must match both the impedance and the cable diameter.
Three coaxial families: BNC for CCTV, F-type for TV, N for RF – each with its impedance.RF coax comes in two impedances. 50 ohm is used for radio, test and instrumentation; 75 ohm for video, satellite and cable television.
Mixing them causes reflections that appear as loss, not as an obvious failure.
Common Types
| Cable | Impedance | Typical use |
|---|
| RG58 | 50 ohm | Short test leads |
| RG213 | 50 ohm | Radio installations |
| LMR-400 | 50 ohm | Low-loss runs |
| RG6 | 75 ohm | Satellite, CATV |
| RG11 | 75 ohm | Long 75 ohm runs |
Loss rises with frequency, so a cable that is fine at 100 MHz may be unusable at 2.4 GHz.
Choosing
- State the frequency range, not just the application
- Check loss at the top frequency
- Match connector to cable diameter
- Consider bend radius in cabinets
- Keep runs as short as practical
The bend radius point catches installers who choose a low-loss cable and then coil it tightly behind a radio.
Real-World Applications
A rooftop antenna feed at 400 MHz uses LMR-400 for low loss over 20 m. A satellite receiver uses RG6 at 75 ohm, where the impedance matches the LNB and the receiver.
Standards and Verification
| Reference | Relevance |
|---|
| IEC 61196 | Coaxial cable |
| IEC 61169 | RF connectors |
| SCTE | Distribution practice |
Loss curves for a specific cable are not provided in the knowledge base. Use the manufacturer's data at your frequency.
References
- IEC 61196-1, Coaxial communication cables
- IEC 61169 series, RF connectors
- SCTE operational practices
Practical Notes
Label 50 and 75 ohm cables differently. They look alike and only one belongs in each system.
Related reading: coax cable — blogs/news/coax-cable
Related reading: white coaxial cable — blogs/news/white-coaxial-cable
Related reading: rf cable extender — blogs/news/rf-cable-extender
Frequently Asked Questions
Does impedance matter at low frequency?
Less, but match it anyway.
Which is lower loss?
Depends on size and frequency, not impedance.
Can I mix 50 and 75 ohm?
No. Reflections follow.
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