What a dB Means
A decibel (dB) is a ratio. For power lost in a cable:
| Loss | Power left |
|---|---|
| 1 dB | 79% (about four-fifths) |
| 2 dB | 63% |
| 3 dB | 50% (half) |
| 6 dB | 25% (a quarter) |
| 10 dB | 10% (a tenth) |
One S-unit on most receivers is about 6 dB, so 3 dB of cable loss is roughly half an S-unit at the other end.
Read This Before You Buy
These figures are the makers’ published loss for new cable with a perfect match. A high SWR adds more loss on top, and old or water-soaked coax can be far worse than its data sheet.
On HF with a run under 100 feet, the difference between good cables is small. At VHF and UHF, and on long runs, it adds up fast; that is where LMR-400 or bigger earns its price.
Names like “RG-8X” describe a family, not one product. Other brands’ versions vary, so check the data sheet for the exact cable you buy.