Formula
LMR-400, LMR-240, and LMR-600 use the Times Microwave attenuation formula, k1 × √f + k2 × f, with f in MHz and the result in dB per 100 ft, at VSWR 1.0 and +25°C. RG-58 is the Belden 8259 table, RG-8X is Belden 9258, and RG-213 is Belden 8267. A frequency printed on those tables is returned as printed. Between rows, log attenuation is interpolated against log frequency. Outside the table, the edge value is scaled by the square root of frequency and marked as extrapolated.
α = k1 √f + k2 f
loss = α × feet / 100 + connectors
Pout/Pin = 10^(−dB/10)
One metre is 1/0.3048 feet. Matched loss is not the extra loss of a high SWR. Put that question on the SWR calculator, and put the cable decibels into the link budget if the path is the thing you are adding up.
Worked examples
LMR-400 at 150 MHz. 1.537 dB/100 ft, within 0.05 dB of the datasheet’s rounded 1.5. At 450 MHz, 2.711 against a rounded 2.7.
Belden 8259 at 100 MHz. 5.4 dB/100 ft, the printed cell. At 400 MHz, 12.4 dB/100 ft.
50 ft of LMR-400 at 150 MHz, no connectors. Half of 1.537 dB.
Reference
| Cable | 100 MHz | 150 MHz | 400 or 450 MHz | Source |
|---|---|---|---|---|
| LMR-400 | formula | 1.537 (table 1.5) | 2.711 at 450 (table 2.7) | Times Microwave formula |
| LMR-240 | formula | 3.014 (table 3.0) | 5.283 at 450 (table 5.3) | Times Microwave formula |
| LMR-600 | formula | 0.964 (table 1.0) | 1.720 at 450 (table 1.7) | Times Microwave formula |
| RG-58, Belden 8259 | 5.4 | interpolated | 12.4 at 400 | Belden 8259 table, VF 66% |
| RG-8X, Belden 9258 | 3.1 | interpolated | 6.6 at 400 | Belden 9258 table, VF 82% |
| RG-213, Belden 8267 | 1.9 | interpolated | 4.1 at 400 | Belden 8267 table, VF 66% |
Questions
How lossy is LMR-400 at 150 MHz?
The Times Microwave formula gives 1.537 dB per 100 ft. The datasheet table rounds that row to 1.5. At 450 MHz the formula gives 2.711 against a table value of 2.7. This page stays within 0.05 dB of those rounded table cells.
Where do RG-58, RG-8X, and RG-213 come from?
Belden 8259, 9258, and 8267 nominal tables. A listed frequency is the printed number. A frequency between rows is a log-frequency interpolation of log attenuation, and the readout says so.
Does this include connectors?
Only the connector loss you type, added once for the whole run. The cable figure is matched loss at the conditions on the data sheet, not a measured reel.
Sources
- Times Microwave LMR-400, LMR-240, and LMR-600 datasheets. Attenuation = k1 √FMHz + k2 FMHz, dB/100 ft, VSWR 1.0, +25°C. LMR-600 velocity is taken as 87% from Times product charts. One catalog extract of the same family prints 85%.
- Belden 8259 (RG-58), 9258 (RG-8X), and 8267 (RG-213) nominal attenuation tables. Listed frequencies are exact. Gaps are log-log interpolation.