For estimating and education only. Results come from the simplified models described below, are provided as is with no warranty, and are not a substitute for engineering analysis, simulation or measurement. Check them before relying on them in a design, purchase or safety decision. See the terms of use.
Equations
\alpha(f) = k_1\sqrt{f_\mathrm{MHz}} + k_2\,f_\mathrm{MHz}dB/100 ft
L_\mathrm{cable} = \alpha(f)\,\frac{\ell_\mathrm{ft}}{100}dB
L_\mathrm{total} = L_\mathrm{cable} + n_c L_c, \qquad P_\mathrm{out} = P_\mathrm{in} - L_\mathrm{total}dB, dBm
\eta = 10^{-L_\mathrm{total}/10}fraction of power delivered
Custom\alpha(f) = \alpha(f_\mathrm{ref})\sqrt{f / f_\mathrm{ref}}conductor loss dominant
Variables
| Symbol | Meaning | Unit |
|---|
k_1 | Conductor (skin-effect) loss coefficient | dB/100 ft/√MHz |
k_2 | Dielectric loss coefficient | dB/100 ft/MHz |
\ell | Cable length | ft |
n_c,\ L_c | Number of connectors, loss of each | —, dB |
Cable coefficients
| Cable | k₁ | k₂ | Velocity factor | Basis |
|---|
| LMR-100A | 0.70914 | 0.00174 | 0.66 | Published |
| LMR-195 | 0.35686 | 0.00047 | 0.80 | Published |
| LMR-200 | 0.32090 | 0.00033 | 0.83 | Published |
| LMR-240 | 0.24208 | 0.00033 | 0.84 | Published |
| LMR-300 | 0.19193 | 0.00033 | 0.85 | Published |
| LMR-400 | 0.12229 | 0.00026 | 0.85 | Published |
| LMR-500 | 0.09659 | 0.00026 | 0.86 | Published |
| LMR-600 | 0.07555 | 0.00026 | 0.87 | Published |
| LMR-900 | 0.05177 | 0.00016 | 0.87 | Published |
| LMR-1200 | 0.03737 | 0.00016 | 0.88 | Published |
| RG-58 | 0.43741 | 0.003648 | 0.66 | Fit, 30–900 MHz |
| RG-8X | 0.35817 | 0.002299 | varies | Fit, 30–900 MHz |
| RG-213 | 0.20474 | 0.002037 | 0.66 | Fit, 30–900 MHz |
Worked example
| Input | Value |
|---|
| Cable | LMR-400, 30 m |
| Frequency | 2400 MHz |
| Connectors | 2 × 0.1 dB |
| Power in | 30 dBm |
| Result | Value |
|---|
| Total loss | 6.711 dB |
| Cable loss | 6.511 dB |
| Connector loss | 0.2 dB |
| Attenuation | 6.615 dB/100 ft |
| Attenuation | 21.7 dB/100 m |
| Power delivered | 21.3 % |
| Power out | 23.29 dBm |
| Velocity factor | 0.85 |
The same 30 m run loses about 12.5 dB in LMR-240 and about 30 dB in RG-58.
Load this example in the calculator
Validity
- New cable at +25 °C, matched at both ends.
- LMR coefficients are the manufacturer’s published fit. RG-58, RG-8X and RG-213 coefficients are a least-squares fit to published attenuation from 30 to 900 MHz (within 6 %); above 900 MHz they are extrapolated.
- RG-type cables vary by manufacturer. Use the datasheet for the cable you buy.
- 0.1 dB per connector is a common planning figure below 6 GHz. Use the connector datasheet when you have it.
Sources
- Times Microwave Systems, Communications Coax Selection Guide (LMR attenuation coefficients; published attenuation for RG-58, RG-8X and RG-213).
- D. M. Pozar, Microwave Engineering, 4th ed., Wiley, 2012. §2.7 (lossy transmission lines).