Calculator Inputs
Preset cable values are typical engineering estimates. Manufacturer data should replace presets when precise acceptance, compliance, or system certification is required.
Example Data Table
| Cable | Frequency | Length | VSWR | Forward Path Loss | Delivered Power | Efficiency |
|---|---|---|---|---|---|---|
| LMR-400 | 450 MHz | 30 m | 1.5:1 | 1.03 dB | 74.2 W from 100 W | 74.2% |
| RG-58 | 144 MHz | 20 m | 1.3:1 | 1.16 dB | 37.4 W from 50 W | 74.8% |
| 1/2" Heliax | 900 MHz | 45 m | 1.2:1 | 1.98 dB | 126.4 W from 200 W | 63.2% |
Formula Used
Attenuation at frequency = Reference attenuation × (Operating frequency ÷ Reference frequency)Exponent
Adjusted attenuation = Attenuation at frequency × [1 + ((Ambient temperature − 20) × Temperature coefficient ÷ 100)]
Line loss (dB) = Adjusted attenuation per 100 m × (Length in meters ÷ 100)
Connector loss (dB) = Connector count × Loss per connector
|Γ| = (VSWR − 1) ÷ (VSWR + 1)
Mismatch loss (dB) = −10 × log10(1 − |Γ|2)
Delivered power = Source power × 10−(Forward path loss / 10) × (1 − |Γ|2)
Efficiency (%) = Delivered power ÷ Source power × 100
This calculator uses practical engineering approximations suitable for planning, comparison, and preliminary design. Replace estimates with measured cable data when final performance margins are tight.
How to Use This Calculator
- Select a cable preset or keep it as a starting point.
- Enter operating frequency and feedline length.
- Choose the input power unit and enter the source power.
- Enter load VSWR to include mismatch effects.
- Add connector count and estimated connector loss.
- Set ambient temperature and temperature coefficient if needed.
- Use reference mode for modeled cable behavior or direct mode when manufacturer attenuation at the operating frequency is already known.
- Press the button to see the result block above the form.
- Use the CSV and PDF buttons to export the calculated results.
Frequently Asked Questions
1. What does feedline loss mean?
Feedline loss is the signal or power reduction that occurs while energy travels through a transmission line, connectors, and imperfect load matching conditions.
2. Why does higher frequency usually increase loss?
As frequency rises, conductor and dielectric effects generally increase attenuation. That is why the same cable often performs much better at lower frequencies.
3. Does VSWR change delivered power?
Yes. Poorer VSWR increases reflection coefficient, reduces absorbed load power, and adds equivalent mismatch loss even when the cable itself stays unchanged.
4. Why include connector loss?
Each connector introduces a small insertion loss. In short runs or precision systems, combined connector losses can become a meaningful part of total path loss.
5. What is velocity factor used for here?
Velocity factor adjusts wavelength and propagation delay inside the line. It helps estimate electrical length, timing, and phase-related behavior in transmission systems.
6. Should I use reference mode or direct mode?
Use reference mode when you know attenuation at a reference frequency. Use direct mode when manufacturer data already gives attenuation at the exact operating frequency.
7. Are the preset cable values exact?
No. They are typical estimates for quick engineering calculations. Actual attenuation varies by brand, construction, installation quality, age, and temperature.
8. Can this replace lab measurements?
No. It is best for planning and comparison. Final acceptance should rely on manufacturer specifications, calibrated instruments, and measured installation conditions.