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About This Tool
In radio communications and amateur radio stations, impedance mismatches between transmitters, coaxial feedlines, and antenna radiating elements create standing waves along the transmission line. Standing waves reflect power back toward the transmitter final amplifier stage, reducing radiated signal strength and increasing line attenuation. This RF calculator converts measured antenna Voltage Standing Wave Ratio (VSWR) to Return Loss (dB), reflection coefficient, and percentage of reflected power, and calculates total matched and mismatched feedline attenuation across common coaxial cable types.
### RF Transmission Formulas - Reflection Coefficient (Gamma): Evaluated from measured VSWR: Gamma = |(VSWR - 1) / (VSWR + 1)| - Return Loss (dB): Logarithmic ratio of incident to reflected power: Return Loss (dB) = -20 * log10(Gamma) - Power Split: Reflected power percentage = Gamma^2 * 100%, with transmitted power = 100% - Reflected Power. - Mismatch Loss (dB): Power loss resulting from impedance boundary reflection: Mismatch Loss = -10 * log10(1 - Gamma^2) - Feedline Attenuation: Scaled with frequency based on nominal 450 MHz benchmark ratings: Cable Loss = Loss_100ft * sqrt(Frequency_MHz / 450) * (Length_feet / 100)
### Worked Numeric Scenario Consider a 70cm amateur radio station operating on 432 MHz with a 50-foot run of Times Microwave LMR-400 into an antenna with 1.35:1 VSWR: - Measured Antenna VSWR: 1.35:1 - Reflection Coefficient: (1.35 - 1) / (1.35 + 1) = 0.149 - Return Loss: -20 * log10(0.149) = 16.54 dB - Reflected vs Transmitted Power: 2.22% reflected, 97.78% transmitted - Mismatch Loss: 0.10 dB - Matched Cable Attenuation (50 ft LMR-400 at 432 MHz): 1.32 dB - Total Transmission Line Loss: 1.32 + 0.10 = 1.42 dB (over 72% of transmitter power reaches the antenna radiator).
### Client-Side RF Privacy All electromagnetic equations evaluate directly in your browser session using JavaScript math functions. No callsigns, operating frequencies, station coordinates, or equipment parameters are transmitted over external networks.
How to Use
- 1
Enter your measured antenna VSWR (e.g. 1.35:1).
- 2
Select your coaxial feedline cable (LMR-400, RG-8X, RG-58, RG-213, or RG-6).
- 3
Input your operating frequency in MHz (e.g. 432) and feedline length in feet (e.g. 50).
- 4
Inspect the computed Return Loss (dB), reflected power percentage, matched cable loss, and total system attenuation.
Frequently Asked Questions
What is an acceptable VSWR for amateur radio and two-way transmitters?
A VSWR below 1.5:1 is considered ideal (less than 4% reflected power). Most modern solid-state transceivers operate safely up to 2.0:1 without thermal power folding, but higher ratios require an antenna tuner to protect final amplifier transistors.
Why does coaxial cable loss increase at higher frequencies?
Coaxial cable attenuation rises with the square root of frequency due to skin effect resistance in the copper conductors and dielectric absorption in the insulating sheath.
Are my station frequencies or equipment details uploaded to a server?
No. All standing wave calculations and cable loss formulas run strictly inside your local web browser.