RF & electrical tools for amateur radio operators
All calculations run locally in your browser — nothing you type here is sent anywhere.
Enter any two of V, I, R, P — solve for the other two
V = I×R | P = V×I = I²R = V²/R
Estimate feedline loss by cable type, length, and frequency
Loss figures are typical published dB/100 ft values for these cable families, interpolated across frequency — actual loss varies by manufacturer, cable age/UV exposure, and connector quality. For critical work, check your cable's own datasheet. Add roughly 0.1–0.2 dB for a typical two-connector run, not included below. The SWR field applies the standard mismatched-line-loss correction (loss beyond the matched-line figure caused by reflections at a non-50Ω load); leave it blank if you don't know it.
Dipole, quarter-wave vertical, and J-pole starting dimensions
These use the standard 468/f and 234/f rule-of-thumb constants (feet, f in MHz), which already include a typical end-effect shortening for wire antennas. Real resonant length also depends on wire/tubing diameter, height above ground, and nearby objects — treat these as a cut-slightly-long starting point and trim to resonance with an antenna analyzer or SWR meter. J-pole feedpoint tap position is the most sensitive dimension and is only an estimate.
Power ratio, absolute power (dBm/dBW), and voltage ratio conversions
What this does: a decibel (dB) doesn't measure an amount by itself — it measures how many times stronger or weaker one signal is than another. Hams use dB for amplifier gain, antenna gain, feedline loss, and comparing signal reports. There's no Calculate button here — each pair of boxes below is linked live: type a number into either box and the other one fills in instantly. Each section already has a working example filled in so you can see it happen — try changing a number and watch the other one update.
Use this when you know how many times more or less power you have — like "my new amp doubled my output" — and want that in dB, or the other way around.
Example shown: doubling your power is +3 dB. Quick reference — 3 dB ≈ ×2 power, 6 dB ≈ ×4, 10 dB = ×10. Formula: dB = 10·log10(ratio)
Use this for a single, absolute power level — for example, turning your transceiver's 100-watt output into dBm or dBW so you can compare it against a spec sheet or a gain figure.
Example shown: 100 W = 50 dBm = 20 dBW. dBm is referenced to 1 milliwatt, dBW to 1 watt — dBW is always dBm minus 30.
Use this for voltage or field-strength ratios — like comparing two RF voltmeter readings — not power. (Power and voltage use different dB formulas, which is why this section is separate from #1.)
Example shown: doubling voltage is +6 dB (not +3 dB — that's the difference from section 1). Formula: dB = 20·log10(ratio)