Chris Collins

Tools · electronics

RF and wireless calculator

Convert dBm, watts, and microvolts; cut antennas for any band; work out free-space path loss, link margin, and Fresnel clearance; calculate LoRa time-on-air and duty cycle; and look up Wi-Fi, BLE, and Zigbee channels.

Added · updated 13 October 2026

How to use it

  • Power units — dBm = 10·log₁₀(P/1 mW). Voltages assume a 50 Ω load (V = √(P·R)), so 0 dBm = 224 mV RMS = 107 dBµV.

  • Wavelength and antennas:

    • λ = c/f.

    • A quarter-wave whip is λ/4 and a half-wave dipole is λ/2, each times a shortening factor of about 0.95 for end effect. Cut long and trim.

    • Coax stubs and baluns use the cable’s velocity factor: 0.66 for solid polyethylene, about 0.8 for foam.

    • The band list fills in 433, 868, 915 MHz, 2.4 and 5.8 GHz, GPS, NFC, and amateur bands.

  • Link budget:

    • Free-space path loss: FSPL = 20·log₁₀(d) + 20·log₁₀(f) + 20·log₁₀(4π/c). That is about 80 dB at 2.4 GHz over 100 m.

    • Received power = EIRP + Rx gain − cable losses − FSPL. Margin = received power − sensitivity.

    • The maximum range solves for the distance that leaves your fade margin.

    • The first Fresnel zone radius at the midpoint is √(λ·d₁·d₂/D). Keep 60% of it clear of obstacles.

    • The Earth’s bulge (with the standard 4/3 Earth-radius refraction factor) matters beyond a few kilometres.

    • Free space is the best case: walls and trees add 3–20 dB each.

  • LoRa airtime uses the Semtech SX127x formula:

    • Symbol time Tsym = 2^SF/BW.

    • Preamble = (n + 4.25)·Tsym.

    • Payload symbols = 8 + max(⌈(8·PL − 4·SF + 28 + 16·CRC − 20·IH)/(4·(SF − 2·DE))⌉·(CR + 4), 0).

    • Low-data-rate optimisation (DE) switches on automatically when symbols exceed 16 ms.

    • Check value: SF7, 125 kHz, 4/5 coding, a 20-byte payload, and an 8-symbol preamble give 56.6 ms.

    • Duty cycle: EU868’s 1% rule means waiting 99× the airtime after each transmission. US915 instead caps a single transmission at 400 ms.

    • The sensitivity estimate is −174 + 10·log₁₀(BW) + 6 dB noise figure + the SNR limit for the SF.

  • Channels tabulates Wi-Fi 2.4 GHz (2412 + 5·(n − 1) MHz, channel 14 = 2484) and Wi-Fi 5 GHz (5000 + 5·n MHz, with DFS channels marked). It also lists BLE’s 40 RF channels with their link-layer indices — advertising channels 37/38/39 sit at 2402, 2426, and 2480 MHz, between the Wi-Fi channels — and IEEE 802.15.4 channels 11–26 for Zigbee and Thread.

Check your local regulations for power, duty-cycle, and band limits; the presets are common values, not legal advice.

Sharing

Share link copies a URL that opens this tool with your current settings — every value it remembers between visits, packed into ?s=. Opening a link replaces your saved settings for this tool. You can also write a short link by hand with sf, bw (LoRa spreading factor and bandwidth in Hz), for example /tools/rf/?sf=9&bw=125000.

For detailed antenna designs, feedline loss and SWR, matching networks, the Smith chart and LC filters, see antennas, feedline and SWR, matching networks, Smith chart and RF filters and attenuators.

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