# Ohmforge RF Calculator: Antenna, Link Budget & VSWR

> Dipole and quarter-wave lengths, free-space path loss and link margin, VSWR ↔ return loss. Three everyday radio calculations in one place: antenna element lengths for a frequency (half-wave dipole, quarter-wave monopole, coax stubs), a Friis link budget with free-space path loss, received power, margin and the maximum range for the fade margin you want, and the conversions between VSWR, return loss, reflection coefficient and mismatch loss — also straight from a load impedance.

Part of usefulweb.app (https://usefulweb.app/). Part of [Ohmforge](https://ohmforge.usefulweb.app/) (ownerApp: ohmforge). Results are deterministic calculations from the inputs and the listed assumptions; the API returns exactly what the web page shows.

## Use it
- Human interface: https://ohmforgerfcalculator.usefulweb.app/ (interactive tool at https://ohmforgerfcalculator.usefulweb.app/tool)
- JSON API: POST https://ohmforgerfcalculator.usefulweb.app/api/v1/calculate with a JSON object of inputs (schema: https://ohmforgerfcalculator.usefulweb.app/api/v1/tool)
- OpenAPI: https://ohmforgerfcalculator.usefulweb.app/openapi.json
- MCP (Streamable HTTP): https://ohmforgerfcalculator.usefulweb.app/mcp - tool `ohmforge_rf_calculator`

## Inputs
- mode (select): Calculation
- frequencyMHz (number): Frequency - Antenna lengths and link budget.
- distanceM (number): Distance - Link budget: distance between the antennas.
- txPowerDbm (number): Transmit power - 20 dBm = 100 mW, 30 dBm = 1 W.
- txGainDbi (number): Transmit antenna gain
- rxGainDbi (number): Receive antenna gain
- txCableLossDb (number): Transmit cable and connector loss
- rxCableLossDb (number): Receive cable and connector loss
- otherLossDb (number): Other losses - Walls, bodies, foliage, polarisation mismatch, rain — whatever free space does not include.
- sensitivityDbm (number): Receiver sensitivity - Weakest signal the receiver decodes at the data rate you use (datasheet).
- fadeMarginDb (number): Fade margin you want - Reserve for fading and obstacles; the maximum range is computed for it.
- endEffect (number): End-effect factor (antenna) - Shortening of a thin wire element because of the end capacitance; 0.95 is the usual approximation (thicker elements need less).
- velocityFactor (number): Velocity factor of the element (antenna) - 1 for bare wire in air; insulated wire about 0.95–0.98.
- cableVelocityFactor (number): Velocity factor of the coax (stubs) - Solid PE coax (RG-58, RG-213) 0.66, foam PE about 0.80–0.85.
- mismatchFrom (select): You know (mismatch)
- value (number): Value (VSWR, return loss or |Γ|) - VSWR ≥ 1, return loss in dB as a positive number, or |Γ| from 0 to 1.
- loadR (number): Load resistance R - Mismatch from a load impedance.
- loadX (number): Load reactance X - Positive = inductive, negative = capacitive.
- z0 (number): Reference impedance Z0

## Method
- Wavelength λ = c/f with c = 299 792 458 m/s. Half-wave dipole = λ/2 × k × VF and quarter-wave monopole = λ/4 × k × VF, where k is the end-effect factor (0.95 reproduces the ARRL rule 468/f(MHz) ft within 0.2 %) and VF the velocity factor of the element. Coax stubs use the cable's velocity factor. All lengths are starting points: trim the real antenna with an analyser.
- Free-space path loss FSPL = 20·log10(4π·d·f/c) between isotropic antennas (ITU-R P.525). Friis link budget: received power = Ptx − Ltx + Gtx − FSPL − Lother + Grx − Lrx; margin = received power − sensitivity. The maximum range is the free-space distance at which the margin equals the fade margin you ask for.
- The first Fresnel zone radius at mid-path is ½·√(λ·d); keeping 60 % of it free of obstacles is the usual rule for line-of-sight links.
- Mismatch: |Γ| = (VSWR − 1)/(VSWR + 1) = 10^(−RL/20); Γ = (ZL − Z0)/(ZL + Z0) for a load impedance; mismatch loss = −10·log10(1 − |Γ|²); reflected power = |Γ|².

## Limits
- Free-space values: real links lose more to the ground, buildings, foliage and multipath, and the received power fluctuates (fading). Transmit power and antenna gain limits are set by your country's radio regulations — this calculator gives no licensing or legal advice.