---
title: "Calculate the Fresnel zone radius at the midpoint of a radio link. Given frequency in MHz, link distance in kilometres, and zone number (1-5), returns the Fresnel zone radius in metres and feet, plus the 60% clearance threshold used in practical link engineering. The first Fresnel zone defines the region where most RF energy travels; obstructions within 60% of this radius cause significant signal degradation. Feeds into link_budget for path clearance analysis."
method: POST
path: "/api/fresnel_zone"
tags: ["rf"]
---

# Calculate the Fresnel zone radius at the midpoint of a radio link. Given frequency in MHz, link distance in kilometres, and zone number (1-5), returns the Fresnel zone radius in metres and feet, plus the 60% clearance threshold used in practical link engineering. The first Fresnel zone defines the region where most RF energy travels; obstructions within 60% of this radius cause significant signal degradation. Feeds into link_budget for path clearance analysis.

`POST /api/fresnel_zone`

## Request

- Base URL: `https://calc.engineer`
- URL: `https://calc.engineer/api/fresnel_zone`
- Auth: none declared

## Request body

- object
  - `freq_mhz` number, required: Operating frequency in megahertz (MHz)
  - `dist_km` number, required: Total link distance in kilometres
  - `zone_number` number: Fresnel zone number (1-5, default 1)

## Response `200`

Computed result for fresnel_zone

- object
  - `result` object, required
    - `radius_m` number, required: Fresnel zone radius at midpoint in metres
    - `radius_ft` number, required: Fresnel zone radius at midpoint in feet
    - `clearance_60pct_m` number, required: 60% Fresnel zone clearance (practical minimum) in metres
    - `wavelength_m` number, required: RF wavelength in metres
  - `tool` string, required
  - `version` string, required
  - `computed_at` string, date-time, required
  - `chain` object, required
    - `feeds` unknown[]
      - unknown
    - `accepts_from` unknown[]
      - unknown

## Other responses

- `400`: Validation error or invalid JSON
- `404`: Tool not found

---

[API](https://skmtc.dev/calc/apis/calc-engineer-api.md) · [All operations](https://skmtc.dev/calc/apis/calc-engineer-api/llms.txt) · [OpenAPI document](https://skmtc.dev/calc/apis/calc-engineer-api/revisions/8830480ec488?raw)