diff --git a/Cargo.lock b/Cargo.lock
index a48f856..83cb19f 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -9,6 +9,10 @@ version = "0.1.0"
 [[package]]
 name = "apsis-antenna"
 version = "0.1.0"
+dependencies = [
+ "apsis-core",
+ "apsis-rf",
+]
 
 [[package]]
 name = "apsis-atmosphere"
diff --git a/crates/rf/apsis-antenna/Cargo.toml b/crates/rf/apsis-antenna/Cargo.toml
index 4a6c61d..8962588 100644
--- a/crates/rf/apsis-antenna/Cargo.toml
+++ b/crates/rf/apsis-antenna/Cargo.toml
@@ -11,3 +11,5 @@ publish = false
 workspace = true
 
 [dependencies]
+apsis-core.workspace = true
+apsis-rf.workspace = true
diff --git a/crates/rf/apsis-antenna/src/lib.rs b/crates/rf/apsis-antenna/src/lib.rs
index 5fa7153..b6f70af 100644
--- a/crates/rf/apsis-antenna/src/lib.rs
+++ b/crates/rf/apsis-antenna/src/lib.rs
@@ -1 +1,54 @@
 //! Antenna radiation patterns and gain models.
+//!
+//! An antenna does not radiate equally in every direction. This crate models
+//! how the gain varies with angle for the two shapes that appear in almost
+//! every satellite link: a parabolic reflector, whose beam is a narrow
+//! pencil, and an idealised element with a broad pattern.
+//!
+//! Three beam width conventions are used in practice and they are not
+//! interchangeable:
+//!
+//! - The half power beam width is the angle between the two directions where
+//!   the gain has fallen to half its peak, which is three decibels down.
+//! - The null to null beam width is the angle between the first two zeros on
+//!   either side of the main lobe. It is roughly twice the half power width
+//!   for a uniformly illuminated aperture.
+//! - The first sidelobe level is the peak gain outside the main lobe. For a
+//!   uniformly illuminated circular aperture it is about minus 17.6 dB.
+//!
+//! Every function here takes or returns angles in radians and gains as
+//! [`Gain`], so a result can be fed straight into a link budget.
+//!
+//! Sources:
+//!
+//! - Parabolic aperture gain: `G = eta (pi D / lambda)^2`, where `eta` is
+//!   the aperture efficiency, typically between 0.55 and 0.70 for a
+//!   well adjusted dish.
+//! - Half power beam width of a circular aperture: `theta = k lambda / D`
+//!   with `k` about 1.02 for uniform illumination and close to 1.2 for
+//!   typical illumination tapers.
+//! - The idealised element pattern: ITU-R Recommendation M.1853, which gives
+//!   `G = -min(12 (theta / theta_0)^2, G_0)` for the co-polar pattern of a
+//!   simple Earth station antenna.
+//!
+//! ```
+//! use apsis_antenna::ParabolicAperture;
+//!
+//! // A three meter dish at twelve gigahertz has a peak gain near 50 dBi.
+//! let dish = ParabolicAperture {
+//!     diameter: 3.0,
+//!     efficiency: 0.65,
+//! };
+//! let peak = dish.peak_gain(0.025).expect("valid wavelength");
+//! assert!(peak.dbi().expect("positive") > 45.0);
+//! assert!(peak.dbi().expect("positive") < 50.0);
+//! ```
+
+#![forbid(unsafe_code)]
+#![deny(missing_docs)]
+
+mod parabolic;
+mod pattern;
+
+pub use parabolic::{ParabolicAperture, beam_width, floor_angle};
+pub use pattern::{ElementPattern, Pattern, mask_gain, off_axis_attenuation};
diff --git a/crates/rf/apsis-antenna/src/parabolic.rs b/crates/rf/apsis-antenna/src/parabolic.rs
new file mode 100644
index 0000000..e720ee1
--- /dev/null
+++ b/crates/rf/apsis-antenna/src/parabolic.rs
@@ -0,0 +1,141 @@
+//! The parabolic aperture model.
+
+use apsis_rf::Gain;
+use core::f64::consts::PI;
+
+/// A parabolic reflector described by its diameter and aperture efficiency.
+///
+/// The aperture efficiency collects every loss between the power delivered
+/// to the feed and the power that leaves the aperture as a uniform plane
+/// wave: illumination taper, spillover, blockage by the feed and its
+/// supports, and surface error. A well adjusted dish reaches 0.65 to 0.70,
+/// and a small or simply built one is nearer 0.55.
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub struct ParabolicAperture {
+    /// The reflector diameter, in meters.
+    pub diameter: f64,
+    /// The aperture efficiency, dimensionless, in `(0, 1]`.
+    pub efficiency: f64,
+}
+
+impl ParabolicAperture {
+    /// Returns the physical area of the aperture, in square meters.
+    pub fn area(self) -> f64 {
+        PI * (self.diameter / 2.0).powi(2)
+    }
+
+    /// Returns the effective area, in square meters.
+    ///
+    /// This is the physical area scaled by the aperture efficiency. It is
+    /// the area that, if it collected uniformly, would deliver the observed
+    /// power.
+    pub fn effective_area(self) -> f64 {
+        self.area() * self.efficiency
+    }
+
+    /// Returns `true` when the aperture describes a usable dish.
+    pub fn is_valid(self) -> bool {
+        self.diameter.is_finite()
+            && self.diameter > 0.0
+            && self.efficiency.is_finite()
+            && self.efficiency > 0.0
+            && self.efficiency <= 1.0
+    }
+
+    /// Returns the peak gain on boresight at the given wavelength.
+    ///
+    /// The relation is
+    ///
+    /// ```text
+    /// G = eta (pi D / lambda)^2
+    /// ```
+    ///
+    /// which follows from the effective area through
+    /// [`Gain::from_effective_aperture`]. Returns `None` for an invalid
+    /// aperture or wavelength.
+    pub fn peak_gain(self, wavelength: f64) -> Option<Gain> {
+        if !self.is_valid() {
+            return None;
+        }
+        Gain::from_effective_aperture(self.effective_area(), wavelength)
+    }
+
+    /// Returns the half power beam width at the given wavelength, in
+    /// radians.
+    ///
+    /// This uses the uniform illumination relation, which is the narrowest
+    /// a dish of this diameter can be. A real dish with an illumination
+    /// taper is wider; use [`ParabolicAperture::half_power_beam_width`] to
+    /// include the taper.
+    ///
+    /// Returns `None` for an invalid aperture or wavelength.
+    pub fn uniform_beam_width(self, wavelength: f64) -> Option<f64> {
+        if !self.is_valid() {
+            return None;
+        }
+        beam_width(self.diameter, wavelength, 1.02)
+    }
+
+    /// Returns the half power beam width at the given wavelength, in
+    /// radians, allowing for an illumination taper.
+    ///
+    /// A tapered illumination reduces the sidelobes at the cost of a wider
+    /// main lobe. The factor `k` in `theta = k lambda / D` rises from about
+    /// 1.02 for uniform illumination to about 1.2 for the taper that
+    /// minimises sidelobes. A value of 1.15 is representative of a
+    /// well designed dish.
+    ///
+    /// Returns `None` for an invalid aperture or wavelength.
+    pub fn half_power_beam_width(self, wavelength: f64) -> Option<f64> {
+        if !self.is_valid() {
+            return None;
+        }
+        beam_width(self.diameter, wavelength, 1.15)
+    }
+}
+
+/// Returns the half power beam width of a circular aperture, in radians.
+///
+/// The relation is `theta = k lambda / D`, where `k` depends on how the
+/// aperture is illuminated. Uniform illumination gives the narrowest beam,
+/// with `k` about 1.02, and the taper that suppresses sidelobes gives the
+/// widest, with `k` about 1.2.
+///
+/// The result is the full width between the two three decibel points, not
+/// the half width. It is a small angle approximation, so the caller should
+/// confirm that the resulting width is small compared to a radian.
+///
+/// Returns `None` when the diameter or wavelength is not positive, or when
+/// `k` is not positive.
+pub fn beam_width(diameter: f64, wavelength: f64, k: f64) -> Option<f64> {
+    if !diameter.is_finite()
+        || diameter <= 0.0
+        || !wavelength.is_finite()
+        || wavelength <= 0.0
+        || !k.is_finite()
+        || k <= 0.0
+    {
+        return None;
+    }
+    Some(k * wavelength / diameter)
+}
+
+/// Returns the angle at which the squared angle law of
+/// [`off_axis_attenuation`](crate::off_axis_attenuation) reaches the
+/// sidelobe floor.
+///
+/// Beyond this angle the model holds the gain at the floor rather than
+/// letting it fall without bound. The angle follows from setting the
+/// attenuation `12 (theta / theta_0)^2` equal to the floor, which gives
+/// `theta = theta_0 sqrt(floor / 12)`.
+///
+/// Returns `None` when the beam width or floor is not positive and finite.
+pub fn floor_angle(beam_width: f64, floor_db: f64) -> Option<f64> {
+    if !beam_width.is_finite() || beam_width <= 0.0 {
+        return None;
+    }
+    if !floor_db.is_finite() || floor_db <= 0.0 {
+        return None;
+    }
+    Some(beam_width * (floor_db / 12.0).sqrt())
+}
diff --git a/crates/rf/apsis-antenna/src/pattern.rs b/crates/rf/apsis-antenna/src/pattern.rs
new file mode 100644
index 0000000..5a07155
--- /dev/null
+++ b/crates/rf/apsis-antenna/src/pattern.rs
@@ -0,0 +1,244 @@
+//! Off-axis gain patterns.
+
+use crate::parabolic::ParabolicAperture;
+use apsis_rf::{Decibel, Gain};
+use core::f64::consts::PI;
+
+/// Returns the gain of an antenna at an angle from boresight, given a peak
+/// gain and a half power beam width.
+///
+/// The model is the one used for interference analysis in ITU-R
+/// Recommendation M.1853 and adopted in the Radio Regulations. The gain
+/// falls as the square of the off-axis angle until it reaches the peak gain
+/// minus a fixed floor, and stays there beyond. In decibels,
+///
+/// ```text
+/// G(theta) = G_peak - min(12 (theta / theta_0)^2, G_floor)
+/// ```
+///
+/// where `theta_0` is the half power beam width and `G_floor` is the
+/// sidelobe level to fall to, in decibels below the peak. The squared angle
+/// law comes from the first term of the Airy pattern of a uniformly
+/// illuminated aperture, which is a good approximation down to the first
+/// sidelobe.
+///
+/// Returns `None` when the peak gain has no decibel form, or when the beam
+/// width or floor is not positive and finite.
+pub fn off_axis_attenuation(
+    peak: Gain,
+    beam_width: f64,
+    off_axis_angle: f64,
+    floor_db: f64,
+) -> Option<Decibel> {
+    let peak_db = peak.as_decibel()?;
+    if !beam_width.is_finite() || beam_width <= 0.0 {
+        return None;
+    }
+    if !floor_db.is_finite() || floor_db <= 0.0 {
+        return None;
+    }
+    if !off_axis_angle.is_finite() {
+        return None;
+    }
+    let ratio = 12.0 * (off_axis_angle / beam_width).powi(2);
+    let attenuation = ratio.min(floor_db);
+    Some(peak_db - Decibel::from_db(attenuation))
+}
+
+/// Returns the gain that an antenna is required not to exceed at an angle
+/// from boresight, in decibels relative to an isotropic radiator.
+///
+/// This is the reference pattern of the Radio Regulations Appendix 8, used
+/// to check whether an Earth station meets the off-axis emission limits that
+/// protect neighbouring satellites. The pattern is piecewise:
+///
+/// ```text
+/// for  D/lambda >= 100:
+///   G = 29 - 25 log10(theta)            for 1   <= theta < 36 degrees
+///   G = -10                             for 36  <= theta <= 180 degrees
+/// for  D/lambda <  100:
+///   G = 32 - 25 log10(theta)            for 1   <= theta < 48 degrees
+///   G = -10                             for 48  <= theta <= 180 degrees
+///   G = 32 - 25 log10(48)               for theta < 1 degree
+/// ```
+///
+/// The angle is in radians and the diameter to wavelength ratio decides
+/// which branch applies, because a small antenna has a broader main lobe
+/// and therefore a wider range over which the pattern must be controlled.
+///
+/// This function returns the limit, that is the largest gain permitted. An
+/// antenna is compliant when its actual off-axis gain is at or below it.
+///
+/// Returns `None` when the angle is negative, when the ratio is not
+/// positive, or when the angle is not finite.
+///
+/// Source: ITU, *Radio Regulations*, Appendix 8, which reproduces the
+/// reference pattern of ITU-R Recommendation S.465.
+pub fn mask_gain(diameter_over_wavelength: f64, off_axis_angle: f64) -> Option<f64> {
+    if !diameter_over_wavelength.is_finite() || diameter_over_wavelength <= 0.0 {
+        return None;
+    }
+    if !off_axis_angle.is_finite() || off_axis_angle < 0.0 {
+        return None;
+    }
+
+    let large_aperture = diameter_over_wavelength >= 100.0;
+    // The break point is where the pattern reaches its floor, and the cap
+    // angle is where the falling branch begins.
+    let (cap_angle, floor_angle) = if large_aperture {
+        (1.0_f64.to_radians(), 36.0_f64.to_radians())
+    } else {
+        (1.0_f64.to_radians(), 48.0_f64.to_radians())
+    };
+    let head = if large_aperture { 29.0 } else { 32.0 };
+
+    if off_axis_angle < cap_angle {
+        // Inside the first degree the pattern is taken at its value at one
+        // degree, which is the most restrictive point on the falling branch.
+        Some(head - 25.0 * cap_angle.to_degrees().log10())
+    } else if off_axis_angle <= floor_angle {
+        Some(head - 25.0 * off_axis_angle.to_degrees().log10())
+    } else {
+        Some(-10.0)
+    }
+}
+
+/// The shape of an antenna pattern.
+///
+/// A pattern is either described by its geometry, in which case the peak
+/// gain and beam width are derived from the aperture, or given directly as
+/// measured or published values. The enum lets a link budget accept either.
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub enum Pattern {
+    /// A parabolic aperture, described by its diameter and efficiency.
+    Parabolic {
+        /// The reflector diameter, in meters.
+        diameter: f64,
+        /// The aperture efficiency, dimensionless.
+        efficiency: f64,
+    },
+    /// An antenna described directly by its peak gain and half power beam
+    /// width.
+    Direct {
+        /// The peak gain on boresight.
+        peak: Gain,
+        /// The half power beam width, in radians.
+        beam_width: f64,
+    },
+}
+
+impl Pattern {
+    /// Returns the peak gain of this pattern at the given wavelength.
+    ///
+    /// The wavelength is ignored for a directly specified pattern, which
+    /// already carries its peak gain.
+    pub fn peak_gain(self, wavelength: f64) -> Option<Gain> {
+        match self {
+            Self::Parabolic {
+                diameter,
+                efficiency,
+            } => ParabolicAperture {
+                diameter,
+                efficiency,
+            }
+            .peak_gain(wavelength),
+            Self::Direct { peak, .. } => Some(peak),
+        }
+    }
+
+    /// Returns the half power beam width of this pattern at the given
+    /// wavelength, in radians.
+    pub fn beam_width(self, wavelength: f64) -> Option<f64> {
+        match self {
+            Self::Parabolic {
+                diameter,
+                efficiency,
+            } => ParabolicAperture {
+                diameter,
+                efficiency,
+            }
+            .half_power_beam_width(wavelength),
+            Self::Direct { beam_width, .. } => {
+                (beam_width.is_finite() && beam_width > 0.0).then_some(beam_width)
+            }
+        }
+    }
+}
+
+/// A gain pattern constructed from a peak gain and a half power beam width.
+///
+/// This is the form a link budget needs: the two numbers that describe the
+/// beam, plus the model that gives the gain at any angle from boresight.
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub struct ElementPattern {
+    /// The peak gain on boresight.
+    pub peak: Gain,
+    /// The half power beam width, in radians.
+    pub beam_width: f64,
+    /// The sidelobe floor, in decibels below the peak.
+    pub floor_db: f64,
+}
+
+impl Default for ElementPattern {
+    /// Returns a pattern for an isotropic antenna, with a floor that makes
+    /// the attenuation zero in every direction.
+    fn default() -> Self {
+        Self {
+            peak: Gain::ISOTROPIC,
+            beam_width: PI,
+            floor_db: f64::MIN_POSITIVE,
+        }
+    }
+}
+
+impl ElementPattern {
+    /// Creates a pattern from a peak gain and a half power beam width.
+    ///
+    /// The sidelobe floor is set to the value of the Radio Regulations
+    /// reference pattern, 32 decibels below the peak, which is a common
+    /// worst case for a small Earth station.
+    pub const fn new(peak: Gain, beam_width: f64) -> Self {
+        Self {
+            peak,
+            beam_width,
+            floor_db: 32.0,
+        }
+    }
+
+    /// Creates a pattern from a parabolic aperture at a wavelength.
+    ///
+    /// Returns `None` when the aperture or wavelength is not usable.
+    pub fn from_parabolic(aperture: ParabolicAperture, wavelength: f64) -> Option<Self> {
+        Some(Self {
+            peak: aperture.peak_gain(wavelength)?,
+            beam_width: aperture.half_power_beam_width(wavelength)?,
+            floor_db: 32.0,
+        })
+    }
+
+    /// Returns this pattern with a different sidelobe floor.
+    pub const fn with_floor(mut self, floor_db: f64) -> Self {
+        self.floor_db = floor_db;
+        self
+    }
+
+    /// Returns the gain at an angle from boresight.
+    ///
+    /// Returns `None` when the pattern is not usable or the angle is not
+    /// finite.
+    pub fn gain_at(self, off_axis_angle: f64) -> Option<Gain> {
+        let attenuation =
+            off_axis_attenuation(self.peak, self.beam_width, off_axis_angle, self.floor_db)?;
+        Gain::from_dbi(attenuation.db())
+    }
+
+    /// Returns `true` when the beam width and floor describe a usable
+    /// pattern.
+    pub fn is_valid(self) -> bool {
+        self.peak.dbi().is_some()
+            && self.beam_width.is_finite()
+            && self.beam_width > 0.0
+            && self.floor_db.is_finite()
+            && self.floor_db > 0.0
+    }
+}
diff --git a/crates/rf/apsis-antenna/tests/parabolic.rs b/crates/rf/apsis-antenna/tests/parabolic.rs
new file mode 100644
index 0000000..cd11c45
--- /dev/null
+++ b/crates/rf/apsis-antenna/tests/parabolic.rs
@@ -0,0 +1,141 @@
+//! Tests for the parabolic aperture model.
+
+use apsis_antenna::{ParabolicAperture, beam_width};
+use core::f64::consts::PI;
+
+/// A three meter dish with a typical aperture efficiency.
+const DISH: ParabolicAperture = ParabolicAperture {
+    diameter: 3.0,
+    efficiency: 0.65,
+};
+
+#[test]
+fn aperture_area_follows_from_the_diameter() {
+    let expected = PI * 1.5 * 1.5;
+    assert!((DISH.area() - expected).abs() < 1e-12);
+    assert!((DISH.effective_area() - expected * 0.65).abs() < 1e-12);
+}
+
+#[test]
+fn peak_gain_matches_the_closed_form() {
+    // At 12 GHz the wavelength is 0.02498 m, and the gain is
+    // eta (pi D / lambda)^2.
+    let wavelength = 0.024_982_710_7;
+    let gain = DISH.peak_gain(wavelength).expect("valid");
+    let expected = 0.65 * (PI * 3.0 / wavelength).powi(2);
+    assert!((gain.ratio() - expected).abs() < 1e-6);
+    // The result is about 49.66 dBi.
+    assert!(
+        (gain.dbi().expect("positive") - 49.661_764_937_398_814).abs() < 1e-9,
+        "dBi {}",
+        gain.dbi().expect("positive")
+    );
+}
+
+#[test]
+fn a_larger_dish_has_more_gain() {
+    let small = ParabolicAperture {
+        diameter: 1.0,
+        efficiency: 0.65,
+    };
+    let large = ParabolicAperture {
+        diameter: 2.0,
+        efficiency: 0.65,
+    };
+    let wavelength = 0.025;
+    let small_gain = small.peak_gain(wavelength).expect("valid");
+    let large_gain = large.peak_gain(wavelength).expect("valid");
+    // Doubling the diameter quadruples the gain, which is six decibels.
+    assert!(
+        (large_gain.dbi().expect("positive") - small_gain.dbi().expect("positive") - 6.020_6).abs()
+            < 1e-6
+    );
+}
+
+#[test]
+fn a_higher_aperture_efficiency_adds_gain() {
+    let low = ParabolicAperture {
+        diameter: 3.0,
+        efficiency: 0.5,
+    };
+    let high = ParabolicAperture {
+        diameter: 3.0,
+        efficiency: 0.7,
+    };
+    let wavelength = 0.025;
+    assert!(
+        high.peak_gain(wavelength).expect("valid").ratio()
+            > low.peak_gain(wavelength).expect("valid").ratio()
+    );
+}
+
+#[test]
+fn the_beam_width_follows_the_wavelength_over_diameter() {
+    let wavelength = 0.025;
+    let uniform = DISH.uniform_beam_width(wavelength).expect("valid");
+    let tapered = DISH.half_power_beam_width(wavelength).expect("valid");
+    assert!((uniform - 1.02 * wavelength / 3.0).abs() < 1e-12);
+    // A tapered illumination widens the beam.
+    assert!(tapered > uniform);
+    assert!((tapered - 1.15 * wavelength / 3.0).abs() < 1e-12);
+}
+
+#[test]
+fn the_uniform_beam_width_matches_a_known_value() {
+    // A 3 m dish at 12 GHz has a half power beam width near 0.0085 radians,
+    // which is about 0.49 degrees.
+    let width = DISH.uniform_beam_width(0.024_982_710_7).expect("valid");
+    assert!((width - 0.008_494).abs() < 1e-5, "width {width}");
+    assert!((width.to_degrees() - 0.486_7).abs() < 0.001);
+}
+
+#[test]
+fn beam_width_helper_validates_its_arguments() {
+    assert!(beam_width(3.0, 0.025, 1.02).is_some());
+    assert_eq!(beam_width(0.0, 0.025, 1.02), None);
+    assert_eq!(beam_width(-1.0, 0.025, 1.02), None);
+    assert_eq!(beam_width(3.0, 0.0, 1.02), None);
+    assert_eq!(beam_width(3.0, -0.025, 1.02), None);
+    assert_eq!(beam_width(3.0, 0.025, 0.0), None);
+    assert_eq!(beam_width(f64::NAN, 0.025, 1.02), None);
+    assert_eq!(beam_width(3.0, f64::INFINITY, 1.02), None);
+}
+
+#[test]
+fn an_invalid_aperture_produces_no_result() {
+    let zero_diameter = ParabolicAperture {
+        diameter: 0.0,
+        efficiency: 0.65,
+    };
+    assert!(!zero_diameter.is_valid());
+    assert_eq!(zero_diameter.peak_gain(0.025), None);
+    assert_eq!(zero_diameter.half_power_beam_width(0.025), None);
+
+    let bad_efficiency = ParabolicAperture {
+        diameter: 3.0,
+        efficiency: 1.5,
+    };
+    assert!(!bad_efficiency.is_valid());
+    assert_eq!(bad_efficiency.peak_gain(0.025), None);
+
+    let zero_efficiency = ParabolicAperture {
+        diameter: 3.0,
+        efficiency: 0.0,
+    };
+    assert!(!zero_efficiency.is_valid());
+
+    let non_finite = ParabolicAperture {
+        diameter: f64::NAN,
+        efficiency: 0.65,
+    };
+    assert!(!non_finite.is_valid());
+}
+
+#[test]
+fn the_aperture_and_gain_conversions_are_consistent() {
+    // The peak gain and the effective area must describe the same antenna.
+    let wavelength = 0.025;
+    let gain = DISH.peak_gain(wavelength).expect("valid");
+    let aperture = gain.effective_aperture(wavelength).expect("valid");
+    assert!((aperture - DISH.effective_area()).abs() < 1e-9);
+}
diff --git a/crates/rf/apsis-antenna/tests/pattern.rs b/crates/rf/apsis-antenna/tests/pattern.rs
new file mode 100644
index 0000000..92d92f8
--- /dev/null
+++ b/crates/rf/apsis-antenna/tests/pattern.rs
@@ -0,0 +1,249 @@
+//! Tests for off-axis gain patterns.
+
+use apsis_antenna::{
+    ElementPattern, ParabolicAperture, Pattern, floor_angle, mask_gain, off_axis_attenuation,
+};
+use apsis_rf::Gain;
+use core::f64::consts::PI;
+
+const TOLERANCE: f64 = 1e-9;
+
+#[test]
+fn boresight_gain_is_the_peak() {
+    let peak = Gain::from_dbi(30.0).expect("valid");
+    let beam = 0.01;
+    let attenuation = off_axis_attenuation(peak, beam, 0.0, 32.0).expect("valid arguments");
+    assert!((attenuation.db() - 30.0).abs() < TOLERANCE);
+}
+
+#[test]
+fn the_half_power_angle_gives_three_decibels() {
+    // At half the beam width the off-axis angle is half of the half power
+    // beam width, and the squared angle law gives 12 * (0.5)^2 = 3 dB.
+    let peak = Gain::from_dbi(30.0).expect("valid");
+    let beam = 0.01;
+    let attenuation = off_axis_attenuation(peak, beam, beam / 2.0, 32.0).expect("valid arguments");
+    assert!(
+        (attenuation.db() - 27.0).abs() < 1e-9,
+        "{}",
+        attenuation.db()
+    );
+}
+
+#[test]
+fn the_attenuation_grows_with_the_square_of_the_angle() {
+    let peak = Gain::from_dbi(40.0).expect("valid");
+    let beam = 0.02;
+    // Both angles are chosen to stay on the squared angle law, well inside
+    // the angle at which the pattern reaches its floor.
+    let first = off_axis_attenuation(peak, beam, 0.02, 60.0).expect("valid");
+    let second = off_axis_attenuation(peak, beam, 0.04, 60.0).expect("valid");
+    // At the beam width the attenuation is 12 dB; at twice the beam width
+    // it is 48 dB. The rise of 36 dB is what a quadrupling of the
+    // attenuation ratio means in decibels.
+    assert!((first.db() - (40.0 - 12.0)).abs() < 1e-9);
+    assert!((second.db() - (40.0 - 48.0)).abs() < 1e-9);
+    let rise = first.db() - second.db();
+    assert!((rise - 36.0).abs() < 1e-9, "rise {rise}");
+}
+
+#[test]
+fn the_pattern_floors_at_the_sidelobe_level() {
+    let peak = Gain::from_dbi(40.0).expect("valid");
+    let beam = 0.02;
+    let floor = 32.0;
+    // Far off axis the gain is the peak minus the floor.
+    let far = off_axis_attenuation(peak, beam, PI, floor).expect("valid");
+    assert!((far.db() - (40.0 - floor)).abs() < TOLERANCE);
+
+    // The floor is reached at the angle the helper reports.
+    let angle = floor_angle(beam, floor).expect("valid");
+    let at_floor = off_axis_attenuation(peak, beam, angle, floor).expect("valid");
+    assert!((at_floor.db() - (40.0 - floor)).abs() < 1e-9);
+    // Just inside that angle the gain is above the floor.
+    let inside = off_axis_attenuation(peak, beam, angle * 0.99, floor).expect("valid");
+    assert!(inside.db() > at_floor.db());
+}
+
+#[test]
+fn invalid_arguments_are_rejected() {
+    let peak = Gain::from_dbi(30.0).expect("valid");
+    assert_eq!(off_axis_attenuation(peak, 0.0, 0.01, 32.0), None);
+    assert_eq!(off_axis_attenuation(peak, -0.01, 0.01, 32.0), None);
+    assert_eq!(off_axis_attenuation(peak, 0.01, 0.01, 0.0), None);
+    assert_eq!(off_axis_attenuation(peak, 0.01, f64::NAN, 32.0), None);
+    assert_eq!(floor_angle(0.0, 32.0), None);
+    assert_eq!(floor_angle(0.01, 0.0), None);
+}
+
+#[test]
+fn the_itu_mask_uses_the_large_aperture_branch() {
+    // For D/lambda >= 100 the pattern is 29 - 25 log10(theta) between one
+    // and 36 degrees and -10 beyond.
+    let at_ten_degrees = mask_gain(1_000.0, 10.0_f64.to_radians()).expect("valid");
+    assert!((at_ten_degrees - (29.0 - 25.0 * 10.0_f64.log10())).abs() < 1e-9);
+    assert!((at_ten_degrees - 4.0).abs() < 1e-9);
+
+    // The falling branch ends at 36 degrees, where the expression is close
+    // to but not exactly -10 dB. The standard accepts that small step and
+    // then holds the value at -10 dB.
+    let at_break = mask_gain(1_000.0, 36.0_f64.to_radians()).expect("valid");
+    assert!(
+        (at_break - (29.0 - 25.0 * 36.0_f64.log10())).abs() < 1e-9,
+        "at break {at_break}"
+    );
+    assert!((at_break + 10.0).abs() < 0.1, "at break {at_break}");
+
+    let beyond = mask_gain(1_000.0, 90.0_f64.to_radians()).expect("valid");
+    assert!((beyond + 10.0).abs() < TOLERANCE);
+}
+
+#[test]
+fn the_itu_mask_uses_the_small_aperture_branch() {
+    // For D/lambda < 100 the pattern starts at 32 and breaks at 48 degrees.
+    let at_ten_degrees = mask_gain(50.0, 10.0_f64.to_radians()).expect("valid");
+    assert!((at_ten_degrees - (32.0 - 25.0 * 10.0_f64.log10())).abs() < 1e-9);
+
+    // The falling branch ends at 48 degrees, again close to -10 dB.
+    let at_break = mask_gain(50.0, 48.0_f64.to_radians()).expect("valid");
+    assert!(
+        (at_break - (32.0 - 25.0 * 48.0_f64.log10())).abs() < 1e-9,
+        "at break {at_break}"
+    );
+    assert!((at_break + 10.0).abs() < 0.1, "at break {at_break}");
+
+    let beyond = mask_gain(50.0, 120.0_f64.to_radians()).expect("valid");
+    assert!((beyond + 10.0).abs() < TOLERANCE);
+}
+
+#[test]
+fn the_itu_mask_is_capped_inside_one_degree() {
+    // Below one degree the pattern does not continue to rise without bound.
+    let peak_limit = 32.0 - 25.0 * 1.0_f64.log10();
+    for angle_degrees in [0.0_f64, 0.01, 0.5, 0.99] {
+        let value = mask_gain(50.0, angle_degrees.to_radians()).expect("valid");
+        assert!(
+            (value - peak_limit).abs() < 1e-9,
+            "angle {angle_degrees} gave {value}"
+        );
+    }
+}
+
+#[test]
+fn the_itu_mask_rejects_invalid_arguments() {
+    assert_eq!(mask_gain(0.0, 0.1), None);
+    assert_eq!(mask_gain(-1.0, 0.1), None);
+    assert_eq!(mask_gain(1_000.0, -0.1), None);
+    assert_eq!(mask_gain(f64::NAN, 0.1), None);
+    assert_eq!(mask_gain(1_000.0, f64::NAN), None);
+}
+
+#[test]
+fn an_element_pattern_reproduces_the_model() {
+    let pattern = ElementPattern::new(Gain::from_dbi(35.0).expect("valid"), 0.02);
+    assert!(pattern.is_valid());
+    let gain = pattern.gain_at(0.01).expect("valid");
+    // 0.01 is half the beam width, so the attenuation is 3 dB.
+    assert!((gain.dbi().expect("positive") - 32.0).abs() < 1e-9);
+}
+
+#[test]
+fn an_element_pattern_floors_far_off_axis() {
+    let pattern = ElementPattern::new(Gain::from_dbi(35.0).expect("valid"), 0.02).with_floor(30.0);
+    let gain = pattern.gain_at(PI).expect("valid");
+    assert!((gain.dbi().expect("positive") - 5.0).abs() < 1e-9);
+}
+
+#[test]
+fn a_pattern_can_be_built_from_a_dish() {
+    let dish = ParabolicAperture {
+        diameter: 3.0,
+        efficiency: 0.65,
+    };
+    let wavelength = 0.025;
+    let pattern = ElementPattern::from_parabolic(dish, wavelength).expect("valid");
+    assert!(pattern.is_valid());
+    assert!(
+        (pattern.peak.dbi().expect("positive")
+            - dish
+                .peak_gain(wavelength)
+                .expect("valid")
+                .dbi()
+                .expect("positive"))
+        .abs()
+            < 1e-9
+    );
+    assert!(
+        (pattern.beam_width - dish.half_power_beam_width(wavelength).expect("valid")).abs() < 1e-12
+    );
+}
+
+#[test]
+fn an_invalid_dish_produces_no_pattern() {
+    let dish = ParabolicAperture {
+        diameter: 0.0,
+        efficiency: 0.65,
+    };
+    assert_eq!(ElementPattern::from_parabolic(dish, 0.025), None);
+}
+
+#[test]
+fn the_pattern_enum_derives_the_same_values() {
+    let dish = ParabolicAperture {
+        diameter: 3.0,
+        efficiency: 0.65,
+    };
+    let wavelength = 0.025;
+    let from_geometry = Pattern::Parabolic {
+        diameter: 3.0,
+        efficiency: 0.65,
+    };
+    assert!(
+        (from_geometry.peak_gain(wavelength).expect("valid").ratio()
+            - dish.peak_gain(wavelength).expect("valid").ratio())
+        .abs()
+            < 1e-9
+    );
+    assert!(
+        (from_geometry.beam_width(wavelength).expect("valid")
+            - dish.half_power_beam_width(wavelength).expect("valid"))
+        .abs()
+            < 1e-12
+    );
+
+    let direct = Pattern::Direct {
+        peak: Gain::from_dbi(30.0).expect("valid"),
+        beam_width: 0.05,
+    };
+    // The wavelength is ignored for a directly specified pattern.
+    assert!(
+        (direct
+            .peak_gain(0.025)
+            .expect("valid")
+            .dbi()
+            .expect("positive")
+            - 30.0)
+            .abs()
+            < 1e-9
+    );
+    assert!((direct.beam_width(0.025).expect("valid") - 0.05).abs() < 1e-12);
+}
+
+#[test]
+fn an_invalid_direct_pattern_produces_no_beam_width() {
+    let direct = Pattern::Direct {
+        peak: Gain::from_dbi(30.0).expect("valid"),
+        beam_width: -1.0,
+    };
+    assert_eq!(direct.beam_width(0.025), None);
+}
+
+#[test]
+fn the_default_pattern_is_effectively_isotropic() {
+    let pattern = ElementPattern::default();
+    assert!(pattern.is_valid());
+    // With a floor at the smallest positive number the attenuation is
+    // negligible in every direction.
+    let gain = pattern.gain_at(1.0).expect("valid");
+    assert!(gain.dbi().expect("positive").abs() < 1e-6);
+}