E20 Metal Antenna Intermodulation

---
name: e20-metal-antenna-intermodulation
description: "Use when compute the passive intermodulation a metal based antenna generates under ECSS-E-ST-20C clause 7.2.2.4.1 and the performance consequence it carries: categorize every metal junction on the radiating path as a loose contact, a pressure joint, a ferromagnetic plating, an oxidised or contaminated face or a benign welded interface, enumerate the intermodulation products of the transmit carrier set with their coefficient vector, order and frequency, isolate the products landing inside a receive band, scale each one from the measured reference level by its per-carrier order, carry it through the transmit-to-receive isolation, and check the receiver-noise-floor degradation against the allowance that band holds. Trigger: ecss, e-st-20c-clause-7-2-2-4-1, passive-intermodulation-product, metal-antenna-junction-nonlinearity, intermodulation-product-order, receive-band-fall-in, pim-reference-level-scaling, receiver-noise-floor-degradation."
license: Apache-2.0
compliance: STANDARDS-REF
standards:
  - id: ecss
    reference-only: true
gated: false
domain: space-systems
pack: space-systems
compatibility: "agentskills.io SKILL.md; any SKILL.md host (Claude Code, Hermes, OpenClaw)"
metadata:
  domain: space-systems
  subdomain: ecss
  tags: [ecss, e-st-20-electrical-scope, e20-metal-antenna-intermodulation, passive-intermodulation-product, metal-antenna-junction-nonlinearity, intermodulation-product-order, receive-band-fall-in, pim-reference-level-scaling, receiver-noise-floor-degradation]
  version: 0.1.0
  author: Aero Agent Skills
---

# ECSS Electrical Engineering -- Metal Antenna Intermodulation (space-systems/ecss/e20-metal-antenna-intermodulation)

Use when the task is the clause 7.2.2.4.1 concern of ECSS-E-ST-20C --
the passive intermodulation a metal based antenna makes by itself while
transmitting, and what that self-generated interference costs the
receive chain sharing the aperture.

## Domain quick reference

- Passive intermodulation is generated by the antenna hardware, not by
  an active stage. A metal radiating structure carrying two or more
  transmit carriers behaves as a weak nonlinearity wherever the current
  crosses a junction that is not a continuous conductor: a loose metal
  contact, a pressure-contact joint that relies on clamping force,
  ferromagnetic plating such as nickel under the finish, a
  dissimilar-metal contact, or an oxidised or contaminated face. A
  welded joint, a brazed joint and a monolithic machined face are
  continuous and sit in the benign family. Categorizing every junction
  first is what separates a hardware finding from a level calculation:
  a junction in a nonlinear family with no control action on record is
  already a finding, whatever the computed level says.
- An intermodulation product is a signed integer combination of the
  transmit carriers. Its order is the sum of the absolute coefficients,
  and only products with a strictly positive frequency are physical.
  Odd-order products of closely spaced carriers land near the transmit
  cluster; the ones that matter are those falling inside a receive
  band, because the antenna offers them no attenuation at all -- they
  are born on the receive side of the duplexing.
- The product level is not predicted from first principles. It is
  scaled from a measured reference product level taken at a reference
  tone power: each carrier contributes its absolute coefficient in
  decibels per decibel of departure from that reference, which collapses
  to the familiar order-times-delta law when all tones sit at the same
  level. Raising a transmit tone by 1 dB therefore raises a third-order
  product by roughly 3 dB.
- The consequence is measured at the victim receiver. The product level
  reaching the receiver is the junction level less the
  transmit-to-receive isolation; the receiver-noise-floor degradation
  it causes is the decibel rise of the floor once that interferer is
  added to the thermal noise. Several in-band products in one band add
  in the linear domain before the degradation is taken. The allowance
  is a per-band number carried by the link-budget, not a constant.

## Workflow

1. Inventory every metal junction on the radiating path and categorize
   each one into its family. Reject an unrecognised family before it
   enters the assessment, and raise a finding for any junction in a
   nonlinear family without a recorded control action (surface
   preparation, joint redesign, plating substitution, contact-pressure
   qualification).
2. Enumerate the intermodulation products of the transmit carrier set
   up to the order the project analyses, keeping for each product its
   coefficient vector, its order and its frequency, and discarding any
   combination that does not yield a positive frequency.
3. Intersect the product frequencies with the declared receive bands.
   A product with no receive band to fall into is not a performance
   consequence and leaves the calculation here.
4. Scale each in-band product from the measured reference level using
   the per-carrier coefficient law, then subtract the
   transmit-to-receive isolation to get the level at the receiver
   input.
5. Compute the receiver thermal-noise floor from the receive bandwidth
   and noise-figure, add the in-band products of each band in the
   linear domain, and convert to a noise-floor degradation in decibels.
6. Compare the per-band degradation against the allowance that band
   holds. The antenna is compliant only when every band is inside its
   allowance and no junction finding is open.

## Pitfalls

- Reading an empty in-band product list as a clean antenna. It only
  says the enumerated order range produced nothing in band; a
  nonlinear junction with no control action is still an open finding,
  and a higher order may reach the band the analysis stopped short of.
- Applying the order-times-delta law when the tones are not equal.
  With unequal tone powers, each carrier contributes its own absolute
  coefficient times its own departure from the reference tone power;
  using the order alone against the loudest tone overstates the level
  on every asymmetric carrier plan.
- Comparing the junction-level product directly against the receiver
  threshold. The level that matters is after the transmit-to-receive
  isolation; skipping that step condemns hardware that is comfortably
  compliant.
- Assessing each in-band product on its own. Products sharing a receive
  band add in the linear domain first, and a band holding several
  products just inside the allowance can be outside it once they are
  summed.
- Letting a band-edge product escape on a rounding difference. A
  product frequency is a signed sum of carrier frequencies, so a
  product physically sitting on the band edge can land a few units in
  the last place outside it; the membership check absorbs that
  representation error rather than the band being widened.

## Behavior contract (gate 3)

The junction categorization, product enumeration, receive-band
membership, reference-level scaling and noise-floor-degradation logic
is exercised by the gate 3 contract test:
`scripts/test_e20_metal_antenna_intermodulation.py` against
`scripts/e20_metal_antenna_intermodulation_logic.py` (stdlib unittest,
offline, deterministic). Run:
python3 scripts/test_e20_metal_antenna_intermodulation.py

## Compliance

- ECSS standards are freely downloadable (ESA); cite the source and
  paraphrase per standards-map.yaml.
- compliance: STANDARDS-REF, gated: false.