Twinax, Quadrax & Coax Contacts:
Complete Guide to High-Speed Contacts in MIL-DTL-38999 Connectors

One Circular Connector, Three Signal Worlds

A single MIL-DTL-38999 shell can carry 28 V DC power, RS-422 telemetry, and a Gigabit Ethernet backbone all at once. The reason that's possible is the contact — not the shell. Standard size 22D/20/16 contacts are single conductors: one wire in, one wire out. They work fine for power and low-speed signal, but they cannot carry a high-speed differential pair because they don't control impedance through the connector.

For anything that runs above a few hundred megahertz — data buses, video, RF, Ethernet — engineers switch to a family of specialized contacts: coax, twinax, and quadrax. These contacts preserve a precise characteristic impedance from the wire, through the connector, and back out the other side. Pick the wrong one, or treat a high-speed pair like two ordinary contacts, and the signal reflects, corrupts, and drops. This guide explains the three families, their impedances, their contact sizes, and exactly which standard calls for which contact.

Coax vs Twinax vs Quadrax: What's Inside Each Contact

The three families are defined by how many conductors they pack into a single contact cavity, and whether the signal is single-ended or differential.

Coax carries one signal referenced to ground — ideal for RF antennas, GPS, IF feeds, and analog video. Twinax carries a single balanced differential pair inside a shared shield — the workhorse of MIL-STD-1553 and 100Base-T. Quadrax carries four conductors arranged as two matched differential pairs (or a signal pair plus its return) — which is exactly what Gigabit Ethernet needs, since 1000Base-T uses four pairs and fits them into just two size 8 quadrax contacts.

The outer shield on all three does double duty: it provides the return path and, more importantly, it contains the field so adjacent contacts don't crosstalk into each other. That containment is why a high-density 38999 insert can run multiple gigabit links side by side without interference.

The Impedance Map: Which Standard Calls for Which Contact

Impedance is not a preference — it's written into the data standard, and mismatching it causes reflections that destroy the signal. The table below maps the common aerospace and defense data interfaces to their required contact.

Two rules stand out. First, 1553 and ARINC 429 use 78 Ω twinax, not 50 Ω coax — using an RF coax contact on a 1553 bus is one of the most common field mistakes and it corrupts the Manchester waveform. Second, Gigabit Ethernet never rides a single twinax; it needs the four-conductor geometry of a size 8 quadrax, usually two of them per link.

Contact Sizes and the M39029 Part-Number Families

High-speed contacts are defined by the M39029 slash sheets (the same military spec that covers standard AS39029 signal contacts). The size you can use is fixed by the connector's insert cavity:

• Size 8 — the largest high-speed cavity; the only size that fits a full quadrax. Carries Gigabit Ethernet, IEEE 1394b, and large twinax.
• Size 12 — fits twinax and coax; the standard home for 1553 and ARINC 429 twinax and 50/75 Ω coax.
• Size 16 — smaller coax only; used for compact RF and video runs where density matters more than bandwidth.

The M39029 dash number encodes size, contact type, and gender — for example M39029/28 and /58 are size 12 coax, while the /90-series and /101–/108 sheets cover size 8 twinax and quadrax. When you cross-reference a competitor's contact, the dash number is what you match: it tells you the cavity size and the impedance family at a glance. A quadrax contact for Gigabit Ethernet cannot be swapped into a size 12 cavity, and a size 12 coax will not carry a 1000Base-T signal — the geometry simply isn't there.

The Electrical Parameters That Actually Matter

Beyond impedance, four parameters decide whether a high-speed contact performs in the field:

• Insertion loss — signal attenuation through the mated pair, measured in dB at the operating frequency. Budget this into your link margin, especially at Gigabit speeds.
• VSWR / return loss — how much signal reflects back at the impedance discontinuities. A poor VSWR looks like intermittent noise to the receiver.
• Crosstalk (NEXT / FEXT) — leakage between adjacent contacts. The shield containment of coax/twinax/quadrax is what keeps dense inserts quiet.
• Propagation-delay skew — the timing difference between the two legs of a differential pair. Gigabit Ethernet tolerates only a few picoseconds of skew before the eye closes.

These are the specs to ask for on a datasheet. A "high-speed contact" with no insertion-loss and skew data is a signal contact with a marketing label — ask for the numbers before you commit.

ARINC 600 / ARINC 801 and the Modular Avionics Path

For systems that need many high-speed channels in one box connector, the industry moves beyond circular 38999 to modular rectangular connectors: ARINC 600 (rack-and-panel avionics), ARINC 801 (fiber optic), and EN 4165 / ARINC 809 (modular Ethernet). These use the same quadrax contacts as 38999 but arrange them in modular blocks, so a single connector can mix power, twinax 1553, quadrax Gigabit Ethernet, and fiber termini in one shell. Understanding the contact families carries straight across — the quadrax contact that works in a 38999 shell is electrically the same one that populates an ARINC 600 module.

Five Mistakes That Kill High-Speed Links

• Using 50 Ω coax where 78 Ω twinax is required — corrupts MIL-STD-1553 and ARINC 429 buses.
• Wiring a differential pair as two separate #22D contacts — no controlled impedance, so reflections and skew destroy the signal.
• Putting Gigabit Ethernet on a size 12 twinax — 1000Base-T needs quadrax geometry in a size 8 cavity.
• Ignoring pair skew — a mismatched cable or contact leg closes the eye diagram at 1 Gbps.
• Forgetting the insertion-loss budget — every mated contact pair eats margin; count them all when sizing the link.

Manufacturer Cross-Reference: Who Supplies High-Speed Contacts

Airoadcon High-Speed Contact Products

Airoadcon supplies a high-speed contact family for MIL-DTL-38999 (J599) shells, covering the full impedance map above:

• 78 Ω twinax contacts (size 12) — MIL-STD-1553B and ARINC 429 data buses, matched to M17/176 twinax cable
• 100 Ω quadrax contacts (size 8) — Gigabit Ethernet (1000Base-T) and ARINC 664/AFDX backbones, two contacts per link
• 110 Ω quadrax contacts (size 8) — IEEE 1394b FireWire for IFE and camera systems
• 50 Ω / 75 Ω coax contacts (size 8/12/16) — RF, GPS, antenna and analog video feeds
• Mixed-layout inserts — power + signal + twinax/quadrax/fiber in a single 38999 shell
• 2–4 week lead time on standard configurations; full cross-reference support for existing Amphenol/TE/Glenair part numbers

For cross-reference assistance — mapping an Amphenol, TE Deutsch, Glenair, or Radiall high-speed contact part number to an Airoadcon J599 equivalent — send your part number and your data-interface standard. We verify impedance, cavity size, and insertion-loss performance before production orders.