Amphenol  MIL-DTL-38999 Series I, Series II and Series III are high-reliability circular connector families for aerospace, military and harsh-environment equipment. Series I uses a scoop-proof bayonet interface, Series II prioritizes low profile and reduced weight, while Series III uses a self-locking threaded interface for the highest vibration, shielding and environmental performance.

The three series share many contact technologies, but their mechanical interfaces are different and Series I, II and III do not intermate with one another.

Quick Comparison

What Is MIL-DTL-38999?

MIL-DTL-38999 defines miniature, high-density circular connectors with removable crimp contacts for demanding electrical interconnect applications. The family supports signal, power, coaxial and other specialized contact configurations.

Common characteristics include:

• Rear-insertable and rear-release crimp contacts

• Multiple contact sizes and insert arrangements

• Environment-resistant construction

• Keying options to prevent incorrect mating

• Conductive shell finishes for EMI shielding

• Panel receptacles and cable plugs

• Operation across wide temperature ranges

• Aerospace and military qualification options

Series selection determines the physical mating interface. The complete part number then defines shell style, finish, shell size, insert arrangement, contact gender and polarization.

MIL-DTL-38999 Series I: Amphenol LJT

Amphenol LJT connectors represent MIL-DTL-38999 Series I. They combine a bayonet coupling with a scoop-proof shell design.

Key characteristics

• Three-point bayonet coupling

• Scoop-proof receptacle interface

• Rear-release crimp contacts

• Environmental sealing

• Medium- and high-density contact layouts

• Conductive shell-to-shell mating

• Compatible with corresponding MIL-DTL-27599 Series I interfaces

What does scoop-proof mean?

A scoop-proof connector has a sufficiently deep receptacle shell that guides the plug before its contacts can reach the mating contacts. This reduces the risk of bent pins or damaged inserts when the plug is introduced at an angle.

This feature is useful in applications involving:

• Frequent field maintenance

• Limited visibility during mating

• Dense pin arrangements

• Mobile military equipment

• High-value avionics hardware

Advantages

• Faster bayonet mating than a multi-turn threaded connector

• Better contact protection than Series II

• Reliable operation in harsh environments

• Established use in aircraft and military equipment

• Available with breakaway and specialized configurations

Limitations

• Deeper profile than Series II

• Bayonet coupling may be less resistant to severe vibration than a properly mated Series III connector

• Does not mate with Series II or Series III

Typical applications

• Aircraft avionics

• Flight-control equipment

• Radar systems

• Military communications

• Navigation equipment

• Ground-support systems

• Rugged test equipment

Series I is appropriate when quick bayonet mating and scoop-proof contact protection are both important.

MIL-DTL-38999 Series II: Amphenol JT

Amphenol JT connectors correspond to MIL-DTL-38999 Series II. They use bayonet coupling but feature a shorter, lightweight shell intended for installations where space is limited.

Key characteristics

• Three-point bayonet coupling

• Low-profile shell

• Reduced connector length and weight

• Rear-release crimp contacts

• Environment-resistant configurations

• Medium- and high-density insert arrangements

• Compatible with corresponding MIL-DTL-27599 Series II interfaces

Main advantage: low profile

Series II was developed for aerospace installations where connector height and weight must be minimized. Its shorter receptacle interface occupies less space behind or around an equipment panel.

This makes it suitable for:

• Compact avionics enclosures

• Equipment with limited panel depth

• Dense aircraft electronic bays

• Weight-sensitive equipment

• Legacy aerospace platforms using JT interfaces

Advantages

• Compact axial length

• Generally lower weight

• Quick bayonet mating

• Broad range of established insert arrangements

• Suitable for many standard avionics connections

Limitations

• Not scoop-proof

• Contacts receive less shell protection during initial mating

• Generally not the first choice for severe vibration

• Does not mate with Series I or Series III

Typical applications

• Aircraft instrumentation

• Compact avionics

• Communication modules

• Navigation systems

• Electronic control units

• Legacy aircraft wiring

Series II is usually selected when maintaining an existing JT interface or when low-profile bayonet coupling is the main mechanical requirement.

MIL-DTL-38999 Series III: Amphenol TV and CTV

Amphenol TV metal-shell and CTV composite-shell connectors correspond to MIL-DTL-38999 Series III. Series III uses a self-locking tri-start threaded coupling and is generally the preferred choice for severe environments and many new high-reliability designs.

Key characteristics

• Tri-start threaded coupling

• Self-locking anti-decoupling mechanism

• 100% scoop-proof design

• Enhanced EMI shielding

• Improved interfacial sealing

• High-temperature vibration resistance

• Aluminum, stainless-steel and composite shells

• Numerous finish and contact options

• Standard shell sizes from 9 through 25

Amphenol describes Series III as its highest-performance MIL-DTL-38999 platform for general-duty and severe-environment applications. 

Advantages

• Strong resistance to vibration and accidental decoupling

• Scoop-proof contact protection

• Improved EMI/RFI performance

• Broad selection of materials and finishes

• Composite options for reduced weight

• Operation up to +200°C in applicable configurations

• Support for power, high-speed, filtered and fiber-optic derivatives

• Wall-mount, jam-nut and cable-plug shell styles

Limitations

• Threaded mating is not as immediately disconnectable as a bayonet interface

• Shell and accessory selection can be more complex

• Series III does not mate with Series I or Series II

• Higher-performance configurations may cost more

Typical applications

• Modern military aircraft

• Helicopters and UAVs

• Military ground vehicles

• Radar and electronic warfare

• Missile and weapons systems

• Marine electronics

• High-vibration engine interfaces

• Rugged industrial equipment

Series I vs Series II

Both Series I and Series II use bayonet coupling, but their mechanical priorities differ.

Series I vs Series III

Series I and Series III are both scoop-proof, but they use different coupling systems.

Series II vs Series III

Series II prioritizes compact installation, while Series III prioritizes rugged performance.

FAQ

Which Amphenol names correspond to Series I, II and III?

Amphenol Series I is LJT, Series II is JT, and Series III is TV for metal shells or CTV for composite-shell versions.

Which series is scoop-proof?

Series I and Series III are scoop-proof. Standard Series II JT connectors are not.

Which series is the smallest?

Series II generally provides the shortest and lightest standard profile. Actual size and weight depend on shell style, shell size and material.

Which series is best for severe vibration?

Series III is generally the preferred choice because of its self-locking tri-start threaded coupling and high-vibration design.

Can a Series I plug mate with a Series II receptacle?

No. Series I, II and III mechanical interfaces are different and do not intermate.

Which series is best for a new aerospace design?

Series III is usually the strongest starting point for new severe-environment designs. Series I or II may still be appropriate when bayonet coupling, low profile or legacy compatibility is required.

Conclusion

Choose Amphenol Series I LJT for scoop-proof protection with fast bayonet mating, Series II JT for a lightweight and low-profile bayonet interface, and Series III TV/CTV for the strongest combination of vibration resistance, shielding, sealing and product options.

Before ordering, verify the series, shell style, shell size, insert arrangement, contact gender, key orientation, finish and backshell requirements. DataStorageIC  supplies Amphenol MIL-DTL-38999 connectors for prototype, maintenance and production requirements.