I haven't sold a TT21 in many years.  It doesn't work for ADS-B OUT and isn't available in stock in the USA.  I recommend the TT22 in ALL new installations.

Trig TT21 Class 2 Mode S Transponder with Built-in Altitude Encoder
The TT21 is a very interesting new transponder.  It is a Mode S transponder with a built-in altitude encoder, and a remote head - at a great price! 

Important Note
This Trig TT21 does not meet the FAA 2020 ADS-B Out requirement for powered aircraft (that gliders are exempt from), and therefore also may not be suitable to provide ADS-B out in gliders should gliders lose that exemption in future. The higher power Trig TT22 (below) does meet the FAA 2020 ADS-B Out requirements (when used with an approved GPS source). Both the Trig TT-21 and TT-22 are usable as basic transponders (without 1090ES Out) in the USA, now and after 2020.

The TT21 is a Class 2 transponder - certified for use up to 15,000 feet.  The TT22 (below) is a Class 1 transponder - certified for use up to 35,000 feet.  If you plan to fly over 15,000 feet, you will probably want the TT22 (below).

With a front panel controller smaller than a standard instrument, and a remote transponder that is not much bigger than a pack of cards, the TT21 can fit into the smallest of aircraft – and yet it is fully certified and compliant with all the transponder requirements.

  • Low Weight - less than 500 grams!
  • Minimal Panel Space
  • Low Power Consumption
  • 130 Watts Nominal Output
  • Integrated Altitude Encoder
  • Simple Installation
  • Easy to use rotary knobs
  • Support for 1090ES ADS-B Out

The TT21 system is the smallest, lightest, Mode S general aviation transponder. This two part system, which includes an integrated altitude encoder, can fit into the smallest of aircraft – and yet it is fully certified and compliant with all the transponder requirements.

The front panel controller can be installed in a standard 2¼ inch instrument cut-out, or where space is really tight it can fit a compact cut-out just 45mm high. The TT21 transponder itself is also only 45mm high, and can be installed anywhere convenient in the airframe.

Mode S for Light Aviation
The TT21 transponder is an ED-73B Class 2 compliant Mode S level 2 datalink transponder, with support for extended squitter. It meets all the requirements for Mode S elementary surveillance transponders in Europe for aircraft flying below 15,000 feet and with airspeeds below 175 knots. It can be used for both IFR and VFR flight.

The TT21 also supports 1090 MHz Automatic Dependent Surveillance Broadcast (ADS-B) extended squitter, known as “ADS-B out”. This provides improved airborne surveillance, vital information for airport ground surveillance, and is a key feature of future airspace plans. An appropriate GPS receiver is required to enable the ADS-B capability.

Flexible Installation
The TT21 system can be easily fitted to aircraft even when there are space and weight constraints. The front panel controller fits into any panel, and takes less space than a standard instrument. Only a few centimetres are needed behind the panel to allow room for the connector and the static port interface. This means that the TT21 transponder is not just suitable for conventional light aircraft; it can be fitted into gliders, antique aircraft, and flex-wing microlight panels. The control panel is splash proof for open cockpit installations.

A lightweight cable runs from the control panel to the transponder unit which can mounted anywhere in the aircraft. The transponder unit then connects to the antenna and aircraft power supply. A quick release latch secures the main unit to the mounting tray, allowing it to be easily removed for security.

Low Power Design
The TT21 has one of the lowest power consumptions for any transponder. With typical power consumption of around 5 watts, it can operate from battery power for gliders or balloons for long duration flights. In powered aircraft you can be sure that the TT21 is not making heavy demands on your electrical system.

Operating Controls
The front panel controller has easy to operate rotary controls, and a simple layout. The clear LCD screen shows the Squawk Code, reported Flight Level, and operating mode of the transponder.

An altitude encoder is built into the controller, so no extra boxes are needed to complete the transponder system.

Finally, the controller also acts as the configuration module for the combined system. With the controller remaining attached to the aircraft, swapping the TT21 transponder box is possible without needing any reprogramming of airframe settings.

Light Weight
The TT21 system weighs less than 500 grams, with less than 100 grams in the front panel controller. For all aircraft low weight translates into more payload and better efficiency, but for microlight aircraft where certification weight limits are tight the low weight of the TT21 system may make it possible for the first time to fit a transponder.

TT21 Specifications
Class 2 Mode S Level 2 Datalink
ETSO 2C112b, C88a, TSO C112, C88b
ED73B, DO160E, DO178B level B, DO254, DO181C, DO260A
Supply Voltage  (DC):
10 – 33 V (works down to 9 V)
Typical Current Consumption:
0.15 A (at 14V)
Active: 0.28 A (at 14V)
Transmitter Power:
130W nominal at connector
Operating Temperature:
-40 to +70 C
Cooling Requirement:
No fan required
1.0 lb (450 g)
Controller: 64 x 45 x 45 mm 
Transponder: 62 x 45 x 141 mm


  • TT21 Transponder Unit (box)
  • TC20 Control Unit (User Interface)
  • Mounting Tray for TT21 transponder unit
  • Adapter for mounting the TC20 in a 57 mm (2 1/4") instrument hole (with mounting screws)

  • Mounting screws for mounting the TC20 without the use of the adapter for 57 mm instrument hole
  • Pneumatic tubing for connection to static port (connects to TC20 Control Unit)
  • Tee connector for pneumatic tubing
  • 2 x Straight connector for pneumatic tubing
  • DB-25 m Connector and backshell for TT21 unit
  • DB-9f Connector and backshell for TC20 unit
  • Hose clamps for securing pneumatic tubing in aircraft
  • TNC connector for antenna (connects to TT21 box)
  • Installation Manual
  • Operating Manual

Not Included:

  • Wiring harness for connecting TT21 to TC20 and 12V power.  Wiring details are in the installation manual (see link below).  All connectors are included, but they are not wired together.  Pre-made wiring harnesses are available.  You can see details here.


    TT21 and TT22 Brochures, Operating Manuals and Installation Manuals

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    • Brands Trig
    • Product Code: Trig-TT21
    • Availability: In Stock
    • $2,320.00