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Overview
I stock a variety of transponders and transponder accessories as you can see in the table below.  Further down this page is a transponder comparison table. 

Transponder Overview Article
Eric Greenwell has written a very nice article which gives an overview of transponder terminology and available products.  This article was originally published as two articles in Soaring magazine in February and March 2002,
and updated January, 2008 for publication on the Soaring Safety Foundation web site.  Eric flies with a Becker transponder in his motorglider. 
The article is available here: http://www.soaringsafety.org/prevention/articles.html.

Transponder Antenna Tips
DG has an excellent document on their web site with details on how to mount transponder antennas to gliders. 
http://www.dg-flugzeugbau.de/Data/TN-DG/service-infos/2004-53-e.pdf

Antenna Cables
I offer transponder antenna cables made by Larry Goddard in several lengths.  They include a TNC connector for connection to the transponder and a BNC connector for connection to a transponder antenna.  They are available in 3 m (9.84 feet) and 5 m (16.4 feet) lengths.

Transponder Antennas
Several transponder antenna options are listed on the Antennas page.  The RAMI rod style and "shark fin" styles are the most popular.  I recommend the shark fin style because I recall from my fluid dynamics courses that the two worst types of bodies in regard to drag are spheres and cylinders.  For installations inside fiberglass, wood and fabric gliders, or in the vertical fin in carbon fiber gliders (with the fin made of fiberglass) you may want to consider the L2 antenna from AAE.

Aviation Transponder Interrogation Modes
Below is a link to a Wikipedia page which explains the difference between Mode A, C and S transponder interrogation modes and other interesting tidbits.
http://en.wikipedia.org/wiki/Aviation_transponder_interrogation_modes

Size Comparison
Below is a photo showing just how much smaller the TT21 is than a Becker ATC4401 transponder.  The Becker transponder and ACK A-30.4 altitude encoder are on the left.  The TT21 transponder and TC20 control unit are on the right.  Click on the image to view a larger version.

Transponder Products Table
Manufacturer
Click on link to see details
Description

Photos
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Logo

ACK Altitude Encoders
Advanced Aircraft Electronics, Inc. Communication, Navigation and Transponder Antennas for use inside fiberglass, wood, and fabric aircraft, and in the vertical fin of carbon fiber aircraft with the fin made of fiberglass.
Becker Mode C and Mode S Transponders
Comant Antennas for transponders
R. A. Miller Industries, Inc. (RAMI) Transponder Antennas
Trig TT21 and TT22 Mode S Transponders with built-in altitude encoder and remote head

 

Transponder Comparison Table
Thank you very much Darryl Ramm and Eric Greenwell for your help creating this table and the notes below.
Please see the notes below the table.
 
  Becker
ATC 4401-1-175
Becker
ATC 4401-1-250
Becker
ATC 6401-2-(01)
Becker
ATC 6401-1-(01)
Trig
TT21
Trig
TT22
Nominal Output Power (measured at rear connector) 175 Watts 250 Watts 150 Watts 250 Watts 130 Watts 250 Watts
Price for transponder only $2070 $2270 $2903 $4990 $2280 $2295
Altitude Encoder Requires External Requires External Requires External Requires External Built-in Built-in
Price for transponder, altitude encoder, and wiring harness $2486 $2686 $3319 $5406 $2553 $2568
Mode* Mode C Mode C Mode S Mode S Mode S Mode S
Class** Class 2 Class 1 Class 2 Class 1 Class 2 Class 1
Level*** N/A N/A Level 2 es Level 2 es Level 2 els Level 2 els
Maximum Altitude**** 15,000 ft 50,000 ft 15,000 ft 50,000 ft 15,000 ft 35,000 ft
Input Voltage 9.5 to 32.2 VDC 9.5 to 32.2 VDC 10 to 33 VDC 10 to 33 VDC 9 to 33 VDC 9 to 33 VDC
Input Current (standby) 0.27 A at 13.75 V 0.27 A at 13.75 V 0.22 A at 14 V 0.22 A at 14 V 0.15 A at 14 V 0.15 A at 14 V
Input Current (typical) 0.40 A at 13.75 V (175 W) 0.50 A at 13.75 V (250 W) 0.43 A at 14 V 0.43 A at 14 V 0.28 A at 14 V 0.34 A at 14 V
Input Current (standby) - including ACK A-30.4 altitude encoder 0.37 A at 13.75 V 0.37 A at 13.75 V 0.32 A at 14 V 0.32 A at 14 V Built-in Alt. Encoder Built-in Alt. Encoder
Input Current (typical) - including ACK A-30.4 altitude encoder 0.60 A at 13.75 V (175 W) 0.70 A at 13.75 V (250 W) 0.63 A at 14 V 0.63 A at 14 V Built-in Alt. Encoder Built-in Alt. Encoder
How is it programmed with the aircraft's unique ICAO address, etc? Programming not Required Programming not Required Address Module Required
Programmed by Dealer
Address Module Required
Programmed by Dealer
Data entered using buttons
and knobs on control unit.
Data entered using buttons
and knobs on control unit.
1090ES/ADS-B data-out
($3500 certified GPS required at this time)
N/A N/A 1090ES/ADS-B out 1090ES/ADS-B out   1090ES/ADS-B out
1090ES/ADS-B data-in for traffic data N/A N/A no no no no
Dimensions 61.3 x 61.3 x 204 mm
(2.413 x 2.413 x 8.03 inch)
61.3 x 61.3 x 204 mm
(2.413 x 2.413 x 8.03 inch)
61.3 x 61.3 x 205 mm 61.3 x 61.3 x 205 mm Controller: 45 x 45 x 64 mm
Transponder: 62 x 45 x 141 mm
Controller: 45 x 45 x 64 mm
Transponder: 62 x 45 x 141 mm
Weight 725 g (1.598 lb) 725 g (1.598 lb) 800 g 800 g 1.0 lb (450 g)
(less than 100 g for controller)
1.0 lb (450 g)
(less than 100 g for controller)

* Mode

Mode S transponders also support Modes C and A capabilities.  Mode C transponders also support mode A capabilities.

** Class:

  • Class 1 Transponders are higher output power than class 2 devices and must deliver between 18.5 dBW (125 W) and 27.0 dBW (500 W) at the transponder antena.
  • Class 2 transponders are lower power than class 1 devices and must deliver between 18.5 dBW (70 W) and 27.0 dBW (500 W) at the transponder antenna. Class 2 transponders are limited by regulations to VFR use, below 15,000 ft MSL and below 175 knots airspeed.
  • For the definition of a decibel (dB) see http://en.wikipedia.org/wiki/Decibel
    For the definition of decibel watt (dBW) see http://en.wikipedia.org/wiki/Decibel_watt

*** Level:

Transponder Level and extended capability letter codes are standard ways of describing the the capabilities of Mode S transponders. This is not relevant to Mode A or Mode C transponders. In practice all current Mode S transponders of interest to glider pilots are Level 2 devices. For future compatibility with ADS-B pilots should look for extended squitter support which provides 1090ES/ADS-B data out and possibly Data-in capabilities. The option code "e" below for extended squitter and whether data out only or data-out and data-in is provided is important for future ADS-B compatibility. All much of the rest can effectively be ignored. The level specification is sometimes accidently confused with the transponder class, so be careful reading specifications. For the technically interested the definitions of the different levels are below.

Level 1 (Comm A) No datalink capability - the transponder can receive 56 bit messages, and reports pressure altitude and and supports Mode A, and Mode S selective addressing.
Level 2 (Comm B) Level 1 + receive and transmit 112 Bit Messages Level 2A Same as Level 2 without the support air-initiated Comm B messages
Level 3 (Comm C) Level 2 + receive 16 linked 112 bit Extended Length Messages (ELM) segments
Level 4 (Comm D) Level 3 + transmit 16 linked 112 bit Extended Length Messages (ELM) segments

Note that ELM messages are completely different than the "extended squitter" that provides ADS-B/1090ES data-out and possibly data-in capabilities. ELM message types and Level 3 and Level 4 will likely never be useful for gliders, Level 4 transponders require prohibitively high power to ensure high enough data rates.

In addition to a transponder level there are capability letter codes that describe additional transponder capabilities. The ones on interest to glider pilots are:

  • e - Extended squitter, the ability to send ADS-B/1090ES data. This requires an external GPS (certified GPS units only). Mode S transponders with 1090ES capability can always send ADS-B data out but may not be able to receive ADS-B data in to receive traffic information. The actual data out and data-in capabilities for each model are shown out in the table above.
  • l - Elementary surveillance, i.e. basic position reporting derived from SSR radar, enhanced in the case of a Mode S environment with higher resolution (25') pressure altitude, unique ICAO aircraft ID and other Mode S capabilities. Note that this is different than 1090ES/ADS-B data-out capabilities.
  • n - Enhanced surveillance, transmission of a state vector including ground speed by the transponder. No transponders of interest for gliders currently include this capability. Note that this is different than 1090ES/ADS-B data-out capabilities.
  • s - Surveillance Identifier (SI) code. Used by Mode S transponders and Mode S equipped SSR to limit interrogations to specific transponders.

    Typical Mode S transponders of interest to glider pilots will be Level 2es or Level 2els devices.

**** Altitude Limits

The 15,000 foot altitude limit is a regulatory limit (see Class 2 in the ** note above).  The Trig TT21 will work to 35,000 feet and the ACK A-30.4 Altitude Encoder used with the Becker transponders will work up to 30,000 feet.

Other Notes:

Most transponders installed in gliders are Class 2 devices because of their lower cost and reduced power consumption. Nominal power outputs for different transponder appear higher than those required by the Class requirements to allow for typical antenna cable losses.

It is the aircraft owner's responsibility to ensure that any transponder is correctly installed. If in doubt check with you local A&P. As a part of any transponder installation, including in experimental aircraft, altimeter/encoder tests per FAR 43 Appendix E and radio frequency signal tests per FAR 43 Appendix F are required. For non-IFR aircraft, there is no ongoing requirement for the FAR 43 Appendix E test, except they need to be repeated any time the static pressure system is worked on. Every two years the FAR 43 Appendix E radio frequency signal check must be repeated. Altimeter/encoder calibration is *not* checked in these ongoing "Appendix E" tests. You can confirm the encoder altitude on the altitude display of most modern transponders, and always confirm current altitude on initial contact with ATC.

This information is for the United States, regulations and certification may be different in other locations.

Some transponders have connections for GPS input.  It is not OK to connect just any GPS to the device.  Only certified GPS units may be used.

Although some new transponders incorporate ADS-B functionality, it is not possible to simply connect your PDA to the unit and display traffic information on the soaring software in the PDA.  That functionality is not currently available.

Cumulus Soaring, Inc.
Paul E. Remde

8661 Connelly Place
Savage, Minnesota 55378
USA


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