While transponder replacement ranks near the bottom of most owners upgrade list, eventually-and often unexpectedly-you’ll be writing a check for a few grand on a transponder upgrade. you’ll likely rely on your shops recommendation for replacement options but there are several issues and a handful of models to consider. A proactive replacement may even be warranted given the modern technology found in many new models.

Heres a review of the current market offerings in ATC transponder gear and a review of the important accessories that should be addressed during installation.

Think Long-Term

Ease of installation shouldnt be the primary consideration when selecting a replacement transponder. You could shop the market for a used, exact-replacement for that ancient Cessna model only to face another failure next month. When an old model fails, our advice is to bite the bullet and replace it with a new unit. This usually includes replacing the interface wiring (also 20-plus years old) and antenna system thats integral to the transponders performance.

L-band transponder antennas are either fiberglass blades or a rod and ball. The

blade style, which looks like a small shark fin, generally offers higher performance and increased durability. One careless swipe with the cleaning brush during a belly-degrease job will trash the relatively fragile rod and ball antenna. Since transponder and DME equipment operate on the same frequency, they share the same design but use dedicated and independent antennas. Garmin markets a blade-style L-band kit for $250.

Replacement of the transponder antenna and installing new twin-shield, low-loss cable that connects it to the transponder can represent a large portion of the upgrade cost depending the antennas location on the airframe. Ask your shop if they plan to completely rewire the new transponder or saddle it up with the old existing wiring, using adapter connectors. Using existing wiring can save money but we think replacing the wiring is the best, long-term solution. The antenna and cable installation is directly related to system performance, so skimping on it shortchanges the upgrade.

The same can be said for the encoder or Mode-C altitude-reporting portion of the system. If its quality or operability is in doubt, consider replacing it, too. The shop can test the encoder and offer their advice on whether its a keeper or not.

Garmin Leads the Pack

Garmin transponders can play an integral role in the modern radio stack. They output RS232 serial data as we’ll as high-speed Arinc for interfacing with panel-mount GPS navigators. When its all connected, the combination can provide

impressive automation. The transponders can also feed pressure altitude data to GPS navigators over the serial stream and listen to select Garmin GPS systems so the transponder automatically enters ALT mode on the takeoff roll. This connection is optional and requires extra effort during installation.

Garmins entry-level model is the $2195 GTX320A. don’t let the traditional, mechanical front end fool you. All of Garmins transponders are solid-state which results in several benefits including no warm-up delay, cooler operating temperature, reduced power consumption, and longer lifespan. Gone is the expensive, heat-inducing cavity tube found in many vintage models. The surface-mount design of the GTX320A is helpful in tight applications where panel depth is an issue. The rear chassis measures a short six inches with an overall weight of two pounds.

For a step up in features, the $2495 digital GTX327 is the perfect match for a wide range of aircraft. It has all the modern amenities you could want in a transponder. It borrows the GTX320As short-chassis design. For aircraft that spend most of the time in flight levels, the GTX327 is a popular choice for a back up to a primary, Mode-S model. If a Garmin IFR GPS upgrade is in your long-term plans, the GTX327 lays the groundwork for later automatic timer operation and transponder-mode selection, activated by the GPS ground speed. With pressure altitude readout on the display, you’ll always know what the Mode C is reporting.

Garmins $3500 flagship GTX330 is a Mode S unit with datalink traffic. Its still a popular choice and, in our view, the most cost-effective route to traffic-minding capabilities. If it werent for the name on the bezel, you couldnt tell it apart from the GTX327, but it does require 11.25 inches of panel depth. The GTX330 has the pressure altitude readout and timer features found on the GTX327, but adds OAT and density altitude (when interfaced with a remote sensor) and a basic altitude minder that chides “leaving altitude” when you deviate off a set altitude. Voice prompting requires interface with the aircraft audio system via the same output that provides TIS traffic callouts.

The GTX330s pictorial traffic alerting is familiar TCAS-like symbology and connects

via Arinc 429 digital output to a variety of displays, including the Garmins GNS 400- and 500-series navigators, the GMX200/MX20 MFD, Avidyne displays and even on the Garmin GPSMAP 396/496 portable systems.

Bendix/King

The hugely popular KT76A model is still offered in the Bendix/King line, at $1978 list. Its regarded as the workhorse of the transponder world. But it isn’t exactly a modern unit. Using a cavity tube to transmit its 200 watts of power, a serious bench repair can yield a hefty invoice. If a direct replacement is the only feasible option for you and the existing wiring is in good order, a new KT76A at $1978 isn’t a bad idea. Just don’t expect much in the way of gee-whiz automation.

A better option, in our view, is the $2535 KT76C. Its essentially a KT76A with a digital front-end, featuring push-button code entry, pressure altitude readout and automatic VFR button. It uses the same wiring as a KT76A so retrofit is seamless, except for a new mounting tray for some cases.The KT76C still uses a cavity tube and is a surface-mount design, which can make for a difficult bench repair in the field. Were also surprised Honeywell hasnt made the switch to solid-state transmitters for their lower-end transponders, as Garmin has.

For higher-end Mode S applications, Honeywell offers the mature KT70-the first Mode S unit available for general aviation aircraft. It has a digital front end with all the necessary Mode S addressing. It connects to a remote configuration module, which stores the aircraft data (tail or flight ID number, speed and weight class, etc.). Weve recently witnessed failure of this module in a helicopter which caused

tail-chasing troubleshoot. Were fonder of Garmins Mode S design, which stores the aircraft data within the transponder itself. Still, the $5926 KT70 is a quality machine, in our view.

The KT73 is a Mode A/C/S transponder with TIS datalink traffic capability (the equivalent to Garmins GTX330) at $5446. TIS traffic symbology from the KT73 can be streamed to Honeywells KMD250, KMD550 and KMD850 multi-function displays. The KT73 was designed to fit into the same mounting rack utilized by the KT76A and KT70 models. Using similar controls as the KT70, including rotary-knob code entry, digital display and pressure altitude readout the KT73 is fully solid state and outputs 200 watts of power without a cavity tube.

Narco Avionics

As the old Narco AT50-series transponders die from old age, Narco has a worthwhile replacement option through their AT165 Value Series. The Mode A/C AT165 is fully solid-state and features a digital display with rotary knob squawk tuning, automatic VFR button and weighs less than two pounds. The unit will slide into an existing AT50/AT50A/AT150 installation without the need for rewiring. You want to be sure the existing wiring is in good condition, of course, but for no-frill pilots with basic VFR airplanes the Narco Value Series is worth considering.

Narco also makes the AT165/C model, which replaces Cessna/ARC transponders without the need for rewiring.

The AT 165 is also available with two screens to add an altitude alert function, pressure altitude readout, multiple flight timers with audible alerts and reversionary dual displays for redundancy. With 250 watts of solid-state transmitter power and a surface mount design, the AT165 is considered a fresh design. Most of the industry has wondered what Narco has been doing the past 10 years, to which the answer is designing new transponders.

Becker Avionics

German avionics manufacturer Becker Avionics offers two models for applications where space is an issue. The $2265 ATC-4401 is a Mode-A/C model that fits into a 2.25-inch instrument cutout. It weighs 1.6 pounds and has a digital LCD display, automatic VFR button and an integral, on-screen bus voltage monitor.

The BXP-6401 is a Mode-S model, certified for operation up to 15,000 feet and also mounts in a 2.25-inch cutout. It has a street price of around $2500. For sport aircraft and Warbird applications, the Becker offerings are worth considering. Our experience is that Becker-USA offers good support and rapid turn-around for repairs from its base in Miramar, Fla.

Squawk Digital

Transponder installations have become more complex given the higher-level of technology and interface ability. Be sure to budget for extra work-connecting to your GPS, replacing old coaxial cable and antenna, yoke-mounted ident switches and controls for a dual-transponder setup. Also, think about putting the transponder in easy reach if you plan to use the flight timer or other features often.

Our top pick for a ground-up replacement Mode A/C transponder is Garmins GTX327. Given its feature set, reasonable cost, modern circuitry and physical size, we think its a prudent option for nearly any aircraft that needs a transponder.

Larry Anglisano is Aviation Consumers Avionics Editor.