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