Transponder Buyer’s Guide

Reliable transponder performance is critical in the ADS-B environment. Garmin and Sandia are top picks.


You still need‭ ‬a solid-performing transponder now that ADS-B Out is the rule‭. ‬As proof‭, ‬we’re hearing field reports of new and existing ADS-B Out systems flunking performance reports because of Mode C transponder issues‭. ‬The lesson learned for those purchasing remote or add-on ADS-B Out technology is that nursing an aging analog transponder might not be even a short-term solution‭.‬

Luckily there are some affordable‭ ‬solutions‭, ‬especially when carefully‭ ‬shopping the used market‭. ‬If you have a new or existing kit to equip for the ADS-B mandate‭, ‬you’ll pay a premium for a one-box solution‭, ‬but it may be the better long-term plan‭. ‬Here’s a scan of the current market‭.‬

Start at the Antenna

Since Mode A/C/S transponder performance relies so heavily on its L-band antenna system‭, ‬that’s a good place to start‭. ‬If you’re assembling a new kit‭, ‬you have the advantage of running new low-loss and shielded coax cable and new antennas‭. ‬Don’t skimp‮—‬we favor fiberglass blade antennas over the aluminum rod and ball design‭. ‬Check the Comant CI-105‭ ‬as one example of a proven blade model‭.‬

If you’re replacing an old transponder‭, ‬still work your way up to the panel by starting at the antenna‭. ‬If it’s a fiberglass antenna‭, ‬check it carefully‭: ‬Does it have cracks in the casing‭? ‬If it’s in the line of fire of the exhaust stack it might even be melted‭, ‬if not loaded with blow-by oil and grime‭. (‬Here’s a tip for you guys still building‮—‬don’t put any antenna immediately downstream of the exhaust or crankcase breather‭.) ‬For best antenna performance‭, ‬it pays to pull it‭ ‬off for a good inspection‭ (‬and keep it clean‭) ‬and consider changing the signal cable if it’s old RG58‭ (‬the black stuff‭). ‬Even the best coax is prone to deterioration over time‭, ‬especially if it hasn’t been secured properly or protected from moving parts inside the airframe‭.‬

Use the required two-year pitot static and transponder inspection as an opportunity to gauge just how healthy the transponder is.

Outside Help

And if you’re simply unsure whether to‭ ‬replace the transponder or its supporting antenna system‭, ‬take the airplane‭ ‬to a trusted avionics shop and pay close attention to the results of the system’s biennial certification‭, ‬which is governed by FAR 91.413‭. ‬During this test‭, ‬technicians are looking for specific performance characteristics to determine the‭ ‬overall health of the system‭. ‬Ask the‭ ‬shop if your system barely passed or fell well within specs‮—‬this is vitally important‭. ‬Many issues related to poor performance are caused by old antennas and cabling as well as creaky transponders‭, ‬and these tests will help you determine where to look and‭ ‬what to fix‭. ‬

Sometimes it’s worth the one-hour bench labor charge for a closer look and a better understanding of its future.

Don’t forget the Mode C altitude‭ ‬encoder‮—‬another critical component‭ ‬in the system‭. ‬As a primer‭, ‬our rule of thumb is to replace the encoder and the transponder at the same time‭. ‬Encoder‭ ‬technology has improved over the‭ ‬years‭, ‬with most current-production models requiring minimal warm-up‭ ‬time and infrequent calibration‭. ‬Sandia even makes a transponder with integral encoder‭, ‬while Garmin offers a tiny add-on module‭. ‬Smart‭.‬

With all that said‭, ‬let’s take a look at each manufacturer’s wares‭. ‬


This is the same company that makes the popular Stratus portable ADS-B receivers sold by Sporty’s‭. ‬It earned certification for its Stratus ESG rack-mounted‭ ‬ADS-B transponder a few years ago‭. ‬When Appareo began developing the ESG‭, ‬it predicted‭ (‬based on FAA stats‭) ‬that NextGen equipage trends would‭ ‬favor 1090ES transponder solutions and it was right‭, ‬especially given the aging‭ ‬fleet of Mode A/C transponders‭. ‬But‭ ‬there was another strategy‭.‬

From the beginning‭, ‬Appareo engineers were sharply focused on federated avionics interfaces‭, ‬which are‭ ‬radio stacks that might have a variety of avionics brands‭, ‬both old and new‭. ‬It also focused on the price-sensitive‭, ‬lower-end of the retrofit market‭. ‬Many of these targeted buyers likely have Stratus portable ADS-B receivers‭, ‬which boosts brand recognition‭.‬

The Appareo Stratus ESG interfaces with Stratus portable ADS-B receivers.

The Appareo 1090ES-based ESG‭ ‬transponder doesn’t have an internal‭ ‬ADS-B receiver‭, ‬but it does have internal WAAS GPS‭, ‬eliminating the need for an external GPS input‭. ‬Also recognizing that buyers‭ ‬would want a seamless ADS-B In solution‭, ‬Appareo designed the transponder to interface with a new hard-wired‭, ‬remote version of‭ ‬the Stratus portable‭, ‬called the 2i‭.‬

Unlike the models in the original Stratus line‭, ‬the 2i isn’t portable and requires a connection to the electrical bus‭. ‬The benefit is a cleaner installation‮—‬no wires‭ ‬strung across the glareshield and no‭ ‬windshield mounts to deal with‭. ‬Mount it behind the panel or anyplace that’s convenient‭. ‬Connecting the 2i‭ (‬and also the portable 2S/3‭, ‬with an optional interface wiring kit‭) ‬with the ESG provides auxiliary power‭, ‬GPS signal and also the‭ ‬ADS-B reception from external antennas‭. ‬As with the portable Stratus receivers‭, ‬the 2i displays traffic‭, ‬weather and AHARS on the ForeFlight app through a Wi-Fi connection‭. ‬Of course‭, ‬if you want‭ ‬to move the Stratus portable receiver‭ ‬between aircraft‭, ‬you’ll want the 2S‭. ‬If not‭, ‬we favor the 2i’s cleaner interface‭.‬

Appareo has two packages‭: ‬The standalone ESG transponder with installation kit is‭ $‬2995‭, ‬and the ESGi‭ (‬with the 2i receiver‭) ‬is‭ $‬3495‭. ‬You won’t lose any features if you go with 2i over the 2S‮—‬just portability‭.‬


Avidyne’s full-stack retrofit avionics‭ ‬line includes the AXP340‭ ‬transponder‭, ‬which can be connected with the‭ ‬company’s IFD550/540/440‭ ‬navigators for a WAAS position source‭. ‬It’s also approved for use with Garmin’s GNS 430W/530W WAAS navigators‭, ‬plus the newer GTN navigators‭. ‬

The AXP340‭ ‬has no built-in ADS-B receiver‭; ‬you have to buy the company’s‭ ‬Skytrax receiver for that‭, ‬which will‭ ‬display weather and traffic on the IFD navigators‭. ‬It will also display on a wide variety of tablet apps‭, ‬using the IFD’s built-in wireless transceiver‭.‬

The AXP340‭ ‬is designed as a slide-in replacement for the Mode A and Mode C functions of the King KT 76A transponder‭, ‬while also‭ ‬using the King’s‭ ‬mounting rack‭. ‬Avidyne has a remote‭ ‬version of the AXP340‭ ‬called the‭ ‬AXP322‭. ‬Priced at‭ $‬5450‭, ‬the 1090ES AXP322‭ ‬is controlled through the IFD‭ ‬navigators‮—‬a feature that does target‭ ‬Entegra/Garmin-equipped Cirrus models‮—‬because eliminating the transponder from the radio stack frees up space for the larger IFD550/540‭.‬

Becker Avionics

To our surprise‭, ‬we learned in our research‭  ‬that Becker is out of the small-airplane transponder market‭. ‬Known for its compact‭, ‬space-saving chassis designs‭, ‬Becker only‭ ‬offers a 1090ES ADS-B transponder solution for Part 25‭ ‬aircraft‭, ‬with a focus on law enforcement ops‭. ‬

If you’re searching the used market‭, ‬you might find the ATC-4401‭. ‬It fits in a 2¼‭-‬inch instrument cutout and is shallow‭, ‬at‭ ‬1/8‭-‬inch deep‭. ‬It’s fully TSO’d and has an LCD display‭, ‬VFR‭ ‬memory button and a bus-voltage monitor‭. ‬As‭ ‬always‭, ‬we would caution buying discontinued products‭, ‬with some exceptions‭. ‬See the sidebar on the next page for more explanation‭.  ‬


We grouped these brands together‭ ‬because there’s lots of rebranding‭ ‬going on‭. ‬First BendixKing‭, ‬which has advanced the KT 74‭ ‬1090ES ADS-B transponder interface to include compatibility with Garmin‭ ‬WAAS navigators‭ (‬it doesn’t have internal GPS or an ADS-B In receiver‭), ‬plus it connects with the made-by-Avidyne retrofit navigators it rebranded last year‭ (‬the AeroNav models‭) ‬for use as an approved position source‭.‬

The BendixKing KT 74 is a Mode S transponder meant as a slide-in replacement for older units. It still requires an external GPS, though.

It is Trig Avionics that designed and builds the KT 74‭ ‬for BendixKing‭, ‬and still offers the rack-mounted TT31‭ ‬and two-piece TT22‭/‬21‭ ‬1090ES transponders‭. ‬FreeFlight Systems sells the Trig‭ ‬TT22‭ ‬as the RANGR FDL 1090‭ ‬TX‭.‬

The Trig saves space and power but doesn’t have GPS.

The BendixKing KT 74‭ ‬and Trig‭ ‬TT31‭ ‬were designed as slide-in replacements for the King KT 76A‭, ‬KT 76C and KT 78‭ ‬transponders‭, ‬but all require the additional wiring for external GPS input‭. ‬While we’ve always been fond of Trig’s quality and design‭, ‬we think its‭ ‬current line of first-generation transponders are at a disadvantage without‭ ‬internal GPS‭, ‬especially with newer‭-‬generation models from Garmin‭, ‬Appareo and L-3‭, ‬which substantially‭ ‬streamline the interface‭. ‬But luckily‭ ‬there’s a workaround‭.‬

Trig offers the NexNav remote‭ ‬WAAS GPS receiver‭, ‬plus external GPS antenna‭, ‬which is a mandate-compliant system priced at‭ $‬1100‭. ‬It weighs less‭ ‬than 1‭ ‬pound and can be mounted‭ ‬pretty much anywhere in the airframe‭. ‬But add that to the price of the transponder‮—‬and the effort to install the receiver and antenna‮—‬and a model with built-in GPS might be the better option‭. ‬We say put a sharp pencil on it‭.‬

Dynon Avionics

For channeling through its SkyView‭ ‬systems‭, ‬Dynon offers the‭ $‬2200‭ ‬SV‭-‬XPNDR-261‭ ‬remote transponder‭. ‬It’s a 1090ES ADS-B Out unit that meets the 2020‭ ‬mandate‭, ‬but it will require input of the Dynon GPS-2020‭ ‬or another TSO’d WAAS source‭. ‬The Dynon unit‮—‬which‭ ‬outputs 250‭ ‬watts‮—‬weighs just under‭ ‬1‭ ‬pound and connects to each SkyView display through an RS232‭ ‬serial line‭. ‬

The Dynon model is an obvious choice‭ ‬for newly installed or future installations of the company’s EFIS‭, ‬and allows you to save a bit of panel space‭. ‬

Dynon SV-XPNDR-261 with SV-GPS-2020 transponder.


Garmin’s latest GTX transponder product line may seem confusing at first‭, ‬but that’s actually a good thing because there are a‭ ‬wide variety of configurations to choose‭ ‬from‭, ‬depending on desired functions‭.‬

Garmin sold a lot of the GTX 330‭ ‬Mode S units‭ (‬as well as the Mode C‭ ‬GTX 327‮—‬more on that in a minute‭), ‬and while it’s been discontinued‭, ‬units‭ ‬can be upgraded to‭ ‬“ES”‭ ‬for ADS-B‭ ‬Out‭. ‬It will need input from an approved position source since it has no WAAS GPS‭. ‬The factory upgrade cost is‭ $‬1200‭.‬

The GTX 330‭ ‬was replaced by the‭ $‬2995‭ ‬GTX 335‭, ‬an entry-level ADS-B Out model‭. ‬A version of that has internal WAAS GPS and has a‭ ‬list price of‭ ‬$3795‭. ‬Neither version has ADS-B‭ ‬In‭. ‬You’ll need to buy the GTX 345‭ ‬for that‭, ‬which also comes standard with a Bluetooth wireless interface for streaming traffic and weather to a tablet running the Garmin Pilot or ForeFlight apps‭. ‬It also works for displaying weather on the Garmin late-model portable navigators‭, ‬which is the aera 660‭/‬GPS696/796‭ ‬units‭.‬

The GTX 345 has been a top seller because it has both ADS-B Out and In, plus Bluetooth for tablet play.

The GTX 345‭ ‬with internal ADS-B In‭ (‬these are dual-band receivers‭) ‬interfaces with the GTN 750/650‭. ‬You get‭ ‬traffic and weather display‭, ‬including Garmin’s Trend Vector traffic mapping technology‭. ‬You’ll get basic traffic‭ ‬display‭ (‬no ADS-B symbology‭), ‬plus‭ ‬NEXRAD images‭, ‬METARs and TAFs on GNS 530W/430W navigators‭. ‬The‭ ‬transponders won’t display on legacy‭ ‬‭(‬non-WAAS‭) ‬GNS navigators‭. ‬These are simply out of horsepower‭.‬

If you’re out of space for these rack-mounted units‭, ‬Garmin offers remote versions‭. ‬They are tuned directly on the screen of the GTN navigators‭, ‬and on the displays of the Garmin G3X integrated avionics‭.‬

Garmin still offers a traditional‭ ‬Mode A/C unit with the‭ $‬2295‭ ‬GTX‭ ‬325‭. (‬The minimum advertised price‭ ‬is‭ $‬1995‭). ‬It replaces the popular GTX 327‮—‬and is essentially the same unit‮—‬but carries the GTX 335/345‭ ‬display and button layout‭. ‬It would be a good‭ ‬choice if you already have an ADS-B‭ ‬Out solution but an older Mode C transponder that’s bitten the dust‭. ‬It’s biggest competition‭? ‬Probably used 327s‭.‬

Lynx NGT9000

A multifunction Mode S ADS-B transponder‭, ‬the NGT9000‭ ‬was developed by L3‭ ‬Avionics‭, ‬but the product is now supported under the ACSS division of‭ ‬L3Harris‭. ‬As far as we know at press‭ ‬time‭, ‬it’s still being manufactured‭. ‬Some install support is also handled by Aspen Avionics‭, ‬since the smart transponder‭ ‬is compatible with Aspen’s Evolution‭ ‬MFD for display of traffic and weather‭.‬

We’ve favored this system because it works well in stark panels since it does‭ ‬double-duty as an ADS-B Out transponder and also displays weather and traffic on its touch display‭. ‬

The NGT9000 multifunction transponder packs a big punch, but we wonder about its future.

L3‭ ‬never liked to call the NGT9000‭ ‬a transponder because it does a lot more‭, ‬but at its core that’s what it is‮—‬a Mode S Class A1‭/‬A1S transponder‭. ‬The new term is smart transponder‭, ‬with wireless connection to tablets and smartphones‭. ‬The unit uses a remote wireless module for that and is compatible with a variety of third-party apps including ForeFlight‭, ‬but not Garmin Pilot‭. ‬

The NGT9000‮—‬which is available‭ ‬in a variety of flavors‭, ‬including one‭ ‬with internal WAAS‮—‬fits in a standard 6-inch-wide radio stack and measures just under 2‭ ‬inches high‭. ‬It has a footprint‭ ‬similar to most other stack-mounted transponders‭. ‬But unlike most transponders‭, ‬the NGT9000‭ ‬has a multifunction touch color display for weather and traffic overlay‭. ‬

If you’re used to eyeballing large EFIS displays‭, ‬the NGT9000‭ ‬will be small‭ ‬by comparison‭. ‬The data is divided‭ ‬between two drag-and-swipe touchscreens‭, ‬and yes‭, ‬at under 2‭ ‬inches tall‭, ‬the NGT9000’s display may not be the best way to view all that data‭. ‬Still‭, ‬think utility and the unit has plenty of it‭. ‬We wish it could accept position data from an existing GNS‭, ‬GTN or Avidyne navigator so you don’t have to buy the version with the GPS option‭, ‬but it can’t‭. ‬The GPS-equipped model can grab the WAAS signal from the existing WAAS antenna‭, ‬at least‭, ‬so you don’t have to‭ ‬install another antenna‭. ‬That’ll save‭ ‬some install time‭, ‬even though you’ll‭ ‬need to buy an antenna signal splitter‭. ‬The GPS Networking LDCBS series is one for around‭ $‬400‭.‬

A feature we like in the system’s latest software is ATAS aural alerting‭. ‬ATAS surveys and issues traffic alerts for airborne traffic targets at all altitudes‭, ‬making it especially useful in the traffic‭ ‬pattern‭. ‬Frequency congestion results‭ ‬in nuisance alerts‭, ‬so some TAS and TCAS systems won’t scan for traffic‭ ‬below 500‭ ‬feet unless they’re connected to external mode sensors like landing‭ ‬gear squat switches and airspeed sensors‭. ‬With its ATAS‭, ‬the NGT9000‭ ‬self-filters terminal area traffic and gives collision alerts down to ground level‭. ‬Worth noting is a distinct advantage to ADS-B traffic minders compared to TAS simply because of a more complete picture of the traffic threat‭. ‬Using‭ ‬predictive algorithms‭, ‬the NGT9000‭ ‬knows where the threat aircraft is turning‭, ‬what its exact WAAS GPS position is and precisely where it’s going to be in relation to the host‭.‬

The system has two sensitivity levels‭. ‬In the terminal mode‭ (‬below 2000‭ ‬feet‭ ‬AGL‭) ‬you’ll get traffic alerts within‭ ‬12.5‭ ‬seconds prior to what’s called the closest point of approach‭ (‬CPA‭) ‬within 750‭ ‬feet horizontally and 300‭ ‬feet vertically‭. ‬En route‭ (‬above 2000‭ ‬feet‭), ‬the vertical sensitivity increases to 500‭ ‬feet‭. ‬If the CPA is greater than two miles or greater than 850‭ ‬feet vertically‭, ‬it won’t issue an alert‭.‬

You’ll have to wire the unit to an audio‭ ‬panel’s switched or unswitched audio‭ ‬inputs‭, ‬but with the ATAS alerting you’ll‭ ‬hear‭ ‬“Traffic‭, ‬three o’clock‭, ‬one-half‭ ‬mile‭,‬”‭ ‬as an example‭. ‬This obviously can save some time scanning out the windscreen‭. ‬If ADS-B traffic isn’t enough‭, ‬there’s a version of the NGT9000‭ ‬with a built-in TAS receiver‭. ‬That will require a top-mounted antenna‭, ‬in addition to the traditional‭ ‬L-Band antenna‭.‬

If your kit has a WX500‭ ‬Stormscope‭, ‬the NGT9000‭ ‬will work with it‭. ‬It connects with an RS232‭ ‬serial interface‭, ‬and the Stormscope is controlled and‭ ‬displayed on a dedicated page in the‭ ‬NGT9000‭. ‬You can view cell mode or strike mode‭, ‬overlay lightning data on the map page‭, ‬plus control the 200-mile range‭. ‬Since the WX500‭ ‬can drive two displays via its twin serial output ports‭, ‬you can still keep it connected to an‭ ‬existing system if you have one‭.  ‬

Sandia Aerospace

New Mexico-based Sandia has two transponders‭. ‬The‭ $‬3200‭ ‬STX 360‭ ‬Sentinel has ADS-B Out and In‭, ‬plus an LCD color display traffic‭ ‬and FIS-B METAR‭. ‬There’s also wireless Bluetooth for more complete FIS-B weather display‭. ‬There’s‭ ‬a remote version‮—‬the STX 360R‭. ‬At‭ ‬press time‭, ‬certification is pending‭.‬

But for Mode A/C ops‭ (‬no ADS-B‭ ‬Out‭), ‬the STX 165‭ ‬is fair game‭. ‬It’s a space-saver with a‭ ‬½‭ ‬3‭ ‬ATI bezel‭, ‬and has an OLED display that does well in‭ ‬sun-splashed cabins‭. ‬What we really‭ ‬like about the STX 165‭ ‬is its built-in altitude encoder‭. ‬It’s also equipped with a pressure altitude display‭, ‬a three-function timer and has an option for OAT probe input‭. ‬

The STX 165R is the remote version for‭ $‬1800‭. ‬It works with MGL‭, ‬GRT‭, ‬Dynon and Advanced Flight displays‭. ‬It’s lightweight‭, ‬at 1.16‭ ‬pounds‭, ‬and only draws 200ma of current at 28‭ ‬volts‭.‬


You might not have heard of German-based TQ‭, ‬but its avionics ride aboard airliners‭, ‬including Airbus and others‭. ‬At AirVenture 2020‭, ‬German-based‭ ‬TQ-Aviation was showing its newly certified KTX2-S transponder‭. ‬It’s a Mode S‭, ‬Class 1‭ ‬unit and has a high-contrast‭ ‬TFT color display‭. ‬It fits in a small‭ ‬instrument cutout‭, ‬so it saves space‭.  ‬We‭ ‬suspect you’ll see more products from‭ ‬this company as it ventures deeper into the GA market‭.‬


If you’re installing your own transponder‭, ‬remember that it will need to be certified per the FAR’s two-year transponder check‭. ‬Same with the altitude encoder‭, ‬if you’re installing a new one‭.‬

In all‭, ‬there are good solutions on‭ ‬the current market‭. ‬If you’ve already installed ADS-B Out‭ (‬maybe you have‭ ‬a skyBeacon 978‭ ‬UAT solution‭, ‬for example‭) ‬and need to replace a failed‭ ‬Mode A/C transponder‭, ‬we favor the‭ ‬Garmin GTX 325‭. ‬We also like the‭ ‬Sandia STX 165‭ ‬because it has a built-in encoder‭, ‬smart features and saves space‭. ‬As we reported in the sidebar‭, ‬keep your eyes‭ ‬open for a well-maintained Garmin‭ ‬GTX 327‭. ‬It could be the ultimate‭ ‬money saver‭. ‬

If you need both a transponder and compliant ADS-B Out‭, ‬our top pick is the all-in-one Garmin GTX 345‭ ‬series‭. ‬With wireless Bluetooth‭, ‬compatibility with Garmin Pilot and ForeFlight apps‭, ‬plus a rugged‭, ‬no-nonsense user set it’s obvious why shops report that it’s the top-selling ADS-B solution to date‭.

Photos: Larry Anglisano and courtesy the manufacturers.


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