Intermitent Mode C on SV-261 transponder

wolf1

New Member
Joined
Aug 14, 2010
Messages
11
Location
SOCAL
SV-261 Mode C altitude readout missing, reported by tower. Installed SV-261 transponder and 2020 GPS puck. I also have ADSB-470 installed. Initially loaded firmware 14.0 and subsequently loaded 14.1 followed by 14.1.1. Got all the right configuration indications as shown in installation manuals. A check with tower initially showed good information and altitude readout until I reached 5 miles from airport at which time the altitude readout became intermittent and eventually was lost completely. Turning back to the airport the altitude readout was again visible at about 5 miles and was observed all the way to touchdown. All other information, code, GS, etc seemed to be OK. If all info was lost, I would suspect antennae wiring but only altitude missing leads me to believe its a internal problem in the SV-261. Any ideas??
 

dynonsupport

Dynon Technical Support
Staff member
Joined
Mar 23, 2005
Messages
13,227
Wolf,
This sounds like maybe it's a new airplane? If so, have you done the 91.413 bi-annual transponder check yet? It is required on brand new airplanes before they first operate the transponder because it finds installation issues.

Your description sounds exactly like a bad antenna or coax leading to putting out minimal power. It takes a better signal to noise ratio to pick up the Mode C code than it does the Mode A or just a secondary reply. Ground Speed is calculated from only primary returns so that is not transmitted from the transponder. There's no way in the transponder for just the Mode C to fail, and with a SkyView the only way to lose ALT data is if the AHRS is offline. You'll see "NO ALT" in the transponder status area in the top bar too.
 

n456ts

Do not write below this line
Joined
Jan 24, 2015
Messages
161
For situations like this, can the Dynon transponders dissipate up to full signal reflection back into the unit?
 

hstaton

New Member
Joined
Dec 2, 2011
Messages
52
Location
Jacksonville, FL
When I had this issue on a previous airplane, the problem ultimately found was an unknown faulty connector in the cable that was hidden behind a panel. This would tend to support a cabling/high SWR issue as the problem.
 

dynonsupport

Dynon Technical Support
Staff member
Joined
Mar 23, 2005
Messages
13,227
Yes, the transponder can handle any SWR and is not damaged by having a bad connection.

In many cases, we can actually report this error as well. Check the transponder setup page and see if you have any errors. This gets reset every power cycle so you need to do it after the transponder in running a bit (and not in SBY or GND).
 
Top