DME vs. Transponder

[sleddriver71]

In a King Air 90 thatI occasionally fly, the DME will not function unless the transponder is in the standby or off position. I am aware that both DME and transponder are VHF so I am thinking for some reason the transponder is overpowering the DME. If anyone has had any experiences like this or any ideas as to what might be wrong...

 

[joe_pilot]

I had a similar problem in a 172. When I got within about 500' AGL, the DME would stop working. It drove me crazy. The mechanics checked it out and found no problems. So when I was at the avionics shop at PWA picking up a radio, I casually mentioned it to an avionics tech. He said that the DME pulse operates on a frequency that is sympathetic or harmonic to the one that the transponder uses. The next time I turned off the transponder at about 1000' AGL and the DME worked just fine.

Unfortunately, I don't have a solution for you as I left that job a few weeks later.

Sorry man.

-JP

 

[JimNtexas]

Transponders operate at 1030 and 1090 mhz. DME operates between 962 and 1213 mhz. They are both radar type pulsed modulated signals.

Most transponders have a 'dme suppression' line that prevents the dme from interferring with the transponder. If this wire is lose or missing then the dme and transponder may not play well together.

It is also possible for damaged or corroded coaxial cable and/or antennas to cause these systems to interfer with each other.

Time for a visit to your friendly neighborhood avionics shop.

 

[Groucho]

DME / Transponder

I am not sure how old the aircraft is but very often problems like these are not related to the black boxes but very possibly the antennas and co-axial cables that they are connected to.

Inspect the antenna, mounting, and sealing. An antenna can "float off ground", that is, no longer have a good ground to the airframe if significant corrosion gets going on the antenna / base seal.

Inspect the coaxial cables and their respective connectors for broken insulation or fluid damage. We had a similar problem at our ailrine and found a leak under the galley had ruined the coax cable connection to the DME antenna.

If one unit is not working when the other is there may very well be due to interference caused by an antenna or antenna feed line fault.

My .$02

 

[erj-145mech]

I had installed a KN-62 DME and a KT-76 in my T-6G. In the wiring instructions it showed a jumper wire that went between the two units to prevent what you are describing. Make sure that the two units are wired correctly. Is this a new installation, or did it just start happening?

 

[sleddriver71]

Thanks for all the great replies. In response to a couple of questions posed, this particular King Air E90 is an LW-100, so I think it is about a 1975 model. And to answer the other question, the problem between the xponder and DME just started happening a couple of days ago.