dme antenna location

[Pilot 2000]

Hi, my DME is often intermittent (looses signal). I have 3 antennas on the belly (see pic): transponder(with GDL-82 adsb), DME (Narco 890) and marker beacon(which is somewhat useless nowadays ). I am worried that the intermittance comes from the landing gear shielding the dme antenna. I was thinking of swapping MB antenna with the DME - what do you think of that?

 

[denverpilot]

Has the antenna system including likely ancient coax been tested to see if it’s gone faulty? There’s test gear for that.

Same for the performance of the DME. All DME really is, is essentially a transmitter that hits a transponder on the ground and times it’s own return transmission round trip time. There’s test equipment for both testing on the bench and on the aircraft.

Swapping the ATC transponder and DME coax temporarily is a fairly common but lazy test trick to see if an antenna and coax have a fault, but it’s smarter to just test it.

What’s the DME doing when it doesn’t work correctly? How far from the station are you usually? Always off exactly 90 degrees from the station doing a DME arc?

That’s the only possible way the landing gear is blocking anything and at the power levels of DME and typical distances, highly unlikely to last for long. Far more likely the radio has a fault, or the coax and antenna system does.

 

[Pilot 2000]

The coax to the dme antenna is fairly new rg400. I noticed that on some headings the signal is lost, but if you change the heading, it's restored. This DME radio is only 25 watt, so perhaps landing gear interference is at fault?

 

[denverpilot]

Not totally far fetched, I suppose. Problem is you swap the transponder for the DME location you just move the problem to the transponder.

But it does have a lot more power than 25w so any airliners you’re about to run into would be able to hear it in their TCAS receiver.

What range is it failing at?

 

[Pilot 2000]

I was thinking of moving the dme antenna aft and move the mb antenna forward and moving the dme antenna back.
Currently it was failing at 25 miles range, and esarly.

 

[Jim Carpenter]

Interesting......certainly could be interference from the gear, but, there are lots of different aircraft with a similar location-setup for antennae without problems. You picture looks like a fairly clean airplane, but sometimes oil and soot on the antennae has been known to interfere with transponder or DME signals, though maybe more of an issue with the bare metal stub-and-ball type. Also, you did mention a correlation between heading and signal, but, recall that DME signals can be overwhelmed by too much "traffic" trying to interrogate the ground station at the same time. Do you fly in a busy area, could it just be overcrowding of the specific frequency? (Though I imagine, with more and more aircraft dumping DME units for GPS, this may be a diminishing problem).

 

[denverpilot]

Adds some more lossy coax run but not a huge amount. At those frequencies RG-400 would lose a dB or so, which might offset any unmasking of the antenna.

I’d have to look up RG-400 loss numbers to nail that down, that’s a SWAG. Just something to think about.

This is why most swap attempts use the two forward antennas. In our airplane when we were lazily swapping the DME and Transponder to troubleshoot the DME eventually died, proving the problem was in the radio all along. Power supply failure.

Wasn’t worth fixing. Got a GPS instead.

 

[Doug Reid]

Do you know someone with the same model that will let you swap for a test to see if the problem follows the DME ?

Looks to me like your DME and transponder antenna are compatible....you could swap antennas, but I have a feeling it is in the DME.