Take a quick look at how the advanced technology at Penn Avionics will save you troubleshooting time and money.

The customer arrives at Penn Avionics, Inc with a communication radio problem. The #2 com radio has poor range on both transmitting and receiving.

The radio is pulled for a bench test, and no problems are found. Output power is good, receiver sensitivity is better than spec.

 

We have an aircraft wiring problem. Using our advanced test equipment, we can track it down quickly.

 

 

 

 

 
The bench test confirmed the  #2 communication radio is good, so it is re-installed back in the aircraft.

Using a spectrum analyzer, Ken performs a simple radio check. A technician in the customer's aircraft transmits on the #2 communication radio while Ken monitors the field strength on the spectrum analyzer.

The spectrum analyzer is located in our bench repair area and is connected to a outside antenna. When an aircraft transmits on our ramp area, the spectrum analyzer will display the amount of RF energy and frequency the aircraft radio is radiating. This is a great overall test of the comm radio, wire harness, and antenna system (all at once!).

The spectrum analyzer confirms the #2 communication radio output power is about 25% of what it should be. Ken notes that although the power output is weak, the transmission is very clear and readable. We now know we have an antenna system problem with the #2 com radio.

Antenna problems can be very time consuming to track down without the right test equipment.

Ken has brought a portable TDR (time domain reflectometer) out to the customer's aircraft and connected it to the antenna port on the #2 comm radio tray

The TDR will send a pulse down the antenna cable and then display the reflections on an oscilloscope type screen. The display will show the actual distance in feet to a discontinuity and if the discontinuity is an open or a short.

Based on the TDR display, Ken has found that the antenna cable on the #2 comm radio has a short to ground about 3.5 feet from the #2 comm tray. The distance information is obtained by measuring the distance between the first positive going pulse, to the 2nd negative going pulse. Since the 2nd pulse is a negative going pulse below the trace baseline, we know the pulse is showing the location of a short to ground.

 

 

 

 

 The customer's aircraft is a Cessna C-182 which has the antenna coax running from the radio tray, along the lower inside windshield edge, then up the door post to connect to the antenna mounted on top over the cabin. The TDR has told us to start looking at the com#2 coax about 3.5 feet from the tray. That location would be around the lower windshield area near the door post. Ken opens up the left side interior panel to inspect the coax.

This photo is a close up of the lower inside windshield edge. The rivets you see are holding the outer windshield attach faring. The coax has a tear in the outer jacket that is shorting the coax shield to the center conductor. This short is causing the poor performance of the #2 com radio. The damage to the coax was caused when the rivets were drilled out to replace the windshield (the windshield was replaced about 1 year before the #2 com radio began having problems). The drill bit grabbed the coax and nicked it, but the mechanic did not realize it had happened.

Rather than trying a repair to the old coax, Ken ran a fresh piece  from the radio tray to the antenna. The #2 comm radio was tested again and checked out good for power and receive sensitivity. The customer was on his way with a minimum down time.

 

Copyright 2003 Penn Avionics, Inc Rev 7/5/2003