Weak Navs & Inoperative Marker Beacon

[MBDiagMan]

I did my second instrument lesson Friday and I asked the instructor if we could try an approach just so I could check my avionics.

I have a NARCO NAV 120 VOR/LOC and a NARCO NAV 121 VOR/LOC/GS/MarkerBeacon reciever. I also have a NARCO CP136M audio panel that is supposed to deal with the Marker Beacon signal.

Both VOR receiver's have to be pretty close to the VOR to work. I am thinking that this might be an antenna cabling problem. I hope to get some time to check out the antenna cables when it's not 105 degrees out. There are two cat whiskers on the tail. At this point I assume that they are two separate antennas, one for each receiver

The MB has a switch with a test position on the audio panel and it all seems dead. I had no MB tone when flying over it either. There is also an MB indicator in the NAV 121, but I didn't notice if it came on. I think my instructor was watching it all closely, and would have said something if the MB indicator on the NAV would have lit up. If it didn't light up there, then you would think that the MB problem is in the NAV.

Thanks for your thoughts.

 

[SCCutler]

Don't be surprised if both NAV receivers are on the same antenna with a splitter. In any event, I bet strongly on the problem being antenna (most likely, dirty connectors or bad grounding).

How did the glideslope work?

 

[MBDiagMan]

Glideslope didn't work either. From the get go I have suspected an antenna problem. There might be a section of coax that is acting as a weak antenna.

I was at North Texas Regional (Graysone) and I think the frequency was 111.7. I will have to look at the plate, but that would be the freq on which the GS/LOC/MB are all picked up correct?

Thanks for the response Spike!

 

[flyingron]

No. All the frequencies you mentioned are distinct and. The localizers are in the same band as the VORs (108.10 thru 111.95 Mhz, though the directional information is modulated differently).

The glideslope is transmitted on frequencies between 328.6 and 335.4 MHz. Similar to DME, these are paired with the LOC frequencies so that when you select the appropriate LOC frequency, the corresponding GS is selected in that receiver. The antenna for the GS is frequently different (you might find it on the nose of a twin, or up on the top of the windshield) than the LOC/VOR receiver one.

The marker beacon is transmitted on 75MHz (always). The frequency of the modulating tone tells the MB receiver which light to flash (I/M/O). This is almost always a seperate antenna on the bottom of the aircraft (usually a bent whip).