VOR Antennas

[Spiff]

I have a M4 180V and it left the factory with VOR/LOC antennas on the vertical stab. They were not hooked up to anything in the panel but did have a cable running up to the panel. I am the current owner and have an avionics shop trying to get them to talk to a Garmin GTN650. It will not receive the VOR signal more than 10 miles from the VOR in flight and their tester indicates a fault 10 feet back which puts it near the tail. We can't figure out how to get up in the vertical stab to check the connection to the antennas without cutting fabric. Does anyone have any suggestions?

Thanks

[Scottish.Maule]

The antenna base is a wood block inside the vertical stabiliser that locates the wire antennas themselves and their securing nuts. The co-ax cable running to it is crimped onto a terminal for each wire antenna - one for the wire core and one for the shielding. Also further down the vertical stabiliser there is another earthing point for the co-ax screening where it is crimped and screwed onto one of the metal ribs in the tail. Unfortunately the only way to access these connections (which presumably is where your problem lies) is to cut open the fabric 😬😬[/img]

[TimB]

The VOR antenna is mounted to a micarta insolating block in the tail, one can use an ohm meter and check for continuity from the BNC connector case[ shield ] and to each VOR wire on the tail and should have a connection, if you do not then a connector or balun is damaged the center pin should not have any connection at the BNC or it is shorted.

[andy]

TimB, is the balun inside the tail a Pawsey Stub? If so, I would think that measuring with an Ohmmeter from the BNC shield to either of the dipole antenna leads would show a short but that measuring from the center conductor of the BNC connector to either of the dipole antennas would also show a short. Maybe the balun is different than this one:

[TimB]

The VOR wires are connected to the shield through the balun, the center pin and wire are not connected at all any were, see AC 43.13 2B, Chapter 3, Par 312, Fig 3-6.

[andy]

Thanks, TimB. Very enlightening. I've never seen a balun wired that way. I'm guessing that with the antenna elements presenting a DC short to the coax ground shield, it protects the avionics and cabin occupants better from a lightning strike on the airframe.

I guess the best way to determine if there is a problem with the VOR antenna would be to have an avionics shop use something like the NAV-401L test set described in this article http://aea.net/AvionicsNews/ANArchives/ ... SOct03.pdf. Is there a simpler, cheaper way?

I've wondered the same thing about my VOR antenna since the KX-155 NAV receiver seemed to be less sensitive on an ILS approach than I expected. When I had my GTN 650 installed, the shop said that the KX-155 NAV receiver wasn't operating properly. They ended up sending it out for repair and a capacitor was replaced. That seemed to correct the problem. Now I can compare the sensitivity of the KX-155 to the GTN 650 in VOR mode. However, they both use the same antenna with a splitter.