I got one from my local avionics shop.... about $40 IIRC
In my case, I upgraded an Apollo Loran to a slide-in replacement GX-55.
The installation instruction gave a set of Comm frequencies to transmit on... if any of these frequencies caused a loss of GPS signal, the installation needed to be tweaked. I had a comm. antenna too close (since moved) and a notch filter in that co-ax solved the problem.
Since it's a "plug-in" device, it's somewhat easy to add later as long as you can get to the coax. behind the panel. In my case I had a straight coupler in line, so I just replaced it with the notch filter.
Do your installation instructions give you the specific comm. frequencies to test?
When I ordered my GNC300XL from Stark, I asked about the notch filter. He said he didn't think it was really necessary. He said he would throw one in with it, but I think it came with the unit anyway.
You were right again. There is a gizmo in the GNC 300XL materials which must be a notch filter. I didn't recognize it as such because it was labled something else in German.
You were right again. There is a gizmo in the GNC 300XL materials which must be a notch filter. I didn't recognize it as such because it was labled something else in German.
Just to be clear, the notch filter goes in line with the Comm. antenna cable. You might need 2 filters if you have 2 comms. Put in in line first with the comm. that has it's antenna closest to the GPS antenna.
