I have a magnet VHF antenna base I am rebuilding. It has a plastic cover that encapsulates the whole base. Inside it has a magnet that has a center hole. On top of the magnet is a metal washer. The antenna mast holder is held by a bolt that passes through the center of the magnet hole, but it is insulated from the magnet itself with plastic washers. The center conductor of the coax was attached to the center mast holder. This effectively isolates the the center conductor from everything and it is good to go. However, the coax shielding wire was kind of just sitting in there. I don't know if it has a specific place or not. Should it terminate to the metal washer on top of the magnet? It was / and will be isolated and not anywhere near the center conductor.
Where does the coax shielding terminate inside of a mag mount base?
- Thread starter JSMachine
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RFI-EMI-GUY
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The older, perhaps better mag mounts had metal foil between the magnet and vehicle body which was connected to the shield. Otherwise you are relying on the coax as a counterpoise. Mag mounts work better at VHF and above where the capacitance of the mount to vehicle body is significant. If the coax shield is unterminated in yours, that is a pretty bad mount.
The ones I have taken apart usually have the ground wedged between a metal ring around the magnet and the plastic case, or just between the magnet and the plastic if no ring around the magnet.
So there was metal foil on the bottom. I had to remove that to get into the base. The washer I memtioned is attached to the magnet on top, and the same diameter of the magnet.
So I could attach the coax to that washer? I suppose it would act as a sort of ground plane.
The metal foil on the bottom of the plastic housing was touching the bottom of the magnet. I didn't think magnets were conductive, but I could be wrong.
So when it is reassembled, I am thinking the shield could be attached or soldered to the metal washer - which is currently glued or somehow attached to the magnet; then replace what foil was there with some metal foil air duct tape...
So I could attach the coax to that washer? I suppose it would act as a sort of ground plane.
The metal foil on the bottom of the plastic housing was touching the bottom of the magnet. I didn't think magnets were conductive, but I could be wrong.
So when it is reassembled, I am thinking the shield could be attached or soldered to the metal washer - which is currently glued or somehow attached to the magnet; then replace what foil was there with some metal foil air duct tape...
That's exactly what I would do.
