I was doing some testing of mobile antennas on my F-150 this weekend. I had a mag mount on the roof, and a door-edge mount on the rear door. Both had NMO mounts, so I could easily move antennas between the mounts. Here is what I found:

I had an HT (Kenwood TH-F6A) connected to an antenna switch, connected to the two antennas. I tuned to a weak repeater. I found that I could hear it with the roof-mounted antenna, but not with the door-mounted antenna. I could switch the antennas between the 2 mounts with the same result, so it was the mount or the mount location making the difference.

When I opened the door with the antenna on it (moving the antenna and mount away from the chassis), I suddenly could copy the repeater with either antenna. I had the bottom of the door-mounted antenna about 4 inches below the roof line. moving it up about 2.5 inches made a huge difference in reception.

So if you are using one of the door-edge mounts, you may want to experiment with the placement.

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