OK, I made a temporary arrangement to test where the problem lies, and eliminated N-to-F adaptor by connecting RG6 cable directly to antenna without any connector.
Now graphs & VRS range plot show improvement, but still far less than Cantenna.
Not sure, but the improvement may very well be because RG6 coax length is now 1 meter / 3 feet instead of 4 meters / 12 feet, and the FA antenna is now closer to window, but about 6 feet / 2 meters lower than Cantenna.
https://farm2.staticflickr.com/1482/24112432403_a6a5f4fa61_c.jpg https://farm2.staticflickr.com/1552/24645800311_0f0b9645d1_c.jpg
