VHF FM is line of sight only, baring any tropospheric ducting. The formula to calculate distance is Distance equals 1.41 times the square root of height. Height is in feet. Distance is in miles. 30 ft is 7.22 miles. 40 ft is 8.91 miles. 15 ft is 5.4 miles. This is the max distance expected, no matter what gain antenna is used. Curvature of the earth cannot be overcome with a higher gain antenna. A higher gain antenna will overcome line and connector losses of the system.
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