I asked Bob Grove about this a couple of years ago in the "Ask Bob" column of monitoring times. He said stacking 2 of the beams would only give you another 3 db of gain. It would give you more directivity than anything else. It would be a narrow reception path, probably nulling out the reception from the sides quite drastically.
Well we spent all day yesterday on the roof, took down the scanner beam took it to a friend who's a expert antenna man,, one of the wires on the antenna was touching the main frame and was grounding it out we bent the wire andput it up alot higher hooked it to advance preamp, now this thing rocks, ran all new coax, now I'm reaching out 50 - 70 mile. and I was thinking about junking this antenna..........
