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> Given the experience of this group, I would be interested in opinions >regarding the practical use of 40m element(s) (eg, C4, C4SXL) at only 50 >feet. Before you reach for your keyboards, QTH is an island in Narragansett >bay west of Newport, RI. We are about 800 feet uphill from the water at 60' >ASL, so the top of the tower would be at ~110' above the salt water >surrounding the island. > Obviously most of the literature and theory prefer to have it higher, but >could being close to the water mitigate this somewhat? Don't think I've ever >read anything about this particular situation. > 50' is the height limit in accordance with town regulations. Alternative >of course is not to bother with 40m and go with a C3. A couple of comments: You should get a copy of YT, the terrain analysis program that comes with the current edition of the antenna book. Then model your lines of sight to the key DX destinations. The software compares the lobes that your antenna will generate by reflection off your sloping foreground with the elevation angles needed for the path in question -- very enlightening, and it's free. You should also have a look at Dave Leeson's book "Physical Design of Yagi antennas," especially chapter 10, on siting. He is a great fan of sloping foregrounds, and has the math in his book for determining where reflection occurs for a given take-off angle and tower height. I just did a very quick and dirty YT run of your terrain and a 2-element 40m yagi at 50 feet. It's clear that the ~ 15:1 sloping foreground gives you usefully more gain at lower angles than the same antenna height on flat terrain, even though the first lobe peaks quite high (>30 degrees) with either foreground. The effect is to broaden the bottom edge of the first lobe, where most of the DX propagation is. Interestingly, you can see the shoreline in the model -- it's where the pattern stops being formed by the slope and starts being formed by the flat seawater! But the bottom line is, go for the 40-meter capability - you won't regret it. 73, Pete Smith N4ZR n4zr@contesting.com Don't forget to update your entry on the World Contest Station Database, now at http://www.contesting.com/stationdatabase.htm -------------------------------------------- Force12Talk mailing list provided as a service by Force 12 Antennas, Inc. Force 12 Web site: http://www.qth.com/force12 Submissions: send to Force12Talk@qth.com To unsubscribe: send a blank e-mail to Force12Talk-leave@qth.com Force12Talk Message Archive: http://www.qth.com/force12/list/force12talk For problems with the list, contact force12@qth.com |
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