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>M/M) and elevated cliff top locations close to the sea. Yes, this is a common misunderstanding. One of the key differences is the height of the vertical over effective ground (salt water in this case). As you raise the vertical's feedpoint, you start compressing the main lobe, and also start increasing a second high-angle lobe. A vertical will split lobes as you raise the feedpoint in the same manor of a Yagi - this is roughly 1 lobe for every wavelength high. If you take a high band vertical and have it on an elevated cliff, the antenna will have lots of nulls at useful take off angles. If the arriving signal angle changes, you could get a lot of fading from the antenna. Low angle gain will be good, but one of the key advantages of verticals mounted right at sea level is that you get one big fat lobe which helps to reduce fading, and you get good signals from all arriving angles. Mounting a vertical on a ship could also be similar mounting it on a cliff, but it is very hard to determine where ground really is on a ship :) We did some modeling for one of the original 4M1X locations, and found that a slightly elevated site (50') that was set a little back from the beach (150') could actually be OK on the low bands 80/160. Its all a function of wavelength from the water, and effective height in terms of wavelength. One of the interesting things that we noticed in the modeling was that the HIGH angles were effected more than the low angles in this example. It was a bit surprising, but N6BV dug into it, and said it makes sense. (right now its 1:30 AM, and I cant remember the theory behind it). Anyway, gotta run. N6BT and I were just talking a few days ago that we need to finish some of the vertical testing we started a few years ago (you can find some of it on my web site), and have already outlined some tests to further validate vertical stuff. 73, Kenny K2KW -------------------------------------------- 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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