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On 4/25/2017 7:36 PM, Michael Black wrote:
On Tue, 25 Apr 2017, rickman wrote: On 4/25/2017 3:19 PM, Dave Platt wrote: In article , rickman wrote: Yesterday I couldn't think of the term for the antenna style they use in the commercial marine VHF antennas, but I believe it is called "co-linear" or something like that. It is a bunch of coax sections connected inner to outer at specific lengths. I have never seen a Ham recommend using that type. But I guess Hams go more for permanent installations with ground planes of some type. The co-linear needs no ground plane I believe. "Colinear" or "collinear", depending on whom you read. They're basically a vertically-stacked array of individual radiating sections. http://www.rason.org/Projects/collant/collant.htm There are quite a few commercial ham antennas which use this approach (the "Stationmaster" probably being the best known). They're fairly popular for use on the 70 cm ham band, and some repeaters and base stations use them on 2 meters. This design is generally used when you want a substantial amount of directional gain, and are willing to pay the price (length) for it. I don't think this design would be a great choice for a kayak antenna, because the individual coax sections in the "stack" are a half-wavelength long (at the coax's velocity factor) and there are usually quarter-wave sections at the top and bottom. The shortest 2-meter collinear (one half-wave section and two quarter-wave) would be 2 meters in length - over six feet - and a marine VHF antenna wouldn't be much shorter. With a collinear of the type shown in the above link, you'd need to mast-mount it up some distance - the bottom quarter-wave tube is RF-hot, and if its bottom end is near water (or anything grounded) it would tend to de-tune the antenna. As others have noted, the OP really doesn't need a high-gain antenna. I think the real problem is this antenna for 2 meter operation is 20 feet long! For marine VHF it can't be used on shore, so hanging it from a tree would not work. When you say using a single half wave section wouldn't be much different from a marine VHF antenna, what type of antenna would a marine VHF antenna be? I thought they used a colinear design. Someone pointed out the classic groundplane antenna, with radials. But, that's just a variant on the basic dipole, where there are two elements of the same length. But the form of the groundplane puts the radiation more where you want it. Yes, that has some potential for a portable antenna by making the radials hinged and letting it all fold up and slide into a tube. I expect I'm not the first to think of this. I know there are Ham events where the bring equipment into the field and have contests. For mobile, the whip antennas are often just a variant on the groundplane, except the body of the car acts instead of the radials. There are longer antennas, but still single pieces, that provide some gain, but more important, don't need a ground plane, which of course is hard to find in a kayak that isn't made of metal. So those are longer whips, with some matching in place. The collinear is like stacked dipoles, providing more gain, but needing more height, and of course the matching stubs stick out the side. They are fine on a tower, not so useful on a kayak. Out the side? I haven't see that yet. -- Rick C |
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