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.

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.

Michael