On Tue, 25 Apr 2017 15:39:57 -0400, rickman wrote:

I think the real problem is this antenna for 2 meter operation is 20
feet long!

Yep. The problem with the alternating coax cable antenna design is
that only every other 1/2 wave section radiates. The result of half
the radiation is half the gain. Or, as you've noticed, the antenna is
twice as long as it might be with phasing elements between 1/2 wave
sections.

For marine VHF it can't be used on shore, so hanging it from
a tree would not work.

I've shoved them into a fiberglass radome filled with urethane foam.
Works nicely hanging from a tower. I guess it might also work hanging
from a tree.

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.

Lots of ways to build a marine VHF antenna. The key is the length.
They come in 3ft, 6ft, 9ft, and 12ft lengths. The shortest are for
top of the mast, where low gain is needed to compensate for pitching
and rolling. The 12ft is for deck mounting, where the effects of
pitch and roll are somewhat less.

Inside the radome is usually a coaxial sleeve dipole with various
numbers of 1/4 wave phasing sections. You start with a coaxial
antenna something like this:
http://www.hamuniverse.com/w4bwsverticalbazooka.html
That's good for about 2dBi gain if the manufacturer bothers to use a
brass sleeve instead of folding the braid over the outside of the coax
cable. For more gain and lower radiation pattern, put another 1/4
wavelength sleeve 1/4 wavelength below the first sleeve and connected
to the coax shield at the top:
http://www.w8ji.com/Image1/Skirt_feed.jpg
I think that page was lifted from an early version of Henry Jasik
"Antenna Engineering Handbook".

The sleeve does not need to be cylindrical such as in the Isopole type
antenna:
https://www.google.com/search?q=isopole+antenann&tbm=isch. You can
keep adding them until you run out of space. However, there's a
catch. As the gain goes up and the vertical radiation pattern goes
down, the usable transmit bandwidth also goes down. Coaxial antennas
are better than most, but you still have to be careful not to add so
many elements that the VSWR is too high at the transmit band edges.

Also, some junk out the
http://www.thehulltruth.com/marine-electronics-forum/343939-here-s-what-shakespeare-5206-vhf-antenna.html
I've seen worse. I won't mention the manufacturer, but there's one
that's nothing more than a length of copper tape stuck to the inside
of a fiberglass radome, with a very narrow band matching system in the
base. Some are just a 5/8 wave antenna with a matching transformer in
the base.

Incidentally, marine antennas are very much like land mobile
commercial antennas. However, there's one difference in the coax
cable. Marine antennas use various schemes to prevent water from
wicking up the braid by capillary action, such as silicone grease
filler in the braid or having the outer jacket reflowed into the
braid. The idea is to not have any air gaps inside the cable that
might fill with water. However, I doubt the kayak will be in the
water long enough for this to be a problem, but if your friend has
money, it wouldn't hurt to spend a few dollars extra for marine grade
coaxial cable.



--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558