On Sat, 22 Apr 2017, Fred McKenzie wrote:

In article , rickman
wrote:

Can you explain what would be preferable about the J-pole antenna? One
big disadvantage is that it appears to be more like 6 foot long for 2
meter use. That could be rather heavy and clumsy on a kayak.

Rick-

You are correct about the size. On the other hand, any separate antenna
for the VHF Marine band will be a bit clumsy on a kayak.

The "J" end of a J-pole is used to match a cable to the high impedance
of the end of a half wave element. One advantage of a vertical half
wave entenna is that it directs more energy towards the horizon,
compared to a quarter wave ground plane antenna.

Jeff Liebermann's analysis sounds like the best approach to VHF on a
kayak. But no matter which way you go, you are transmitting from a
point close to the water. You are limited to line-of-sight to another
kayak. Range to a base station will be mostly determined by the height
of the base station antenna.

And with a kayak, the issue is less about "antenna gain" than that there's
nothing with height on a kayak, and it's very low in the water.

Figure out some sort of "mast" and the problem is close to solved. Once
you have height, the actual antenna type matters less. I seem to recall
George Dyson put sails on some of his really big kayaks, so that's an
option, and the mast would provide some height for the antenna.

Michael

In article , rickman wrote:
A kayaking friend was asking me about antennas for marine VHF radios.
Someone recommended this type of antenna.

http://www.amateurradio.bz/2m-70cm_v...e_antenna.html

I guess my first question is what would this be dual band? Is it
because it works over a range from 2 meters to 70 cm? Or is it somehow
tuned for both bands at the same time but not a lot in between?

The latter... it's a dualbander. Electrically, it's effectively two
dipoles mounted side-by-side. The longer arms of the elements are
tuned for 2-meter operation, the shorter ones for 70 cm.

It would probably not work terribly well on marine VHF - those
frequencies are far enough above the 2-meter band that the 2-meter
elements on this antenna will be too long. The radio would see a high
SWR.

In principle you could build a similar dipole, with one set of
elements, somewhat shorter than the 2-meter ones on this example,
which would be resonant on the marine VHF frequencies. You'd reduce
the element lengths by a factor of about 146/157.

This antenna would need to be vertically mounted, so would be a little
clumsy sticking three foot up from a kayak. Light weight would be
better than heavier. The construction plans call for stainless steel
material. Any reason to not use aluminum?

Fresh-water kayaking, or bay/sea kayaking?

Aluminum antenna elements do not survive well around salt water (or
salt air near the coast). They corrode, often quickly and
enthusiastically and without a word of apology or shame. Maintaining
good electrical connections to aluminum is quite difficult under those
conditions. Stainless steel is used for marine antennas for the same
reason it's used for boat fittings.

Is diameter important? To
use aluminum I would want to use something thicker than 1/8 inch and
likely hollow.

The length of the elements will need to be changed somewhat, if you
use thicker-diameter tubing. For any given resonant frequency, a
very-thin element (wire or rod) needs to be longer; thick tubing
needs to be shorter.

Thicker-diameter elements tend to give you a broader SWR bandwidth,
which is often a good thing - the length becomes less critical.

Would it be ok to make it with some
sort of couplers for the bends?

Yes, but there's no need for bent elements in your case. This antenna
uses dual elements (connected via the bend) only because it's a
dualband antenna. A single-band antenna can use single pieces of
tubing.

However - as you note, this antenna design is going to be clumsy for a
kayak. Any half-wave dipole for the marine frequencies is going to be
almost a yard long. To complicate matters, a center-fed dipole like
this needs to be mounted "out to the side", with the feedline running
away at a 90-degree angle (as is show in the picture of the antenna
mounted on a mast). If you try to mount this up vertically above the
kayak, with the feedline running down right next to the lower dipole
arm, the feedline will "de-tune" the antenna quite badly and it won't
work well.

If you really want to build a home-made antenna for this band,
consider a "twin-lead J-pole" design. These are just about the same
length, overall, but they're fed at the bottom. You can make them
from a piece of coax, and a length of old-style 300-ohm "twinlead"
antenna cable. A light and simple antenna system would be one of
these twinlead J-poles (cut and tuned for the marine VHF frequencies),
taped to a simple piece of wooden dowel which is then mounted on the
kayak. They're not perfect antennas, but they're easy to make, light,
the antenna itself can be rolled up for storage, and they're cheap.

https://m0ukd.com/calculators/slim-j...le-calculator/

Just plug in the center frequency (157 MHz should do) and it will show
you the dimensions.

Or, as others have suggested, buy a commercial marine VHF antenna.

On Fri, 21 Apr 2017 00:28:47 -0400, rickman wrote:

Starting over from scratch...

A kayaking friend was asking me about antennas for marine VHF radios.

What problem is your friend trying to solve? My guess(tm) is that he
wants more range from what he's getting on his VHF marine handheld
radio. This is a common problem because the typical rubber ducky
helical antenna used on handheld radios has very low gain and is
lacking a proper counterpoise.

The first improvement would be to replace the rubber ducky antenna
with a 1/4 wave vertical stiff wire antenna on 156Mhz. It would be
about 48cm (18.9 in) long. That will provide a little gain and not be
too long and unwieldy. There are various such antenna available that
can be cut down to length. I would need to know the radio model and
RF connector type in order to be more specific.

Then next improvement would be a counterpoise ground element. In ham
radio circles, it's called a "tiger tail".
http://www.hamuniverse.com/htantennamod.html
https://www.google.com/search?q=tiger+tail+antenna&tbm=isch
It's just a length of 1/4 wave insulated wire dangling opposite the
antenna element. You could get fancy and use 3 or 4 such counterpoise
elements, but the improvement over a single element is minimal and not
worth the effort.

If the kayak was of a larger or ocean going variety, it might be
possible to mount a larger marine antenna like this:
http://www.n0lx.com/kayak.html
It's for HF, but would serve as a suitable mount for a VHF antenna.
A key component is the mount:
https://www.westmarine.com/antenna-mounts
I suggest something like this bolted to a backing plate:
https://www.westmarine.com/buy/west-marine--dual-axis-ratchet-mount-nylon--1986645?recordNum=12
The thread fits most larger marine antennas.
https://www.westmarine.com/vhf-antennas

Light reading:
"VHF Marine Antenna Fundamentals"
http://www.boat-project.com/tutorials/vhfant.htm

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

On 4/21/2017 5:37 AM, Jeff wrote:
On 21/04/2017 05:28, rickman wrote:
A kayaking friend was asking me about antennas for marine VHF radios.
Someone recommended this type of antenna.

http://www.amateurradio.bz/2m-70cm_v...e_antenna.html

I guess my first question is what would this be dual band? Is it
because it works over a range from 2 meters to 70 cm? Or is it somehow
tuned for both bands at the same time but not a lot in between?

This antenna would need to be vertically mounted, so would be a little
clumsy sticking three foot up from a kayak. Light weight would be
better than heavier. The construction plans call for stainless steel
material. Any reason to not use aluminum? Is diameter important? To
use aluminum I would want to use something thicker than 1/8 inch and
likely hollow. I believe TV antennas are usually made from 1/4 inch
tubing with maybe 1/16 inch thick walls. I guess that might make it
harder to bend without kinking. Would it be ok to make it with some
sort of couplers for the bends?


This is an entirely unsuitable for marine VHF on a kayak!!

Firstly it is for the wrong frequency, secondly it is unduly complicated
as it is designed to work on 2 frequency bands (not anywhere in
between), and thirdly if has to be mounted on a pole!!!

I didn't find an explanation of the theory of this antenna. How does it
work on two bands?

As far as not working on the right frequency, that's just a matter of
scaling the dimensions, no? 2 meter band is around 146 MHz and VHF
marine is around 156 MHz. Looks like a small adjustment to me.

This antenna is mounted using a piece of plexiglass. Are you saying the
plexiglass is dimensioned to make the pole part of the antenna? I
didn't see a mention of that in the construction.


I would investigate something like a J-pole for marine band which does
not require a ground-plane. The matching section could be enclosed in a
tube with a s/s whip on top.

S/S because aluminium will corrode with sea water.

Who said anything about salt water use?

I seem to recall the longer marine VHF antennas are a type called
"colinear" where multiple elements are connected essentially end to end.
My understanding is the advantage is that they can get more gain by
being longer.

--

Rick C