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Old April 21st 17, 07:10 PM posted to rec.radio.amateur.antenna
Dave Platt[_2_] Dave Platt[_2_] is offline
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First recorded activity by RadioBanter: Dec 2014
Posts: 67
Default Antenna for Marine VHF

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.