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