On 4/21/2017 1:51 PM, Fred McKenzie wrote:
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?

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?

Rick-

I did not watch the video. I believe the antenna is "dual band" because
it happens to also work on a frequency band that is three times the
basic frequency. In other words, 146 and 440. That does not appear to
make any difference with regard to the Marine VHF band. You would
simply scale the dimensions by the ratio of the Two Meter frequency to
the Marine band frequency, 146/156.

The link is not a video. I appreciate the speculation, but I am
researching this and am looking for facts. As others have pointed out,
the design is basically a dipole but with J shaped elements 47.3 and
15.9 cm lengths. Someone else has suggested this is essentially a pair
of dipoles of the two lengths.

Usually a theoretical analysis can be found for any given antenna
design, but I have not found anything other than construction projects
for this design. They don't seem to cover theoretical aspects.

That said, the antenna in the article does not appear to be the best
choice for your application. The "J-Pole" antenna Jeff suggested would
be my choice. Do some research on the J-Pole, but remember to design it
for 156 instead of 146 MHz.

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 C