On Fri, 21 Apr 2017 00:28:47 -0400, 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
No. The above antenna is tuned for 144 to 148MHz. Marine band
transmit is from 156.0 to 157.5Mhz transmit, and 161.975 to 162.6Mhz
for AIS and weather. You might be able to retune the Jpole antenna in
the above article, but my guess(tm) is that VWSR at the band edges is
too high. Another reason you don't see Jpole antennas in marine use
is that vertical radiation angle. Jpoles radiate most of their RF at
the horizon and above, not down. With fairly low gain, that's not a
problem with an antenna on the water line as the boat pitches and
rolls. It's a major problem with mast mounted antennas. It would be
better if the vertical radiation pattern was roughly symmetrical as in
this dual Jpole antenna. It would be interesting to see what a model
of this antenna looked like in 4NEC2.
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?
Nope. It relys on the 3rd harmonic of 146Mhz being roughly on 440MHz.
There's usually a complex matching network on dual band antennas to
help keep the VSWR down, but this one apparently lacks even a balun.
For example, this is part of the guts of a Diamond X-50 dual band ham
antenna (after a friend backed his car into it breaking it in half).
http://802.11junk.com/jeffl/antennas/Misc/slides/x50-01.html
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?
Aluminum is fine. Anodizing might be a problem. Alodyne 1200 is
fine. Most paints are ok. Coat hangers suck. Watch out for
dissimilar metals in contact.
Is diameter important?
Larger outer diameter means wider bandwidth. You can use tubing in
order to get light weight.
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'm too lazy to measure mine, but I would guess(tm) 0.5" diameter and
a very thin 0.031 wall diameter. TV needs all the bandwidth it can
get, so fat pipes are needed. The antenna also needs to be light and
cheap, so thin wall with seams is standard for TV.
I guess that might make it
harder to bend without kinking.
We used 6061-T6 for antennas. You can bend it in a tight turn if you
seal the ends and fill the tubing with sand:
http://www.wikihow.com/Bend-Aluminum-Pipe
Would it be ok to make it with some
sort of couplers for the bends?
Sure. The bend does not need to be contiguous. A bar with two holes
drilled in it for the two elements should work (and be adjustable).
Suggestion: Look at various commercial VHF antennas and build
something similar. You'll be amazed at how crude they are inside. One
common antenna (I forgot the maker and model) used a fiberglass radome
with a length of 1/4" wide copper tape stuck to the inside of the tube
for driven elements.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558