Making my own VHF antenna

[Doug Whitman]

I am installing a VHF radio in a 1971 Venture 21. I am handy, and wan
to make my own antenna (mostly, so I can easily repair it whenever i
gets whacked on the trailer). I have read some helpful posts (Larr
and Gary) on this forum which have generated some followup questions.

Using a borrowed SWR meter, I tested the old coax and antenna, an
promptly threw them in the trash. I made a 1/4 wavelength antenna
which worked so well, I am thinking about making it permanent. M
questions are related to this idea.

I understand the advantages of using a 3 dB antenna on a sailboat. I
this 1/4 wave whip, giving the same donut pattern as the commercial
dB antennas?

Is there any advantage in making the whip the same gauge wire as th
center conductor of the COAX?

I can easily make some ground radials at any angle. Would this improv
the transmission pattern?

Marine antennas have gain ratings such as 3 dB, 6dB. What are thes
numbers based on? What would be a 0 dB marine antenna?

I understand the basic fiberglass antenna is a vertical 1/2 wavelengt
dipole with a 1/4 wavelength whip above and a 1/4 wavelength coaxia
tube below surrounding the transmission line. I am running m
transmission line up the inside of my mast, so in a sense, I alread
have a 25 foot coaxial element (fed from the top and grounded at th
bottom) with a 18 inch whip element above. This seems like a usefu
configuration, but I suspect I am missing something.

Comments?

Dou

 

[Larry]

make my own antenna

http://www.hamuniverse.com/jpole.html

http://users.marktwain.net/aschmitz/antennas/jpolecalc.html
Super J-pole. Has 3 db more gain than regular Jpole.

http://www.alpharubicon.com/elect/jpolejaden.htm

http://n4bz.org/ham/jpoles.htm
Build the twinlead version out of 300 ohm TV antenna twinlead and wrap it
up with some emergency coax. Stow it in a little box, buried in a
cabinet. That way when the main antenna fails, during that storm in the
night, you can bring out the little twinlead J-pole VHF antenna and plug
it into your marine VHF walkie talkie, haul it up the mast with any
lanyard from the top and tie the line off to the handrail to get the
jpole away from the metal bits, like the mast. Voila, a $1 emergency VHF
marine antenna. Every boater should have one.

http://www.w4zt.com/jpole/
Don't use the 2 meter ham band measurements in any of these jpoles. Use
one of the calculator webpages in javascript and substitute the
measurements it suggests when you put in 157 Mhz into the webpage
frequency box. See how neat the portable J-pole for a walkie talkie is
made at the bottom of this webpage....emergency comms, even from the
liferaft with any length of some kind of pole or even flown by balloon or
kite to get altitude. I'm in Charleston, SC. We borrowed an advertising
helium balloon that looks like a little blimp from a car dealer and tied
a j-pole to the bottom of it with 250' of RG-58 cheap coax. Payed out to
the end of the coax roll, up 250' AGL, our range to repeater stations on
VHF 2 meter band was about 200 miles! Perhaps a kite and balloon, like
the old WW2 LF grinder distress radios had, would also be a great idea in
a liferaft to extend VHF range.

http://www.vcars.org/tech/J_Pole2M.html

http://www.dxzone.com/catalog/Antennas/J-Pole/
Here's a whole webpage of URLs to Jpole websites....(c;

As the center and shield of the coax cable are BOTH hooked to the copper
pipe (or twinlead loop), these antennas are very safe to use for the
radios. There is no static voltage feeding to the radio, etc. They
measure a dead short with an ohm meter across the coax cable
connector....

Bon Appetit!

 

[Mic]

http://gallery.wdsg.com/Marine-Antennas

Pictorial and comparison chart