On Thu, 2 Sep 1999, Eric Johnson wrote:
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> West Wight Potter Website at URL
> http://www.lesbois.com/wwpotter/
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> > I've noticed in several references that increasing the antenna height on a
> > boat also increases the antenna gain, supposedly increasing its
> > transmitted
> > signal strength.
>
> I always thought >gain< was a function of the antenna design, and has to do
> with the radiation pattern from the antenna becoming more horizontal and
> less spherical, but conventional wisdom holds you don't want a really high
> gain antenna on a sailboat because the horizontal radiation pattern doesn't
> help you much when heeled.
Well said. Most antenna gain figures are relative to an isotropic
radiator which radiates equally in all directions (a sphere). Gain is
obtained by squishing the straight up and down radiation into the more
horizontal plane. This can be accomplished in many ways, one of the more
popular ways is to use several half wave radiators in phase making a
collinear array. These are placed one above another in the typical
commercial vertical antenna, increasing the length of the antenna for each
one.
> I think west marine recommends a 3db gain for
> monohull sailboats, while I think I've seen antenna designs with as much as
> 9db gain for trawlers and multihulls.
Since the Potter is not really designed to be sailed at a big heel angle,
I would think that the higher gain antennas would be fine on it. Even the
highest gain verticals have plenty of radiation at 10 - 15 degrees off the
horizontal.
> Since VHF is basically line-of-sight, height increases the range for the
> same reason a taller light can be seen further.
The only negative of increasing height is that you need more feed line to
feed it. At VHF frequencies 50 ft. of RG58U has a loss of 3db. That
means that the cable has eaten half your power!
> But I don't believe height itself increases gain.
There is a factor called "height gain" which is manifested in how loud
your signal will heard at a given distance no matter what the gain of the
actual antenna is. It is usually considered 0 up to about 30 ft. so
it is probably not important on a potter.
> > What is the maximum height that I can put my antenna
> > without blowing out or overloading the radios of the adjacent boats?
>
> I wouldn't worry about it if your running normal (<=25watt) power. As a
> courtesy to others you should try to low power setting to reach your target
> before trying the high power. I've never heard of anyone blowing out an
> adjacent boats radio.
If his radio's front end is that delicate, he needed to trade it in
anyway.
> or are you pulling our legs again?
>
Rich Gort W7JVL
WWP19 #202 SAYOKO
Lake Stevens, WA