Assuming identical setups and a perfectly vertical antenna, what
percentage increase in range would be likely as a result of changing
from a 3db to 6db VHF antenna?

Assuming they are both high enough to hit max range, then by definition,
the 6db would put 2x the power in the direction of the receiving station...

Why do bigger boats use 9DB? they stay more horizontal... why sailboats
use 3 DB? They lean to one side for long periods of time.
BUT... VHF is line of site so if you can't "see" the horizon from the
middle of the antenna, you won't be able get the extra distance...

My experience... I have 1 9Db antennas on my sportfish... I can hit 50
miles from Bimini to Fort Lauderdale ONLY when communicating to the big
guys. (USCG, SEATOW, etc)... Why? THey have very tall antennas and I am
concentrating my 25 watts along a very narrow band along the water.

I could never do that with my older 6 DB antennas.



Sailman wrote:
Assuming identical setups and a perfectly vertical antenna, what
percentage increase in range would be likely as a result of changing
from a 3db to 6db VHF antenna?

What is this 3db,6db,9db etc.
Are the VHF antennas all the same? (excluding heght).
What db stands for??
Pls give me an idea.
tks
AP

"Sailman" wrote in message
om...
Assuming identical setups and a perfectly vertical antenna, what
percentage increase in range would be likely as a result of changing
from a 3db to 6db VHF antenna?

So how should I think about translating "2x the power" into a rough
estimate of increased distance?

Ed wrote in message . ..
Assuming they are both high enough to hit max range, then by definition,
the 6db would put 2x the power in the direction of the receiving station...

Why do bigger boats use 9DB? they stay more horizontal... why sailboats
use 3 DB? They lean to one side for long periods of time.
BUT... VHF is line of site so if you can't "see" the horizon from the
middle of the antenna, you won't be able get the extra distance...

My experience... I have 1 9Db antennas on my sportfish... I can hit 50
miles from Bimini to Fort Lauderdale ONLY when communicating to the big
guys. (USCG, SEATOW, etc)... Why? THey have very tall antennas and I am
concentrating my 25 watts along a very narrow band along the water.

I could never do that with my older 6 DB antennas.



Sailman wrote:
Assuming identical setups and a perfectly vertical antenna, what
percentage increase in range would be likely as a result of changing
from a 3db to 6db VHF antenna?

Db is a comparison of the signal from a perfectly spherical pattern to
the pattern of an individual antenna. Think squshed doughnut. A 3db
antenna radiates a fat small doughnut while a 9db antenna radiates a
flattened and very wide doughnut. Same amount of dough just a different
shape.

The 9db can radiate further perpendicular to the antenna but if the
antenna tilts it radiates into the water or the stars. With the fat 3db
pattern some portion of the signal goes to the horizon no matter which
direction the antenna is leaning.

AP wrote:

What is this 3db,6db,9db etc.
Are the VHF antennas all the same? (excluding heght).
What db stands for??
Pls give me an idea.
tks
AP

"Sailman" wrote in message
om...

Assuming identical setups and a perfectly vertical antenna, what
percentage increase in range would be likely as a result of changing
from a 3db to 6db VHF antenna?




--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

On Tue, 22 Jun 2004 16:42:17 +0300, AP wrote:

What is this 3db,6db,9db etc.
Are the VHF antennas all the same? (excluding heght).
What db stands for??
Pls give me an idea.
tks
AP

"Sailman" wrote in message
om...
Assuming identical setups and a perfectly vertical antenna, what
percentage increase in range would be likely as a result of changing
from a 3db to 6db VHF antenna?

3db, etc. stands for 3 decibels gain. 3 decibel gain is twice the gain.
The next question is usually "gain over what"? More properly, 3 db usually
refers to gain over a 1/2 wave dipole. If specified as "dbi" then it means
decibel gain over isotropic which is a point source.

No, all vhf antennae are not the same, but I wouldn't put too much
confidence in advertised gain. That said, the higher the number, the
greater the gain. That usually translates in greater vertical height in
the case of vhf antennae.

How does it obtain the gain? Consider that an isotropic antenna radiates
power equally in all directions. Certainly, any power radiated straight up
or straight down is wasted - there is nobody there to receive it. Antennae
get their gain by concentrating their power in the horizontal plane. The
radiation pattern resembles a doughnut. The greater the horizontal
"squeeze", the greater the gain. This is done by stacking elements and
using phase to concentrate power horizontally.

db stands for "decibels" Decibels are a logarithmic measure.
--

Larry W1HJF
email is rapp at lmr dot com

Assuming identical setups and a perfectly vertical antenna, what
percentage increase in range would be likely as a result of changing
from a 3db to 6db VHF antenna?


I have both and to tell you the truth the only difference I see is that a boat
on the fringes (noisy signal, low S/N ratio) on the 3 dB gets a bit clearer on
the 6 dB. I assume it does the same general thing for my transmittedf signal.
It isnt as big a deal as you might think, no dramatic range increases. I think,
from my experience,that antenna height is the more dominant factor, not antenna
gain.

On 22 Jun 2004 21:38:22 GMT, (BOEING377) wrote:

Assuming identical setups and a perfectly vertical antenna, what
percentage increase in range would be likely as a result of changing
from a 3db to 6db VHF antenna?


I have both and to tell you the truth the only difference I see is that a boat
on the fringes (noisy signal, low S/N ratio) on the 3 dB gets a bit clearer on
the 6 dB. I assume it does the same general thing for my transmittedf signal.
It isnt as big a deal as you might think, no dramatic range increases. I think,
from my experience,that antenna height is the more dominant factor, not antenna
gain.


As a rough rule of thumb, the length of the antenna must be twice as
long for it to have a 3 db gain over another one. A 6 db gain antenna
would be twice as long as a 3 db antenna. A 9 db antenna would be
twice the length of a 6 db antenna.
If they aren't then they probably don't have the promised gain.

Another way to get gain is to increase the height of the antenna above
ground / water. When you double the antenna height you pick up 6 db
gain at the range limit. Every time you double the height again you
pick up another 6 db.

The only problem is that the height gets out of hand quickly. Doubling
from 20 feet means going to 40 feet. Doubling that means going to 80
feet etc.

Here is a link with a chart that lets you estimate range with
different antenna heights and gain.

http://www.artscipub.com/simpleton/simp.range.html

Regards
Gary

In article ,
(Sailman) wrote:

So how should I think about translating "2x the power" into a rough
estimate of increased distance?

You really can't. Once you hit the Radio Horizon for your antenna
height, any increase in power is just going out over the heads of your
intended listeners. By increasing the power, you will cause your signal
to noise ratio to improve at the Radio Horizon, but you really don't
extend that horizon by increasing the power, or the antenna gain.
The exceptions to the preceeding rule are, that Vhf Signals can, and do,
DUCT in Thermal Inversion Layers, that are common in some places. These
ducts can cause Vhf signals to be heard and communicated on, many
hudreds of miles further than the Radio Horizon. They do, however,
change and disappear quite often, with weather fronts, and shouldn't be
reallied on for communications.

I pioneered a bunch of these Vhf/Uhf Paths in Brystol Bay, Alaska,
for the Salmon Industry, back in the late 70's, that are still in
use today. When a Low Pressure Front moves thru the area, these Radio
Paths go Stoned Dead for a few days untill the thermal Inversion
rebuilds. Path Studies that we did back then, showed that these things
were good for 60 to 70 days out of the 90 day Salmon Season, and that
justifdied using them, untill good phone service was established in the
region. Now they are just a cheap backup for commercial communications.


Bruce in alaska Who gives Bill Forgey, K7KDU all the credit for
coming up with the idea, in the first place....
--
add a 2 before @

"Bruce in Alaska" wrote

Paths go Stoned Dead for a few days untill the thermal Inversion
rebuilds. Path Studies that we did back then, showed that these things
were good for 60 to 70 days out of the 90 day Salmon Season, and that
justifdied using them, untill good phone service was established in the
region. Now they are just a cheap backup for commercial communications.

In really warm waters, the tropo-ducting is an annoyance that brings
unwanted radio traffic from hundreds of miles away, and it does this most
nights in the summertime.

http://www.iprimus.ca/~hepburnw/tropo_xxx.html

Find your ocean and see what the ducting forecast is.

Jack