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.

Here in southern SC, at times we can hear Ft Mason, NC CG down to Ft
Lauderdale and Miami to give those that have never experienced it an idea.

Leanne
s/v Fundy

A small (and often confused) point - a 6 dB increase is 4x the power - not
2x. The formulas a
dB = 10 log10 (P1/Pref)
= 20 log10(V1/Vref)
where P1, Pref are power levels, V1, Vref are voltage (or current, or field
strength) levels. If P1/Pref = 2 , then dB = 3. It all comes about
because P = (V^2)/R.
Derek

"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?

True, a 6db gain is four times the power. But the question was comparing a
3db gain antenna to a 6db gain antenna. The difference is 3db, or two times
the effective radiated power.

Don't forget, RTFQ.

Rusty O

Thanks a lot guys for your info.
Very interesting.
Tks again
AP


"Larry" wrote in message
...
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

In article fJdEc.125914$eu.79506@attbi_s02,
"Derek Rowell" wrote:

A small (and often confused) point - a 6 dB increase is 4x the power - not
2x. The formulas a
dB = 10 log10 (P1/Pref)
= 20 log10(V1/Vref)
where P1, Pref are power levels, V1, Vref are voltage (or current, or field
strength) levels. If P1/Pref = 2 , then dB = 3. It all comes about
because P = (V^2)/R.
Derek

Your are correct in your math, but 4x Power is illusionary when trying
to compare the range difference between the two signals. 4x Power
doesn't get you anything, but better signal to noise at the Radio
Horizon, which is what the original poster asked.......

me

"AP" wrote in :

Thanks a lot guys for your info.
Very interesting.
Tks again
AP


3 dB sounds like a lot....twice the power.....but, alas, look on any
calibrated receiver's S-meter. Look at how LITTLE 3 dB is above the S-9
mark on it. The calibrations are 10 dB apart.

In reality, on the air, 3 dB is undiscernable.

Also, in reality, ON A BOAT...IN THE WAVES, the flat pattern generated by a
super-high-gain antenna squishing the donut of a vertical dipole out flat,
all that power will end up being pointed at the sky on one side of the
wave....and at Davy Jones' Locker on the other side of the wave....making
this whole discussion MOOT!.....in reality.....(c;

Larry W4CSC

"Larry W4CSC" wrote in message
...

Also, in reality, ON A BOAT...IN THE WAVES, the flat pattern generated by
a
super-high-gain antenna squishing the donut of a vertical dipole out flat,
all that power will end up being pointed at the sky on one side of the
wave....and at Davy Jones' Locker on the other side of the wave....making
this whole discussion MOOT!.....in reality.....(c;

Well, this calls for a gimbaled hi-gain VHF anntenna. Or mount one on top of
your gimbaled radar dome (it save you a counterweight). Man, how close an
easy solution sometimes is.....

Meindert