How to calculate the coverage area, over sea, of a fixed coast station (UHF link)?

HI!
I need to calculate the coverage area, over sea, of a fixed coast station (UHF link).
I know it depends on the refraction index also, does anyboby know a formula or a procedure to compute this range?

thanks.

Re: radio link coverage

**broken link removed**

Re: radio link coverage

Pay special attention to the Fresnel zone discussion on the link above. This will be the major cause of limiting range in your situation.

Re: radio link coverage

It would help if you clear a litle bit more about it.
1)Are you trying to create a "link" between two fixed
stations?
2) Or, Are you trying to create a coverage map of
fixed station (link Fixed to mobile)

In the first case, if the link is LOS you would need to
make sure it is LOS must of the time. so will need to
have fresnel clearance and make sure there is no
specular reflection due to sea water. You dont need
a "coverage" area.

In the second case, the free space loss does not aplies
since there is not clearance. In this case the equation
for losses is almost the same, just needs more att per
decade. And, you need a link budget, to account for Rayleigh,
log-normal fading. You will obtain an approximated distance for
coverage. For more precise "coverage" mapping you will
need topographic data and software to apply model as
Okumara-Hata, Cost231, Hikegami, Egli, etc.

Added after 4 minutes:

But, since you say is over sea water, I assuming you
dont need a coverage area since would be circular.
That is why is confusing.

Re: radio link coverage

first of all thanks for all replies.
I will try to explain better my problem:
i want to calculate the max distance from a fixed station (on the coast) to a mobile station (ship) in different (weather) contition.
I needed to know if there is relation between the max distance and umidity (or other parameters) related to atmosphere. Fixed station is on a high cliff. If these relations exist where could I find doumentation?

Hope everything is clear this time,
Thanks again.

Ciao

Re: radio link coverage

Usually under 1 GHz humidity and rain don't paly a big role in attenuation, you can estimate some 0,2 dB /Km.
Take instead a lot of care at the link margin; if you have to transmit data at least 20 dB margin, for voice 10 to 15 dB is enough.

Mandi

Re: radio link coverage

Eventought the link is from a fixed station to a mobile station,
this type of communication is not consider mobile. The reason:
is there not multipath in the mobile sense. But, there is main
trace reflection due sea water.
The analysis is better using regular equations for fixed relay
link. But, here is the catch:
1) Since the first antenna is a lot higher than the second one,
the reflection point in the water is closer to the mobile and
it may be hard to get fresnel cleareance.
2) since the reflection point "move" due to changes in the
height of the mobile, wave reflection plane change
and the orientation of the antenna
change with the movement of the ship the assumption that
a Rice type link may be not apllicable. Maybe a combination of
Rice/Gaussain.

That is translated in a bigger free space loss or a bigger margin.
The problem here is to find how much is that margin. You do not
want to much since is a waste of money. You do not want too
little since the link will be unreliable. If you must be precise then
a research about maritime communications is recommended.
If you dont need to be precise, just a ball park calculation, you can
use 20 dB as FANT proposed, but I would calculate the margin
with an receiver antenna no more than -3dB.