"samersaed" wrote in message
...

- Why are looking at a flat panel directional antenna when you want a
omnidirectional antenna?
Mikek -

First sorry that i forgot to attach the picture of my tower, it's
attached now in this RAR File :
http://www.mediafire.com/?cse8yiyn5d4jtd9 .

I'm Forced to use panel antenna cause the person i'm buying the antenna
from told me that my tower specs can handle nothing but a panel antenna
to get the desired coverage cause if i use another system of antenna the
tower will make a shadow to the transmitted signal and that's an
information i don't trust, the panel antennas are so expensive and heavy
for shipping so i'm asking for a replacement for those antenna that will
work with my tower and gets me an omni-directional coverage.

can you verify this information?

- You might be able to use a simple sleeve dipole, ground plane, or
suchlike
at the top of the mast if that's vacant and you can get an antenna
that's
happy to work as the lightning finial, but the antenna gain will be low
to
the intended service area; possibly less than 0 dBd, particularly in the

case of a ground-plane antenna (upward-tilted VRP: radiation pattern in
the vertical plane). For police communication at similar frequencies in
the UK, four two-element yagis around a pole were used at the top of the

structure, the pole actining as the finial. Usually, two such arrays
were
stacked vertically. The small radius of such an array gives a good omni
HRP (radiation pattern in the horizontal plane). -

Unfortunately i'm renting a place in the tower which is 80 m high and
the parameters of the tower at 96 m high is 2.5m x 2.5m so it's not
gonna be small in radius for the antenna system.


- A service area of 60 km^2 would have a radius of 4.4 km if it were
circular - is yours? The required ERP depends on the required minimum
field strength at the limit of service. For simple spherical spreading,
taking no account of antenna VRPs and reflections/multipath,
E=(7/r)sqrt(ERP) ... which can be found in ITU texts and is easy to
derive
from first principles ... so if you were to use, for example, a minimum
field strength of 5 mV/m (stereo, in a large city with lots of sources
of
interference, using low-gain receiving antennas) at r=4.4 km, then
ERP=9.9W. If your service area isn't circular, say it extends out to 10
km in one direction, then to achieve 5 mV/m at 10 km would require ERP =

51.0 watts. A 600 watt transmitter into an antenna system with a net
gain
of, say -3 dBd (accounting for VRP, HRP ripples, and
combiner/feeder/power-splitter loss), would provide an ERP of 300 watts
so, in the latter case, a margin of 7.7 dB. This would provide some
resilience to multipath fading (although P-P=0, no matter how large P
is!). In other words, you probably wouldn't need a higher-powered
transmitter in this case - but it does depend on the shape of the
service
area, and on your definition of 'service'. -

Ok let us Stick to the 600w transmitter, i'm not an expert in waves
calculations glad to have such help from you, you can see exactly the
picture of the tower and the position of it in this RAR FILE
http://www.mediafire.com/?cse8yiyn5d4jtd9 , attached also a placemark
from google earth for the tower location in the city, Benghazi's surface
has a lot of area exposed to the sea at the north west, and there are
hills at the south east, i only want to cover the residental area in
between which is a cicule of 15 km of radius, getting a wrong pattern
antenna will waste most of my signal into the sea, please recommend me
an antenna system which works with the 600w transmitter and the tower
specs i mentioned ( tower angle 4 leg (leg size = 10 x 10), parameter at
96 m height = 2.5m x 2.5m ) and also for the geographical
charecteristics of the city and also want to get use of the gain of the
antenna i will get to strength the signal ( my tower is the one at the
left )

thanks in advance

--
samersaed


I don't have software to read .rar files - perhaps you could provide links
to .jpg photos that can be read by Windows without needing to download a
reader program.

If the aperture you're renting is at 80 m on the 96 m tower then, clearly,
you don't have access to the top so you can't use a simple omni-directional
antenna. If you mount a half-wave dipole any reasonable distance off the
face of a tower with side dimension similar to the wavelength then the
resulting HRP will be directional (viz the 'shadow' principle you mentioned,
and reflection off it). You may be able to use one or more such directional
patterns if your service area isn't uniform around the antenna but if your
aim is consistent coverage at all angles around the structure then you will
need to mount an antenna off each face, and that's the purpose for which
panel antennas have been designed - to achieve controlled hand-over from one
panel to the next so ripples in the HRP are minimised. It isn't trivial to
predict the radiation patterns of an antenna next to a metal structure and
many manufacturers offer sytlised patterns based on averaged measurements.
One manufacturer has already been named by amdx and there are many others
you can Google (e.g. RFS, Alan Dick, R&S, Harris, etc.).

However, since your service area covers a limited arc, bounded by sea and
hills, it sounds like you don't want an omni-directional HRP at all - more a
precisely-tailored directional pattern. It would probably take range
measurements using a stub tower to get that right - but it all depends on
how precise you need to be (e.g. on the definition of your coverage
criterion).

You will need to do some research into this (or pay someone to do it for
you) if you don't trust the advice you're being given by a supplier. There
may also be regulatory issues to contend with. It goes without saying that
you shouldn't base your purchase on anything written in this or any other
newsgroup!

Chris