"christofire" wrote in message
news

"samersaed" wrote in message
...

Hi Engineers,

I'm Starting a new FM Station Project in BENGHAZI - LIBYA which is 60km2
in area, i was advised to use a transmitting system of a 600w
Transmitter attached to 4 double dipole antenna
(http://www.labelitaly.com/website/akk2.html) as my tower specs are :
tower angle 4 leg (leg size = 10 x 10), parameter at 96 m height = 2.5m
x 2.5m. attached a picture of my tower.

i want to get an omni-directional coverage as my tower is located in the
center of the city and I've been told that unless a panel antenna the
tower will make a shadow or something like that to the broadcast
signal.

i want to know if i'm really forced to use a panel antenna in my system,
why i can't use a high power regular dipole antenna.

and please advice if the transmitter power i'm using will be enough to
cover the 60km2 area.

hope to find help here.

Thanks in advance.




--
samersaed


Nothing attached!

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).

An advantage to using a circular array of panels is that it can be
positioned below the top of the structure (e.g. if the top is used for
television) and you might achieve a minimum of 0 dBd if the handovers
between adjacent panels are well controlled (or fortunate!). At many
stations in the UK, a number of tiers of panels are used in order to
provide gain and to facilitate a split upper/lower half-array system for
resilience. You could try to get the antenna supplier to measure four
panels on a stub tower of the same cross-section as your structure, but if
you're only buying a one-off they might charge you a premium for
perefoming the measurement. It would demonstrate unequivocally what the
final system would achieve HRP-wise, although, depending on the height of
the stub tower (or time-domain gating, etc.) you might still need to infer
the VRP.

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'.

Hope this helps.

Chris

Another reason for using panel elements is to facilitate horizontal
polarisation, which was popular in the UK (but perhaps for a questionable
reason) - nowadays, most stations use mixed or vertical polarisation. When
the majority of the listener-base is located in vehicles, VP is a better
choice principally because of the typical receiving antenna.

Chris