Brought to you by PCS Electronics, www.pcs-electronics.com
53
Typical FM broadcasting antenna setups
Below are several of the typical broadcasting antenna systems that can be encountered worldwide.
Fig. 29: Typical antenna setups
Lets look at system A first. It’s a simple vertical dipole antenna, mounted on a pole. The gain of this antenna is 0dBd and if
we assume that the coaxial cable does not have any losses the ERP of this system equals transmitter power. For example, a
1KW transmitter with this antenna system and perfect coaxial cable (losses=0) would have ERP of 1000W. Radiation
pattern of this system is more-less omni-directional but since the metal pole holding the antenna blocks the signal there is a
null of signal exactly on the opposite side of the pole.
System B has two simple dipole antennas mounted on a pole. The gain of this antenna is slightly less than 3dBd (due to
losses in harness – splitter). If we assume that the coaxial cable does not have any losses the ERP of this system equals
double transmitter power. For example, a 1KW transmitter with this antenna system and perfect coaxial cable (losses=0)
would have ERP of 2000W. Note the antennas are mounted on the opposite sides of the pole to help make radiation pattern
as omni-directional as possible.
System C has four simple vertical dipole antennas mounted on a pole. One of the antennas is behind the pole and is not
visible. Note the antennas are mounted at an angle of 90 degrees between each other to help make radiation pattern as omni-
directional as possible. The gain of this antenna is slightly less than 6dBd (due to losses in harness – splitter). If we assume
that the coaxial cable does not have any losses the ERP of this system equals 4x transmitter power. For example, a 1KW
transmitter with this antenna system and perfect coaxial cable (losses=0) would have ERP of 4000W.
System C has theoretically double the range of the System A although in practice it takes 4-6x increase of power to double
the range. 4x increase of power is equal to 6dB of gain. And you get 3dB of gain by doubling the number of dipoles. So to
upgrade system C to 9dBd you’d need 8 dipoles. And for 12dBd you’d need 16 dipoles. 16 dipoles would in theory increase
your range 4x compared to a single dipole. In practice there would be some losses in combining so many dipoles. You can
use circular dipoles in very similar configurations.
