RTcom Radio Modems User Manual
Page 10
10
Warning: The use of gain antenna in some countries is not permitted. Similarly where ERP
(transmitted power limits) are imposed the actual transmitted power must not exceed the limit stated.
This means that the transmitter output power, less any coaxial feeder losses, plus the antenna gain
must not exceed the specified maximum ERP. Before installing your system please check!
Antenna Types
Antenna types fall into a number of categories. For the majority of applications the choice lies between
portable, fixed, omni-directional and those with gain and directional properties.
True omni-directional antenna in reality do no exist, i.e. those with equal gain in all directions.
However the nearest practical equivalent is the 1/2 or 1/4 wave dipole. The 1/4 wave dipole is by far
the most popular of all antennas and is found in use on most portable apparatus. Where the frequency
is too low and the dipole can become cumbersome and a compressed dipole (helical) antenna has to
be used.
Whatever antenna types you choose please ensure that its impedance is 50 Ohms.
name
Gain and Directional Properties
Comments
1/4 wave dipole
< -0.8dB near omni in the
vertical plane only.
For portable apparatus operating above 400MHz
Requires a ground plane for operation
Helical
< -4 to -10dB near omni in the
vertical plane only.
For portable apparatus < 400MHz
Requires a ground plane for operation
1/2 wave dipole
+ 1.2 to + 1.8dB near omni-
directional in the vertical plane.
For portable and fixed apparatus looking for a low
cost antenna that does not require a ground plane
End fed dipole
0db, near omni in vertical plane
For fixed apparatus that does not require a ground
plane
Colinear
+3dB to +9dB , near omni-
directional in vertical plane
For fixed apparatus that does not require a ground
plane
Yagi
+3dB to +28dB . Beam width
proportional to gain
For point to point links. Where an unwanted signal
needs to be blocked from the opposite direction
choose an antenna with a high front to back ratio