User Manual

VoiceXtender Modem Installer’s Guide

System Planning

Rev. A - 8/97 2-5

System Gain

System gain is the total gain of the radio system, without any consideration of

the antennas or cables. It is the arithmetic difference between the transmitter’s

output power and the receiver’s sensitivity threshold. System gain (G

in the

formula) is measured in dB. To calculate the system gain, subtract the receiver

sensitivity from the transmitter power:

(Transmit Power) - (Receiver Sensitivity) = (System Gain)

For example, the system gain for the VX at maximum power is 127 dB.

(+29 dBm) - (-98 dBm) = 127 dB

Antenna Gain

Antenna gain is the measure of the antenna’s ability to focus the radio

frequency (RF) energy into a preferred direction. Antenna gain (G

ANT

in the

formula) is measured in dBi—the ratio between the power radiated by the

antenna in a specific direction over the power radiated to that direction by an

isotropic antenna fed by the same transmitter. An isotropic antenna radiates a

signal evenly in all directions.

Refer to the antenna manufacturer’s information to determine the antenna gain

for the antenna you will be using. Some antennas are specified in dBd. This

number can be converted to dBi by adding 2 dB.

Cable/Connector Loss

A directional antenna is connected to the VX modem with a coaxial cable. The

cable loss (L

in the formula) is measured in dB, and depends on the cable

length and the type of cable you are using. You can use any 50 ohm coaxial cable

whose loss in dB per hundred feet is low enough (3 - 15 dB) so as not to contribute

significantly to the total link loss. Quality cables with less than 7 dB loss per

hundred feet are sold by Cylink and Cylink distributors.

Free Space Path Loss

Due to reflections from the ground and other objects, the actual path loss

between the transmitting and receiving antennas might differ significantly

from the calculated path loss when both antennas are placed in a “free space”

environment. Unfortunately, the additional loss due to these effects is difficult

to calculate and requires precise knowledge of the geometry of the link and

surrounding materials which, in most cases, is not available.