Radio Frequency (RF) Behaviors & Measurements, Units,

Posted By: technopediasite




Radio Frequency (RF) Behaviors

Gain and Loss

Gain describes an increase in an RF signal’s amplitude. Two sources of gain are:
• External power sources (RF amplifiers).
• Passive sources (reflected signals combining with the main signal to increase overall
signal strength). Various antennas are constructed with different passive gain.

Added power can be a serious problem. If the transmitted radiated power is close to legal

output limits, this added power might violate regulatory limits.

Loss describes a decrease in signal strength. The two main sources of loss within a wireless

system are:

• Loss caused by the impedance of cables and connectors causing power to reflect back,

resulting in the attenuation of total or forward power.

• Loss that occurs during signal propagation where objects cause the RF signals to be

absorbed, reflected, refracted, scattered, or destroyed.

Loss can be intentionally injected by an RF attenuator. Loss must be correctly measured and compensated due to receiver sensitivity.

Impedance

Impedance is defined as the resistance to ac flow, measured in ohms. Impedance in an RF

system is associated with the cabling and connectors within a radio system. An impedance

mismatch results in power being reflected back toward the transmitter.

Voltage Standing Wave Ratio (VSWR)

VSWR is a mathematical expression of the nonuniformity of an electromagnetic field on a

wave guide or RF voltage on a transmission line such as coaxial cable or a connecting device.A standing wave pattern arises when part of the energy of a forward traveling wave is reflected back at a point where there is an impedance mismatch along the transmission path. This causes the forward and reflected wave to add in and out of phase along the length of the transmission path. VSWR usually is defined as the ratio of the maximum RF voltage to the minimum RF voltage along the line.

VSWR is caused when all the devices responsible for transmission of the radio signal do not

match impedance wise. This includes the cabling, connectors, and antenna.

The two numbers relate to an impedance mismatch against a perfect impedance match. The second number is always one. The lower the first number, the better the impedance match.

The effects of VSWR include:

• A marked decrease in the amplitude of the transmitted RF signal.

• The reflected power burning out the electronics of the transmitter if they are not protected

against power being returned.

Several methods of changing or eliminating the effects of VSWR include:

• Proper use of appropriate equipment.

• Tight connections between cables and connectors.

• Use of impedance matched hardware.

• Use of high-quality equipment with calibration reports.

• Never using 75 ohm cable with 50 ohm devices.

Intentional Radiator

Intentional radiator is defined as an RF device that is specifically designed to generate and

radiate RF signals. In terms of hardware, intentional radiator includes the RF device and all

cabling and connectors up to (but not including) the antenna. The power output of the

intentional radiator refers to the power output at the end of the last cable or connector before the antenna.

Equivalent Isotropically Radiated Power (EIRP)

EIRP is the output power of a radio transmitter measured at the antenna. In calculating the

EIRP, use must include the power of the intentional radiator, any amplifiers, connector loss,

cable loss, and gain of the antenna.

Coverage

Coverage is the geographic area within which service from a radio communications system

can be received.

Ranging

Ranging is the measurement of the distance to a remote object (target) from a known

observation or reference point. Within RF transmission, it is the measurement of the transit

time of the RF signal.

Automatic Gain Control (AGC)

AGC is the process or means by which gain is automatically adjusted in a specified manner as a function of a specified parameter, such as received signal level.

Radio Frequency (RF) Measurements, Units, and Conversions

To ensure a system’s power is within standard specifications, the following calculations and

measurements must be performed:

• Power must be measured at the transmitting device.

• Loss and gain of connectivity devices between the transmitting device and the antenna

(cables, connectors, amplifiers, attenuators, and splitters) must be calculated.

• The transmitting power plus or minus the connectivity device calculation will give the

power at the last connector before the RF signal enters the antenna (intentional radiator).

• EIRP is the power at the antenna element (the intentional radiator) plus the antenna gain.

• Each area determines whether RF links are viable without overstepping power limitations

set by regulatory bodies.

Several units of measurement are standard within the wireless industry to reflect the amount

of power. The two most common units are watt and decibel:

• A watt is the basic unit used to measure power. One watt is equal to an ampere multiplied

times a volt.

• A decibel is the measurement of a signal’s loss or gain. The decibel is 10 times the

logarithm of output power to receive power.

The standard power guidelines for wireless local area networks (WLANs) is 1 W of power

from the intentional radiator and a maximum of 6 dB gain from the antenna. This gives a

WLAN an overall power output of 4 W.

While watts provide an absolute measurement of power, decibels provide a relative gain or

loss of power.

3-10 Rule

The relationship of dB loss and gain to power is known as the 3-10 rule. In calculating the

power output of a wireless system, the calculations below can be used as a quick reference:

• 0 dB is equivalent to 1 millionth of a watt (1 mW).

• For every 3 dB of gain, the power is doubled.

• For every 3 dB of loss, the power is halved.

• For every 10 dB of gain, the power is 10 times.

• For every 10 dB of loss, the power is one tenth.

As described above, gain and loss are measured in decibels. There are multiple points within an RF system wherein these gain and loss measurements can be taken.

Power Measurements Using Decibels

Below are the types of decibel measurements used in measuring the power in wireless

systems:

• dBm is a measure of power in milliwatts.

• dBi is the isotropic measurement of power at the antenna.

• dBd is the isotropic measurement of power for a dipole antenna.

• dBc is the measurement of the average power supplied to the antenna transmission line by a transmitter during one RF cycle taken under the condition of no modulation.