Radio Frequency (RF) Link Loss Budget and RF Path Loss Calculation

Posted By: technopediasite



Theory of Propagation:

Propagation is the motion of electromagnetic waves through or along a medium. Radio waves can take different paths for a transmission medium to a radio receiver.Long-distance communications usually use sky waves or direct waves for transmission. Sky waves are defined as waves that are usable due to refraction off of the

ionosphere or troposphere:

• The ionosphere is the portion of the Earth’s upper atmosphere where ions and electrons

are present in quantities sufficient to affect the propagation of radio waves. Normally, the

ionosphere extends from about 48 km to 1000 km (30 mi to 621 mi) above the Earth. At

certain times and locations, however, it may reach even lower. Long-distance, HF 2 MHz

to 30 MHz communications is made possible by reflections of radio waves from ionized

layers in this portion of the Earth’s atmosphere.

• The troposphere is the part of the atmosphere extending from the surface of the Earth to

about 11.3 km (7 mi). Within the troposphere, bending of radio waves by refraction

makes the distance to the radio horizon exceed the distance to the optical horizon.

Tropospheric refraction (bending caused by sudden changes in the characteristics of air in

a lower atmosphere) affects the received signal at distances beyond the radio horizon. The

troposphere is normally used in long-haul military microwave applications.

Direct waves are ground wave components that travel directly from the transmitting antenna

to the receiving antenna. In terrestrial communications, the direct path is limited by the

distance to the horizon from the transmitter. This is essentially LOS distance. It can be

extended by increasing the height of the transmitting antenna, the receiving antenna, or both.

Direct waves are used in point-to-point (PTP) satellite (UHF/SFH) communications. The

direct path also is useful for extraterrestrial. It is useful in air/ground/air communications

because most short-distance air/ground services are now on VHF or UHF.

Short-distance transmissions are used via ground waves and ground reflection techniques.

Ground waves are propagated waves that take three separate paths to the receiver:

• Direct wave

• Ground-reflected wave

• Surface wave

The effectiveness of ground waves depends on the RF, transmitter power, transmitting

antenna characteristics, electrical characteristics (conductivity and dielectric constant) of the

terrain, and electrical noise at the receiver site. LF and VLF transmissions use ground waves for propagation.

When high-powered transmitters and efficient antennas are used, the surface path has a

maximum range of about 500 km (310 mi) at 2 MHz. Surface path range decreases as

frequency increases. About 80 km (50 mi) represents the usual maximum range.

Free Space Path Loss (Dispersion)

Dispersion is the loss incurred by an RF signal due to signal dispersion. Signal dispersion is

the natural broadening of the wave front. The wider the wave front, the less power can be

induced into the receiving antenna. The power level decreases at a rate inversely proportional to the distance traveled and proportional to the signal’s wavelength as it is transmitted and travels through the atmosphere.

The calculation of path loss is performed by using the following formula:

20Log10 (4πd/wavelength)(dB)

Isotropic Gain:

Isotropic gain is the ratio of the signal level at the output of the antenna to that of its input

under a specified set of operating conditions. Isotropic gain is usually expressed in dBi.

6 dB Rule:

One rule commonly used when calculating isotropic gain is called the 6 dB rule:

• Each 6 dB increase in EIRP equates to a doubling of range.

• Each 6 dB reduction in EIRP equates to cutting the range by one half.

Fading:

Fading is defined as the variation (with time) of the amplitude or relative phase, or both, of

one or more of the frequency components of a signal.

Fading is caused by changes in the characteristics of the propagation path with time.

Types of fading are:

• Multipath fading—The propagation phenomenon that results in RF signals reaching the

receiving antenna by two or more paths. The causes of multipath fading include RF signal

reflection and refraction from natural objects (e.g., mountains) and man-made objects

(e.g., buildings).

• Delay spread—The result of multiple reflections of the transmitted signal arriving at the

receiver at different times. This causes the signals to crash into one another, resulting in

the receiver being unable to sort them out.

The effects of multipath fading and delay spread include constructive and destructive

interference, which is a phase shifting of the signal that causes signal loss and distortion.

Link Loss Budget Calculation:

Below is a formula that can be used in calculating link loss for a particular radio path. In

looking at a real system, consider the actual antenna gains and cable losses in calculating the signal power Pr (e.g., dBm) that is available at the receiver input:

Pr = Pt + Gt + Gr – Lp – Lt – Lr

Pt = transmitter power output (same units as Pr)

Gt = transmit antenna gain (dB)

Gr = receive antenna gain (dB)

Lp = free space path loss between antennas (dB)

Lt = transmission line loss between transmitter and transmit antenna (dB)

Lr = transmission line loss between receive antenna and receiver input (dB)

RF Path Loss Calculation:

What Is RF Path Loss, And How Does It Affect The Link Budget?

In order to determine factors such as the required antenna gain levels, radio power levels and reciever sensitivity figures in a wireless system, system designers must first be able to devise the link budget.

But, in order to establish that link budget for a proposed path, it’s important to understand how to calculate RF path loss. This must account for all of the gains and losses from the transmitter, through the medium (free space, cable, waveguide, fiber, etc.) to the receiver in an RF path.

This post explores 8 key steps in calculating RF path loss in order to devise the link budget.

8 Steps To Calculating RF Path Loss And Link Budget

RF path loss includes the attenuation of the transmitted signal as it propagates, as well as the antenna gains, cable and connector losses. Where the losses may vary with time, such as ‘fading’, allowance is made within the link budget.

In order to devise a link budget, it is necessary to investigate all the areas where gains and losses may occur between the transmitter and the receiver. Although guidelines and suggestions can be made regarding the possible areas for losses and gains, each link should be analyzed on its own merit.

The following are recommended steps:

1. Select a frequency – Certain system components are frequency sensitive such as cables, duplexers / couplers, free space path loss, or building material losses. Other components are not frequency sensitive, such as optical fiber, optical connectors and lasers.
2. Determine the signal source’s output power – Will it be coupled directly from a base station or will it be coupled from a repeater?
3. Specify the uplink and downlink system gain.
4. Determine the passive losses in the path – Design for losses at the highest frequency. Include any splitters, coaxial cable and connectors.
5. Select an Antenna – with the designed antenna gain.
6. Compute the free space path loss.
7. Add any additional losses that need to be accounted for – such as indoor walls and partitions or terrain, buildings and vegetation outdoors.
8. Compute the Received Signal level expected. This is then be compared to the receiver sensitivity to establish a pass/fail criteria for the proposed link.