Radio Frequency Technologies

Question 1

What is Radio Frequency propagation?

Answer 1

Radio Frequency propagation is a phrase used to describe how RF waves move through space.

Question 2

How are electrical and magnetic fields aligned to a RF propagating source such as an antenna?

Answer 2

Electric and Magnetic fields oscillate perpendicular to each other.They are both perpendicular to the direction of propagation. Electric field is parallel to the generating wire(antenna) and the magnetic field is perpendicular to the generating wire.

Question 3

What are RF characteristics or properties and what do they define?

Answer 3

The characteristics of an RF wave define properties of the RF waves. Some important characteristics are wavelength, frequency, amplitude, and phase. These properties can be manipulated or modified to allow modulation of information onto the wave.

Question 4

What is wavelength of an RF wave?

Answer 4

The wavelength of a RF wave is calculated as the distance between two adjacent points on the wave. In other word it is the distance between two recurring points in the wave.

Question 5

What happens when an RF wave strikes an object that is large in comparison to the wavelength than when it strikes an object that is small in comparison to the wavelength?

Answer 5

An RF wave would reflect of an object that is larger than its wavelength and would scatter when the object is smaller than its wavelength.

Question 6

What does λ = c / f mean in RF?

Answer 6

The Greek letter lambda (λ) denotes the wavelength of a RF wave. The formula λ = c / f is used to calculate the wavelength of an RF wave provided the frequency of the wave is known (denoted by f in the formula) and c is the symbol for speed of light which is 299,792,458 meters per second.

Question 7

What is Frequency in RF?

Answer 7

Frequency in RF refers to the number of wave cycles that occur in each window of time. Usually measured in one-second intervals, a frequency of 1 kilohertz (KHz) would represent 1,000 cycles of the wave in one second.

Question 8

What is modulation in RF?

Answer 8

Modulation is the process of manipulating a carrier so that it represents meaningful information.

Question 9

What is amplitude of an RF wave?

Answer 9

The amplitude ———-

Question 10

What is Amplitude Shift Keying?

Answer 10

Amplitude Shift Keying is a modulation technique that involves changing or shifting the amplitude of a wave to represent information.

Question 11

What is the phase of a wave?

Answer 11

Phase is the position of a point in time on a cycle of a wave form. Unlike wavelength, frequency and amplitude, phase is not a characteristic of a single RF wave but is instead a comparison between two RF waves.

Question 12

What happens to the strength of received signal when two copies of a wave arrive in-phase?

Answer 12

Two arriving copies of a wave that happen to arrive in-phase will increase the strength of the received signal; however, they will never make the signal stronger than the transmitted signal. Signals weaken as they travel through space and energy is spread over an increasing area.

Question 13

What would happen to two waves that are completely out of phase?

Answer 13

Two waves that are completely out of phase ( 180 degrees out of phase) will cancel each other.

Question 14

What is an RF signal?

Answer 14

RF waves that have been modulated to contain information are called RF signals

Question 15

What are the various RF behaviors of major importance to a CWNA?

Answer 15

Gain, Loss, Reflection, Refraction, Diffraction, Scattering, Absorption, VSWR, Return Loss, Amplification and Attenuation, Wave Propagation, Free Space Path Loss and Delay Spread.

Question 16

What is Gain in RF?

Answer 16

Gain is defined as the positive amplitude difference between two RF wave signals

Question 17

What are the two types of RF gain?

Answer 17

Active and Passive gain. Both types can be intentional and passive gain can also be unintentional.

Question 18

How is Active gain acheived?

Answer 18

Active gain is achieved by placing an amplifier in-line between the RF signal generator (such as an access point) and the propagating antenna.

Question 19

What is passive gain?

Answer 19

Passive gain is not an actual increase in the amplitude of the signal delivered to the intentional radiator, but it is an increase in the amplitude of the signal, in a favored direction, by focusing or directing the output power.

Question 20

What are some of the cases where unintentional passive gain occurs?

Answer 20

Unintentional passive gain happens because of reflection and scattering in a RF coverage area. When the RF signal leaves the transmitting antenna, the primary signal travels out from the antenna according to the propagation patterns for which the antenna is designed. However, this signal may encounter objects that cause reflection and scattering, resulting in multiple copies of the same signal arriving at the receiving antenna. If these signals arrive in phase, they can cause the signal strength to actually increase and this would be a form of unintentional passive gain.

Question 21

What is Loss in RF?

Answer 21

Loss is defined as the negative amplitude difference between two RF signals. Like gain loss can be either intentional or unintentional

Question 22

What is intentional loss in RF?

Answer 22

Due to regulatory constraints the output power of a wireless device is restricted. Attenuators may be used to comply with these requirements. They are placed in line between the access point or bridge and the antenna.

Question 23

What is natural loss in RF?

Answer 23

Natural loss in RF signal occurs in the normal process of RF propagation, which involves spreading, reflection, refraction, scattering, diffraction and absorption.

Question 24

What is signal reflection in RF?

Answer 24

When an RF signal bounces off a smooth non-absorptive surface, changing the direction of the signal it is said to reflect, and the process is known as reflection.

Question 25

What is it called when an RF signal changes speed and is bent while moving through media of different densities.

Answer 25

This phenomenon is called Refraction.

Question 26

What are some of the common cause of Refraction in RF?

Answer 26

Some of the common causes of refraction include changes in temperature, changes in air pressure or the existence of water vapor

Question 27

What is the phenomenon of change in the direction and / or intensity of a wave as it passes by the edge of an obstacle called?

Answer 27

Diffraction

Question 28

What causes Diffraction in RF?

Answer 28

Diffraction occurs because the RF signal slows down as it encounters an obstacle and this causes the wave front to change direction.

Question 29

What are the effects of Diffraction in RF?

Answer 29

Diffraction can cause RF shadow which is results in areas without proper RF coverage. If you are in RF shadow area, you will not be able to receive communications from the wireless network.

Question 30

What is Scattering in RF?

Answer 30

Scattering happens when RF signal strikes an uneven surface - such as when an RF signal passes through a medium that contains small particles. Causing the signal to be scattered instead of absorbed, so that the resulting signals are less significant than the original signal. Another way to state Scattering is simply multiple reflections.

Question 31

What are some mediums that could cause Scattering of RF waves?

Answer 31

Smog, rocky terrain, leafy trees or chain link fencing, rain and dust can all cause scattering of RF waves.

Question 32

What is Absorption in RF?

Answer 32

Absorption in RF is the conversion of RF signal energy into heat. This conversion happens because the molecules in the medium through which RF signal is passing cannot move fast enough to keep up with the RF waves.

Question 33

What are some materials / medium that can absorb RF signals?

Answer 33

Water, drywall, wood and humans are some examples of things that are known to absorb RF signals.

Question 34

What is VSWR?

Answer 34

VSWR stands for Voltage Standing Wave Ratio. It is the measurement of mismatched impedance in an RF system.

Question 35

What is defined as the resistance in an electric circuit and is typically measured in Ohms?

Answer 35

Impedance

Question 36

What is Return Loss in RF system?

Answer 36

When there is a VSWR of 1.0:1, which will always be the case, there is some level of power loss due to backward reflection of the RF signal within the system. The energy that is reflected towards the RF generator or transmitter causes return loss. It is expressed in decibels (dB) of the ratio between the forward current (incident wave) and the reflected current (reflected wave).

Question 37

What can be done to minimize the effects of VSWR and Return Loss in an RF system?

Answer 37

Avoid impedance mismatches in the entire link chain from the radio to the antenna. You would need to use all equipment ( RF transmitters, cables and connectors) with the same ohm rating.

Question 38

What is the common impedance rating in an 802.11 RF system?

Answer 38

The most common impedance rating in 802.11 systems is 50 ohms.

Question 39

What is amplification in RF signals?

Answer 39

Amplification is an increase in the amplitude of an RF signal. It causes an active gain in RF signal, somewhere along the path up to the point of intentional radiator. It is accomplished by using an amplifier.

Question 40

What is Attenuation in RF signal?

Answer 40

Attenuation is the process of reducing an RF signal’s amplitude.

Question 41

What is wave propagation in RF?

Answer 41

The way RF waves move through an environment is known as wave propagation.

Question 42

What is weakening of RF signal due to broadening of the wave front called?

Answer 42

This phenomenon of RF signal dispersion is called Free Space Path Loss denoted by FSPL or FSL.

Question 43

What does the 6-dB rule in RF?

Answer 43

The 6-dB rule states that for every doubling of distance there is an amplitude loss of approximately 6-dB. Inversely, an increase of 6-dB in signal doubles the distance at which the signal is usable. A decrease in 6-dB halves the distance.

Question 44

What is Multipath in RF?

Answer 44

When signals bounce around in an environment through reflection, refraction, diffraction and scattering, they create an effect known as multipath. Multipath occurs when multiple paths of the signal, understood as multiple signals, arrive at the receiving antenna at the same time or within small fraction of a second of each other.

Question 45

What is delay spread?

Answer 45

The difference in time between the arriving of the first and second signal at the receiver in a multipath occurrence is known as the delay spread.