RF Signals Propogation / Antennas

Question 1

What type of interference is it when two or more transmitters are using the same channel?

Answer 1

Co-Channel Interference

Question 2

What type of interference is it when channels are spaced too closely and they overlap?

Answer 2

Neighboring Channel Interference

Question 3

In 2.4GHz, are channels 1 and 2 adjacent channels?

Answer 3

No, adjacent channel are non overlapping so channel 1 and 6 would be adjacent.

Question 4

What type of interference does a microwave cause?

Answer 4

Non-802.11 interference

Question 5

What is free space path loss and what causes it?

Answer 5

RF signal’s amplitude decreases as it travels thru space.

Energy is being spread over larger area as the wave expands.

Question 6

Why is the free space path loss greater in 5GHz than 2.4GHz?

Answer 6

Loss is a function of distance and frequency only. The higher the frequency, the greater the loss.

Question 7

802.11 devices adjust their modulatioon and coding schemes based on RSSI and SNR conditions. What is this scheme selection commonly known as?

Answer 7

DRS (dynamic rate shifting)

Question 8

What are the effect of physical objects on RF signals?

Answer 8

Absorption (weakens signal)
Reflection (bounces signal)
Scattering (disperses signal in many directions)
Refraction (changes signal direction)
Diffraction (bends signal around object)

Question 9

What percentage of Fresnel zone 1 should be free of obstacles on a point-to-point signal?

Answer 9

60%

Question 10

Referring to an antenna radiation pattern, what two names are used for the H Plane?

Answer 10

Horizontal and Azimuth

Question 11

Referring to an antenna radiation pattern, what name is used for the E Plane?

Answer 11

Elevation

Question 12

How do antennas add gain?

Answer 12

By shaping the RF energy as it is propagated into free space. (The higher the gain, the more effective the device at focusing the signal in a certain direction.)

Question 13

Beamwidth is a measurement of an antenna’s focus. How is it calculated?

Answer 13

Find the strongest point on the radiation plot, then the plot is followed in either direction until the value decreases by 3dB (1/2 the strongest power).

Question 14

What is antenna polarization?

Answer 14

The electrical field wave’s orientation with respect to the horizon.

Question 15

What polarization are Cisco antennas designed to use?

Answer 15

Vertical polarization.

Question 16

What are 3 types of omnidirectional antennas and what are their typical gain?

Answer 16

Dipole (2.2 dBi in 2.4GHz, 3.5 dBi in 5GHz)
Monopole (2.2 dBi in both 2.4 and 5GHz)
Integrated (2 dBi in 2.4GHz, 5d Bi in 5GHz)

Question 17

What are 3 types of directional antennas and what are their typical gain?

Answer 17

Patch (6-8 dBi in 2.4GHz, 7-10 dBi in 5GHz)
Yagi (10-14 dBi in 2.4GHz, Cisco doesn’t have for 5GHz)
Dish (20-30 dBi in both 2.4 and 5GHz)

Question 18

What accessory would you use if you needed to add power between the transmitter and the antenna?

Answer 18

Amplifier (might need due to cable loss being to great)

Question 19

What accessory would you use if you needed to reduce power between the transmitter and the antenna?

Answer 19

Attenuator

Question 20

What does a lightning arrestor do and can it prevent damage from a direct lightning strike?

Answer 20

It allows sudden spikes of electricity near an outside antenna to be bypassed to ground and no, it will not protect from a direct strike.