Network Ninja

To understand Wireless Local Area Networks (WLAN) some understanding of WLAN antenna is important (this is some more for dummies type stuff i.e. with no electric/electronic engineering background a.k.a ME). This is important because to setup a WLAN it isn’t just install, config and done, there is a fair bit of optimizing at times and when you don’t understand how antennas work this gets difficult.

There are several terms that you need to understand:

Directionality: In short means the coverage around the antenna.

• A directional Antenna focuses the signal from the Access Point (AP) into a smaller coverage area, resulting in a stronger signal in this direction.
• A Omni-directional Antenna transmits and receives signals in all horizontal directions equally.

Gain: The amount of increase in energy that an antenna appears to add to a Radio Frequency (RF) signal. Cisco Aironet Wireless is standardizing on dBi (gain using a theoretical isotropic antenna as a reference point).

Polarization: The physical orientation of the element on the antenna that actually emits the RF energy. All Cisco Aironet antennas are set for vertical polarization. A vertical (upright) dipole antenna is vertically polarized (as opposed to horizontal).

Multipath Distortion

Multipath interference occurs when an RF signal has more than one path between the transmitter and a receiver. Looking back at this post, you will find an explanation on how radio waves bounce off and are absorbed.

Multipath distortion is where high signal strength yet low signal quality causes data to be unreadable. When an antenna transmits, it radiates RF energy in more than one definite direction. The end result is a desired (most direct) path and one or more alternate paths that could include reflected paths off other surfaces.

At the receiving end the numerous RF waves are combined and may cause data to be unreadable. There is also a possibility that the RF waves can cancel each other out causing radio null or dead spot.

Changing the location of the antenna can change the reflections and diminish the change of multipath interference.

Defining Decibel

Antennas and RF Power are measurement units based in decibels. A decibel (dB) is the ratio between two signal levels (named after Alexander Graham Bell).

dB Milliwatt (dBm): A signal strenght or power level. 0 dBm is defined as 1mW (milliwatt) of power into a terminating load such as an antenna or power meter. Small signals are negative numbers (-78 dBm).

dB Isotropic (dBi): The gain a given antenna has over a theoretical isotropic (point source) antenna. Unfortunately, an isotropic antenna cannot be made in the real world, but it can be useful for calculating theoretical fade and systems operating margins.

Effective Isotropic Radiated Power

Effective Isotropic Radiated Power (EIRP) is defined as the effective power in front of the antenna. The EIRP of a transmitter is the power that the transmitter would appear to have if the transmitter were an isotropic radiator (the antenna radiated equally in all directions).

EIRP = Transmitted power + antenna gain – cable loss

Cable loss is where RF energy carried between the antenna and the radio equipment introduces signal loss. Signal loss is directly proportional to the length of cable used. As the diameter of cable increases, signal loss decreases (cable more expensive). As singal frequency increases, singal loss increases.

To reduce the singal loss, minimize the cable lenght and use low-loss or ultralow-loss antenna cable to connect equiptment (Cisco offers LMR400 20 and 40 feet total loss 1.3 and 3.4 dB and LMR600 100 and 150 feet 4.4 and 6.6 dB).

Notes and Notices:

This is a part of my personal BCMSN notes and research to assist myself in learning and understanding the concepts and theory for the BCMSN exam. I learn by making notes reading and writing things down and wish to file them where I can’t lose them. These notes are not to be seen, judged or mistaken for replacements to Cisco recognized and authorized training which I personally support and attend and suggest you undertake if you are going for the BCMSN Certification.