C HANNELS OF C OMMUNICATION Fig 1.

C HANNELS OF C OMMUNICATION Copper wires Wire pairs Coaxial cables Optic fibres Radio waves Microwaves Satellites

C OPPER W IRES Fig 2.

C OPPER W IRES Cheap Used in the first telephone networks (1876) and still used today. Copper wires transmit electrical current. Current is not constant so produces variations in magnetic fields. Variations in magnetic field produce cross – talk. Cross – talk generates noise and interference. Copper wires need to be spaced apart to reduce effects. Low bandwidth (20kHz). Frequent need for amplification (every 10km).

W IRE PAIRS Fig 3.

W IRE PAIRS Twisted wires carry currents in opposite directions. Opposite currents reduces magnetic field interference. Twisted wires reduce flux linkage. Minimizing area minimizes unwanted signals created by electromagnetic induction. Problems with attenuation. Amplification needed every 5km. Low bandwidth (500kHz). Distorts radio waves of different frequencies and speeds due to dispersion.

C OAXIAL CABLE Fig 4.

C OAXIAL C ABLE Coaxial refers to the common axis of the two conductors. Both conductors are parallel. Most common cable for transmitting TV and video signals. Grounded shield protects core. Data is sent through central copper core. Electric and magnetic fields are confined within the dielectric. Interference from outside noise is reduced by outer shield, grounded shield and dielectric. Good at carrying weak signals. High bandwidth (500MHz) Amplification (MHz = 10km, GHz = 100m). Buried underground which is expensive.

O PTIC FIBRES Fig 4.

O PTIC FIBRES Replacing Coaxial cable High frequency signals (approaching THz) High bandwidth (10GHz) Low attenuation Amplification every 80km Perfect regeneration (Schmitt trigger)

R ADIO SIGNALS Fig 5 Fig 6

S URFACE RADIO WAVES Frequencies of 3MHz, wavelengths ≤ 100m Diffracted by Earth’s surface, therefore following the curvature of the Earth. AM radio transmissions can travel distances of 100’s km. Powerful transmitters at low frequencies of 3kHz can travel 1000’s km. Fig 7.

S KY RADIO WAVES Radio waves of 3MHz up to 30MHz. Radio waves suffer total internal reflection. Wave travels a certain distance from transmitter called ‘skip distance’. ‘Skip distance’ is unreliable due to changes in ionosphere. Severe problems with attenuation. Huge interference due to ions ionosphere. Fig 8

S PACE RADIO WAVES Radio waves of frequencies above 30MHz. Waves travel in straight lines and are not effected by ionosphere (λ = 10m). Used for Earth bound satellite transmissions, FM transmissions and GPS. Fig 9.

M ICROWAVES High frequency waves (GHz) Large bandwidth (100MHz) Multiplexing possible due to large bandwidth. Travel in straight lines, not effected by ionosphere. Reduced attenuation. Fig 10

C OMPARISONS BETWEEN CHANNELS OF COMMUNICATION Channel Carrier Frequency Bandwidth Average distance between amplifers Specific attenuation dB /km Copper wire 20kHz20Hz10km10 Wire pairs 10MHz500Hz5km25 Coaxial cable 2MHz (phone) 1GHz (TV) 500MHz 10km 100m Microwaves5GHz100MHz50km Distance – dependent Optic fibres 0.2THz10GHz80km0.20

S ATELLITES Fig 11.

G EOSTATIONARY S ATELLITES Equatorial orbit approximately 42000km above the Earth’s centre. Expensive to put into space (1963). However ideal for communication. Communication signals need to be in the range of GHz. Large bandwidth means multiplexing is possible. Limited power in satellite means that down-link signal transmission must require low power signals. Up-link signals need to be powerful and have higher frequencies than down-link signals.

G EOSTATIONARY S ATELLITES 13 equatorial countries, 7 have equatorial space. Who owns the space? 1 geostationary satellite can cover 42% of the entire surface of the Earth. 3 geostationary satellites can cover the entire surface, not taking into consideration the polar caps.

P OLAR SATELLITES Orbits poles a few hundred km’s above Earth surface. Can receive, store and retransmit data at a later time. Cheaper to put into orbit and requires less power to up-link signals. GPS Fig 14.

S ATELLITES Communication to rural areas. Environmental concerns. No more cables but increasing space junk. International understanding. No international boundaries leading to international understanding. However there is always extremism Colonizing space.

H IGH BANDWIDTH COMMUNICATION Good points Multiple communications Sharing of information Business Bad points Copyright infringement Extreme views Plagiarism Inappropriate material Spam

P HOTO URL ’ S Fig nm.jpeghttp://gb.fotolibra.com/images/previews/ telegraph-poles-route-66-near-bluewater- nm.jpeg Fig Fig Pair-Conductors.jpghttp://image.made-in-china.com/4f0j00kBYQraIyVWbt/Station-Wire-With-One-Twisted- Pair-Conductors.jpg Fig Fig Radio_masts_with_radio_waves-SPL.jpghttp:// Radio_masts_with_radio_waves-SPL.jpg Fig Fig Fig Fig Fig Fig Fig Fig Fig

S OURCES OF REFERENCE Hamper, C. (2009). Higher Level Physics developed specifically for the IB Diploma. Essex: Pearson Education Limited. Tsokos, K.A. (2008). Physics for IB diploma, fifth addition. Cambridge: Cambridge University Press.