Antenna and Communication Systems: A Technical Overview
Antenna Basics
Antenna: Provides directivity, gain, and concentrates electromagnetic energy towards the receiver. A single element is not used.
Radiation Pattern: Measured in the far-field region where the spatial distribution of radiated power does not depend on distance.
Free Space Gain and Loss: Gf = 1/Lf = (λ / 4 π r) ^ 2
Friis Transmission Equation: Expressed in dB: Pt + Gt + Gr – Lf
Power Calculations:
- P [dBW] = 10log (P [W])
- P [dBm] = 10log (P [mW] / 1mW)
Fractional Bandwidth (FBW): FBW = (fmax – fmin) / f0 * 100%
Quality Factor (Q): Q = f0 / (fmax – fmin)
Multiband and Wideband Antennas:
- Multiband: Well-matched at several unconnected bands.
- Wideband: Well-matched across a wide bandwidth (BW).
Dipoles:
- λ / 4 D = 1.64
- 3λ / 8 D = 1.94
- 5λ / 8 D = 3.33
- λ / 2 D = 2.41
- 3λ / 4 D = 2.17
- λ D = 2.52
Communication Systems
CPDLC and ADS
CPDLC (Controller-Pilot Data Link Communications): Improves the speed and comprehension of messages between controllers and pilots. Does not require transmitting ATC technologies: VHF, HF.
ADS (Automatic Dependent Surveillance): Surveillance and control system that depends on position information recovered automatically.
Coverage and Attenuation
Coverage depends on: Transmit power, receiver sensitivity, antenna types, location, topography, and attenuation.
Rain Attenuation: Affects terrestrial and satellite radio links, affecting frequencies above 6 GHz. Attenuation increases with frequency until the drop is similar to λ.
Attenuation Calculation: L (dB) = α (dB / Km) * D (Km)
Equivalent Noise Power: Pn = FKToB
Tropospheric and Ionospheric Communication
Tropospheric Dispersion:
- Application: Used to save large distances.
- Disadvantages: Requires high transmit power; Deep fading due to multipath propagation; Temporal instability.
Ionospheric Communications (HF): The ionosphere has a great effect on the scope since it depends on the reflection and the sky-wave dispersion. Changes in the ionosphere influence the propagation of MS waves.
TDMA and CDMA
TDMA (Time Division Multiple Access): Requires time synchronization, only for digital modulations, based on transmit bursts.
CDMA (Code Division Multiple Access): The code produces a widening of the band, or code synchronization between transmit and receive. Power control is necessary. CDMA was created for military use.
Intermodulation and Frequency Bands
Intermodulation Block: 88 – 108 KHz is for broadcasting FM.
AMSS (Aircraft Meteorological Service System): Geostationary, one center satellite ground.
VHF / HF: Voice in terminal areas (separation 25KHz, Europe 8.33 kHz), HF remote areas.
Ionospheric Communications (HF): Carrier 2 to 22 MHz, SSB modulation, to cover great distances is not transmitted continuously, Propagation is unreliable; Great day / night: just system provides direct link – ground station.
HFDL and VHF Systems
HFDL
HFDL (HF Data Link): Completing the AMSS. Each ground station provides 3 or 4 HF frequencies. The system can measure the quality of the channel and choose the appropriate frequency. Data rate: 300-1800 bps, vertical polarization, 8.33 kHz: Cannot shrink from 25KHz without changing technology. Prox. replace step VDL.
VHF Systems
VHF / VDL (VHF Data Link):
- VDL2: Frequency band 118-136.9 MHz / 25MHz; D8PSK Modulation; Data rate: 31.5 Kbps, offers A / T point to point, limitation makes the system does not meet the requirements in areas of high density.
- VDL3: 118-136.9 KHz; Modulation D8PSK; Data rate: 31.4 kbps; Point to Point and Point to Multipoint.
- VDL4: 108-136.9 MHz GFSK Modulation, data rate 19.2 KS / s, Enables A / T point-to-point and multipoint and A / C point to point, Meets ATC surveillance is not restricted band, and is used to transmit ADS-B position message VDL4.
GPS and Antenna Polarization
Knowing that GPS radiates signals using RHCP (Right-Hand Circular Polarization), which of the following is false regarding the received power?
- a) If the antenna has linear vertical or horizontal polarization, there is a 3dB loss.
- b) If the antenna has linear horizontal polarization, there is a 3dB loss.
- c) If you own a RHCP antenna, you will receive the maximum power.
- d) If the antenna has no right elliptical polarization, there is a maximum power.
Communication and Refraction
Communication T / A: Reflections and suffer day and night F2. Rain: f > 6GHz.
If a sector is to cover, it is necessary to use various control centers frequencies. Displaced: The aircraft transmits it at the same frequency, and the control center selects the signal with the higher quality site.
AMSS: Geostationary (36000 km).
T / A on VHF: 25 to 8.33 improves performance, but the system will extinguish with VDL.
Atmospheric refraction index: Will reach much higher, can occur at very high levels of interference with small CIR; Tx signal prop softens to form a distance.
Satellite Communication
Link Budget and Rain Effects
Link calculation satellite network. in the number of users due to rain. Limitation on benefits? Downlink.
Satellite Communication: The uplink rain causes a reduction in (C / N) u = at. rain lent them. usually come from the limited transport connections (C / N) d link descend; The downlink is primarily limited by satellite transmit power.
Satellite: The location of the ground station coverage area does not result in big changes; The same diameter earth (satellite) will have a smaller beamwidth antenna for uplink.