12.9.9 Why Are Satellite Antennas So Different from Mobile Phone Antennas?
- Why Can't Every Communication System Use the Same Antenna?
- Why Do Satellite Antennas Need High Gain?
- Why Are Mobile Phone Antennas Almost Omnidirectional?
- Why Are Satellite Dishes So Large?
- Why Don't Mobile Phones Use Reflector Dishes?
- Why Are Radar Antennas Different Again?
- Why Are Wi-Fi Antennas Different?
- Why Is Polarization Important?
- Why Must Satellite Antennas Be Pointed So Accurately?
- Why Are Flat-Panel Satellite Antennas Becoming Popular?
- What Can We Learn from Comparing Different Antennas?
- What Should You Remember?
Short Answer
Satellite antennas and mobile phone antennas are designed for completely different communication environments. A satellite antenna usually communicates with a single, precisely known target located thousands of kilometres away and therefore requires high gain and a narrow beam. A mobile phone, on the other hand, must communicate with many nearby base stations while continually changing location and orientation. It therefore uses compact, low-gain antennas that provide broad coverage rather than highly directional beams. The differences reflect the requirements of the communication system rather than the antennas themselves.
Why Can't Every Communication System Use the Same Antenna?
Every wireless communication system has different design objectives.
Some require:
- maximum communication range;
- wide-area coverage;
- high data rates;
- compact equipment;
- low cost;
- high mobility; or
- rapid installation.
No single antenna can optimise all of these requirements simultaneously.
Consequently, engineers design antennas specifically for the intended application.
Why Do Satellite Antennas Need High Gain?
Communication satellites are located enormous distances from Earth.
A geostationary satellite, for example, orbits approximately 35,786 km above the Earth's surface. Even low-Earth-orbit satellites are typically several hundred kilometres away.
Radio signals therefore experience substantial free-space path loss before reaching the satellite. To compensate, satellite antennas employ high gain by concentrating their energy into extremely narrow beams. This allows:
- stronger received signals;
- lower transmitter power;
- greater communication range;
- improved resistance to interference; and
- efficient frequency reuse.
Why Are Mobile Phone Antennas Almost Omnidirectional?
A mobile phone rarely knows exactly where the nearest base station is located.
Furthermore, the user continually changes position and orientation. The antenna must therefore communicate effectively regardless of how the phone is held. For this reason, mobile-phone antennas are designed to provide relatively broad coverage rather than maximum gain.
Although modern smartphones employ beamforming internally for some frequency bands, they still require wide angular coverage compared with satellite terminals.
Why Are Satellite Dishes So Large?
Satellite communication requires very high antenna gain.
One way of achieving this is to increase the antenna's effective aperture.
Parabolic reflector antennas collect incoming radio waves over a large surface and focus them onto a feed antenna.
Larger reflectors produce:
- higher gain;
- narrower beamwidth;
- greater receiver sensitivity; and
- improved communication reliability.
This is why professional satellite earth stations often employ reflector antennas several metres in diameter.
Why Don't Mobile Phones Use Reflector Dishes?
A reflector dish would be completely impractical for a handheld device.
Instead, smartphones require antennas that are:
- compact;
- lightweight;
- inexpensive;
- capable of operating on several frequency bands; and
- effective regardless of the phone's orientation.
These requirements have led to sophisticated internal antennas that are carefully integrated into the phone's structure.
Although they provide much lower gain than a satellite dish, they are ideally suited to short-range mobile communication.
Why Are Radar Antennas Different Again?
Radar systems have yet another set of requirements.
Instead of communicating with another transmitter, radar must detect extremely weak echoes reflected from distant objects.
Radar antennas therefore require:
- very high gain;
- exceptionally narrow beams;
- accurate pointing; and
- rapid beam steering.
Many modern radar systems achieve these objectives using electronically scanned phased arrays rather than mechanically rotated reflector antennas.
Why Are Wi-Fi Antennas Different?
Wi-Fi systems usually operate over comparatively short distances.
Their objectives are quite different from those of satellite systems. Wi-Fi antennas are designed to provide:
- good indoor coverage;
- moderate gain;
- compact size;
- support for MIMO operation; and
- compatibility with portable devices.
Many Wi-Fi routers contain several antennas, not because greater range is required, but because multiple antennas improve throughput and communication reliability.
Why Is Polarization Important?
Different communication systems often employ different antenna polarizations.
Examples include:
- vertical polarization for many land-mobile systems;
- horizontal polarization for some broadcast services;
- circular polarization for many satellite navigation systems; and
- dual-polarization techniques for modern broadband communication.
Matching the transmitting and receiving polarizations maximises signal transfer while reducing unwanted interference.
Modern satellite and cellular systems frequently exploit dual polarization to increase communication capacity.
Why Must Satellite Antennas Be Pointed So Accurately?
The high gain of a satellite antenna is achieved by concentrating energy into an extremely narrow beam. The disadvantage is that even a small pointing error may significantly reduce received signal strength. Large earth stations may require pointing accuracies better than a fraction of a degree.
Modern tracking systems automatically compensate for satellite motion, wind loading, and mechanical tolerances to maintain optimum alignment.
Why Are Flat-Panel Satellite Antennas Becoming Popular?
Recent advances in phased-array technology have led to electronically steerable flat-panel satellite antennas.
Unlike conventional reflector dishes, these antennas:
- contain no moving parts;
- can track moving satellites electronically;
- are thinner and lighter;
- are easier to install on moving vehicles; and
- require less maintenance.
Such antennas are becoming increasingly common on aircraft, ships, trains, and vehicles communicating with low-Earth-orbit satellite constellations.
What Can We Learn from Comparing Different Antennas?
The comparison between satellite antennas and mobile-phone antennas illustrates an important principle of antenna engineering.
The best antenna is not necessarily the one with:
- the highest gain;
- the narrowest beam;
- the largest size; or
- the most advanced technology.
The best antenna is the one that best satisfies the requirements of its communication system. Different applications require different compromises between:
- gain;
- beamwidth;
- bandwidth;
- size;
- efficiency;
- mobility;
- cost; and
- mechanical complexity.
Successful antenna design therefore begins with understanding the communication problem rather than selecting the antenna first.
What Should You Remember?
- Satellite antennas and mobile-phone antennas are optimised for very different communication environments.
- Satellite communication generally requires high-gain, highly directional antennas because of the enormous communication distances involved.
- Mobile phones require compact, wide-coverage antennas that operate effectively in many orientations.
- Radar, Wi-Fi, broadcast, and satellite systems all employ different antenna designs because they have different operational requirements.
- Polarization, beamwidth, gain, and aperture size are selected according to the application rather than according to a universal optimum.
- Modern flat-panel phased arrays are beginning to replace mechanically steered satellite dishes in many applications.
- There is no universally "best" antenna—only the antenna that best satisfies the requirements of a particular communication system.