Microwave Transmission
Microwaves can propagate through a guided medium, such as a transmission line, which could be cable or waveguide. They can also propagate through an unguided medium as plane waves in free space and through the atmosphere.
In all networks, selecting a physical medium is generally a matter of budget, capacity needs, availability, reliability and how quickly the solution can be deployed. Common options include twisted-pair copper cable, coaxial cable and fiber-optic cable.
In some instances, however, conflicting requirements defeat all these options for instance, capacity requirements may demand a fiber-optic back-haul link, but the budget may not allow for the time and cost needed to install it.
MW transmission holds a unique position as a solution where cost, capacity, flexibility and timing all intersect.
Wave Guide
Microwave energy travels through guided media in different modes. A microwave waveguide with a single conductor is a high pass filter; these structures have a cutoff frequency. Single-conductor options include:
• Rectangular waveguide
• Circular waveguide
• Elliptical waveguide
• Ridged waveguide
• Corrugated waveguide
Attenuation in waveguide can be caused by dielectric loss, if the wave guide is full of dielectric, or by conductor loss due to the metal structure’s finite conductivity. The various modes of operation available depend on the desired frequency, as well as the size and shape of the waveguide itself.
The maximum attenuation values measured in decibels per meter (dB/m)—are published by the International Eletro-technical Commission based in Switzerland (IEC). Waveguide attenuation is published by manufacturer.
For example, an EWP52 elliptical wave guide (attenuation: 3.93 dB/100 m (1.2 dB/100 ft) @ 6.175 GHz will attenuate 2.4 dB over a 61 m (200 ft) length. Microwave waveguides are maintained under dry air or dry nitrogen pressure to avoid moisture condensation that would impede their performance.
Going To The Air
Microwaves display some interesting propagation characteristics that make them ideal for radio transmission. Point-to-point radio links are often the most cost-effective method of transporting large volumes of data in a location without existing copper or fiber-optic infrastructure.
Low in cost and easily installed, network operators don’t have to rely on third party vendors to deploy expensive cables. Highly directional antennas such as parabolic dishes facilitate point-to-point radio links. Lower frequencies (≤11 GHz) can propagate over long distances with larger long-haul antennas, enabling connections to the core network from remote locations.
Higher frequencies above 11 GHz propagate over lesser distances via smaller short-haul antennas, providing connectivity better suited to urban environments where fiber-optic “point of presence” is closer and more accessible.
In all cases, a microwave transmission’s highly-focused line of sight (LOS) beam path allows the reuse of the same frequencies system wide without concern that adjacent links will interfere with each other.
