pes of MW antenna towers 3.4.2 Microwave Tower There are a few different types of towers that are used for mounting microwave (MW) links (shown in Figure 3.38). For smaller antennas that are mounted on top of a building in a city access network, strong poles are used. These usually have a height of 5 m. Simple guyed metallic structures and self-supporting towers are used when a high number of antennas are being mounted (thereby increasing the total weight of the mounted antennas). Engineers should ensure that the number of antennas should not exceed the load bearing capacity of the tower. 3.4.3 Microwave Link Design The main purpose of the link design is to make sure that the microwave network gives high performance in all types of atmospheric conditions. A link design contains four major steps: Path calculations Antenna height calculations Frequency planning and interference calculations Performance calculations However, as the atmosphere is highly unpredictable, all possible loss inducing factors should be taken care of. This makes an understanding of the atmospheric behaviour necessary for microwave engineers. Refractivity and the k-Factor Earth's atmosphere plays a vital role in the propagation of microwaves. Due to the properties of the atmosphere, communication between two microwave links is possible beyond the optical horizon. In a standard atmosphere, pressure, temperature and water vapour contents change (decrease) with (in- creasing) height. This means that the atmosphere contains several minute layers, each having its own pressure, temperature and water vapour content (which of course keeps on changing). This means that a microwave signal passes through many different layers or media. Each of these layers or media has different refractivity, defined by the refractivity index. Microwave Link Planning Refractivity n of a particular media can be defined as the ratio of the velocity of a signal in free space to the velocity of a signal in that medium: velocity of a signal in free space (vacuum) v = velocit