Receiving antenna choice¶

In this part of our project, we will build a receiving antenna so as to collect information from weather satellites. We must choose our antenna according to the signal characteristics such that:
• The type of radio-frequency signal polarization to receive: it defines the orientation of the radio waves in space and the receiving antenna polarization have to fit the one at the emitter in order to maximize the signal strength.
• The gain provided by the antenna must be sufficient in the satellite direction to be able to extract the signal: the antenna should increase the signal strength to improve the image quality.

Moreover, as the frequency of APT signals is between 137MHz and 138Mhz, we must consider the Faraday rotation. This phenomenon is effective for frequencies lower than 5Ghz in the ionosphere. It transforms a linear polarization to a circular one. Consequently, the emitter on the NOAA satellite uses a Right-Hand Circular Polarization (RHCP) to avoid this depolarization. It means our receiver must also use the same polarization.

Following this property, we can consider two main antenna types. Either we can use a directional antenna or an omnidirectional one.

Directional antennas¶

Considering directional ones, Yagi antenna should fulfil the polarization property for instance. The principal benefit is that its radiation pattern is more focused in one direction, so that it can provides a larger gain than omnidirectional antennas, taking into account we use a tracking system. Here is the radiation pattern of this antenna:

Even if we could have better performances due to a larger gain, the use of such a system is more complex and costly. Moreover, some previous studies on APT receivers have shown that we can achieve sufficient results with omnidirectional antennas which are less complex and less costly to build. Consequently, we will focus on these systems.

Omnidirectional antennas¶