SOURCEFORGE » RALF » 2014 Reception of weather images

h1. 6. Receiving antenna

In this second part of the project, a receiving antenna will be built to collect the data information sent by the polar orbiting weather satellites. The two important characteristics  to take into account are the polarization and the frequency used by the transmitting antenna. As it has been mentioned previously, the POES satellites transmit a signal with right hand circular polarization, therefore our receiving antenna must be able to lock the data in RHCP. On +picture XX+, it can distinguish the two different components which form the emitted signal and should be received by the chosen antenna. Besides, the antenna size will depend of the receiving frequency. Therefore, the first step will be to choose a specific POES satellite. 

p=. !{width: 30%}Polarisation.png!

*Figure 6.1*: Right Hand Circular Polarization

In the previous section has been explained the different transmission modes used by the NOAA satellites. In this project will be used the APT format, which send the digital transmission from the satellite in lower flowing traffic than HRPT.  The APT mode sends the desired information in the frequencies from 137.1 MHz up to 137.9125 MHZ depending of the satellite. Currently, the available APT satellites with theirs frequencies are  NOAA 15 (137.62 MHZ), NOAA 18 (137.9125 MHz) and NOAA 19 (137.1 MHz). The chosen satellite for the project is the NOAA 19 with a frequency on 137.1 MHZ.

Therefore, for the reception of polar orbiting satellites will be desirable to have an antenna

which:

> * Exhibits a radiation pattern in the upper hemisphere. (The antenna is directed upwards and has omni-directional coverage).

> 

> * It is sensitive in all directions only to right-hand polarised EM waves.

> * It receives the information on 137.1 MHz.

To get the best choice, a study of three possible antennas have been realized. It compares the advantages and drawbacks of each one and lastly the antenna with greater good characteristics will be chosen and built. In this analysis has been used the SWOT analysis tool where are able to visualize the pros and cons of each one and get the best option.

h2. 6.1 Study of the three possible antennas

> h3. 6.1.1 TURNSTILE ANTENNA 

p=. !{width: 30%}Turnstile_antenna.png!

*Figure 6.1.1.1*: Turnstile antenna [3]

>> Turnstile antenna, also called Crossed dipoles, was created in 1935 by the American engineer George Brown. Turnstile is an omnidirectional antenna that can be used in horizontal, vertical and circular polarization; the latter polarization is used for satellite communications. [1] The antenna consists of the combination of two pairs of dipoles aligned and positioned at right angles to each other in the same horizontal plane. Each pair of dipoles is called a bay. [2] In addition, a current with the same magnitude and in phase quadrature feeds the antenna.

>>The phase quadrature can be done by feed-line techniques or by redefining the size of the dipoles. The most common feed-line technique consist in divide the signal using a two-way splitter and after, one of these two divided signals is progressive phase shifted 90°. On the other hand, changing the dimension of the dipoles and adding reactances in serial to them can modify the phase quadrature.

>>The antenna is built by combining two bays from ½ wavelength to each other and they are excited in phase to modify their radiation pattern. It is illustrated in the picture below.

p=. !{width: 30%}antenna_structure.png!

*Figure 6.1.1.2*: Turnstile antenna structure [2]

>>The vertical radiation pattern is modified because part of it is cancel by the vertical radiation generated by the other bay. It produces a decrease in the vertical radiation energy, but it increases the horizontal one while the vertical angles are also enhance. The Turnstile antenna radiation pattern is shown in the picture below. The image A shows the omnidirectional radiation pattern of a pair-bay. The image B presents the difference between a circular radiation pattern from a pair-bay and a four-bay. Finally, the image C illustrates the final radiation pattern of the Turnstile antenna.[2]

p=. !{width: 30%}radiation_pattern.png!

*Figure 6.1.1.3*: Radiation patterns of a Turnstile antenna [2]

>>There are two modes: Normal mode and axial mode. The normal mode radiates in horizontal polarization due to the position of the orthogonal pairs of dipoles that are in parallel between them and the floor. On the other hand, the axial mode uses circular polarization because the set of dipoles is perpendicular oriented to the receiver signal, in our case, it would be the signal transmitted by the NOAA satellite. Often, this mode is the selected one to satellite communication because by using circular polarization the polarization of the signal does not change with the movement of the satellite in its trajectory. [1]

> h3. 6.1.2 QUADRAFILAR HELIX  

p=. !{width: 30%}QFH.jpg!

*Figure 6.1.2.1*: Quadrifilar helix antenna

>> The second chosen antenna is the Quadrifilar Helix antenna which is considered with a better performance than turnstile. It has composed of two sets of loops which provides a better performance reducing noise and eliminating null spots. The two loops have different sizes and they are called large loop and small loop. The terminals of each loop are fed 180° out of phase, and the currents in the two loops are in phase quadrature (90° out of phase). [XX] {TODO} references

>> By selecting the appropriate configuration of the loops, a wide range of radiation pattern shapes is available, with excellent axial ratio appearing over a large volume of the pattern. Therefore, a good improvement w.r.t. previous antenna is the radiation pattern. Seeing the picture XX, it can be a better reception due to most of the energy received by the satellite on the horizon. Another advantage will be the possibility of use it in low heights. The drawback is the complexity and implementation with respect to the other two antennas. The SWOT diagram for this antenna can be seen in the following figure:

p=. !{width: 30%}RadPatter_QFH.jpg!

*Figure 6.1.2.2*: Radiation patterns of a quadrifilar helix antenna

> h3. 6.1.3 DOUBLE CROSS  

p=. !{width: 30%}DCA.jpg!

*Figure 6.1.3.1*: Double cross antenna

>> Lastly, the third option is the Double Cross antenna (DCA). It is composed on two pairs of crossed dipoles (see _figure 6.1.3.1_). It also improves a better radiation performance with respect the turnstile. 

>> The purpose of the four dipoles is to produce a radiation pattern with great RHCP reception at the zenith axis. In order to achieve this pattern, the properties of two dipoles crossed, spaced a quarter wave and fed in phase have been used. With this pair of dipoles acquire a radiation pattern such as the _figure 6.1.3.2_.   

p=. !{width: 30%}RadPatter0_DCA.png!

*Figure 6.1.3.2*: Radiation patterns of a pair of dipoles crossed, spaced a quarter wave and fed in phase

>> As can be seen, the X axis has a pattern null so it can be filled by adding a second pair of crossed dipoles but these last one should be fed ninety degrees later than the first pair. This phase shift can make adding quarter wave of coaxial cable more in one pair than the other. Once the two pair of dipoles are put correctly, the final radiation pattern can be seen in the _figure 3.1.3.3_ . 

>> Therefore the DCA provides a high gain both upper angles and for the horizon angles. That increases the performance of this antenna. Furthermore, it has nulls for loud angles. This characteristic is very important because the low frequencies, used by APT mode, are also used in terrestrial communications. 

{TODO}

p=. !{width: 30%}RadPatter_DCA.jpg!

*Figure 6.1.3.3*: Radiation patterns of a double cross antenna

REFERENCES:

[1] http://en.wikipedia.org/wiki/Turnstile_antenna#Satellite_and_Missile.2FRocket_Antenna {TODO}

[2] http://www.tpub.com/neets/book10/42o.htm

[3] http://www.astrosurf.com/luxorion/qsl-satellites-reception.htm

[4] http://noaaport.poes-weather.com/