SOURCEFORGE » RALF » 2016 NOAA: receiving weather images

h1. Receiving antenna design

As we have seen in the previous section, the double cross antenna is composed of two crossed pairs of equidistant dipoles spaced a quarter wavelength and offset 30° to the azimuth. Each dipole has a length of half a wavelength. You can see the resulting general antenna appearance below.

p=. 

!{width:30%}DoubleCrossAntenna2.PNG!

h2. Dimensions

To specify the dimensions, we need to calculate the received wavelength. We calculated it for NOAA 19 (137.1MHz) and NOAA 18 (137.915MHz) to see if there is a noticeable difference on the needed dimension:

p=. 

!{width:30%}Wavelength.PNG!

Where:

•	λ is the wavelength (m)

•	c is the speed of the light (m/s)

•	f is the frequency of the received signal (Hz)

As the difference is very small, the antenna can be used for any NOAA satellite.

We can deduce the dipole length and the distance between the dipoles which is respectively half a wavelength and a quarter wavelength: 

p=. 

!{width:30%}Length.PNG!

In compliance with these results we can model our antenna:  

p=. 

!{width:70%}AntennaDesign.PNG!

h2. Polarization