tailieunhanh - FREQUENCY / PHASE EFFECTS OF ANTENNAS

The radiation patterns of the antennas presented in the previous section are for antenna geometries most commonly used. | FREQUENCY PHASE EFFECTS OF ANTENNAS The radiation patterns of the antennas presented in the previous section are for antenna geometries most commonly used. The antenna should be viewed as a matching network that takes the power from a transmission line 50 ohm for example and matches it to the free space impedance of 377 ohms. The most critical parameter is the change of VSWR with frequency. The pattern usually does not vary much from acceptable to the start of unacceptable VSWRs 2 1 . For a given physical antenna geometric size the actual radiation pattern varies with frequency. The antenna pattern depicted in Figure 1 is for the dipole pictured in Section 3-3. The maximum gain is normalized to the outside of the polar plot and the major divisions correspond to 10 dB change. In this example the dipole length in wavelengths is varied but the same result can be obtained by changing frequency with a fixed dipole length. From the figure it can be seen that side lobes start to form at and the side lobe actually has more gain than the main beam at . Since the radiation pattern changes with frequency the gain also changes. BW BW .L 8 Figure 1. Frequency Effects Figure 2 depicts phase array effects which are yet another method for obtaining varied radiation patterns. In the figure parallel dipoles are viewed from the end. It can be seen that varying the phase of the two transmissions can cause the direction of the radiation pattern to change. This is the concept behind phased array antennas. Instead of having a system mechanically sweeping the direction of the antenna through space the phase of radiating components is varied electronically producing a moving pattern with no moving parts. It can also be seen that increasing the number of elements further increases the directivity of the array. In an array the pattern does vary considerably with frequency due to element spacing measured in wavelengths and the frequency sensitivity of the phase .