Antenna Current Distribution Page 6

4.4) Comparing phases of measured difference current with an ideal vector difference current

  The resulting difference current curve can now be compared with an ideal generated vector difference current curve as in figure 18 and it can be seen that there is a phase difference between the two. This is partially due to the electrical length of the Toroidal Ferrite Probe as illustrated in figure 19 below.

Figure 18 Current Phase Comparison

[Phase comparison graph of shorter antenna]


  The horizontal line in figure 19 represents the length of the antenna and the point D the current detection point of the probe. The probe is positioned at point B on the antenna in order to measure current at point B. However, it can be seen that when the INCIDENT current travels from left to right it is sensed at the point B and arrives at the detector (point D) after travelling the length of the probe (B to D). When the sensed signal arrives at the probe (point D) the main signal has arrived at point C along the length of the antenna. So, the signal detected at point D has the phase of point C with the current of point B (ignoring probe losses), thus causing phase error. The REFLECTED signal has a similar problem and when the REFLECTED arrives at the detector (point D) it has the phase of point A with the current of point B. There is therefore, a total phase error introduced of twice the phase shift along the probe (B to D) .

Figure 19 Phase Comparison

[Diagram of phase comparison]

  The fact that the phase shift in the probe is causing phase error can be proven in a simple way. An antenna of twice the length of the original is constructed and the measurements repeated. The frequency is now halved in order to obtain the antenna centre frequency. This means that the phase shift along the probe is halved because its electrical length remains constant. The phase shift error in measurement should also be halved (this was in fact more than halved because some other minor errors e.g. mechanical positioning of the probe and radiation have also been reduced) and produce an improved curve match. The result of the longer antenna can be seen in figure 20 below and it can be seen that the phase shift error is greatly reduced and there is little point in making further measurements.

Figure 20

[Phase comparison graph of longer antenna]


4.5) Calculation of INCIDENT and REFLECTED Currents

  Now that a curve similar to a sinusoidal wave as stated in the text books has been obtained, it is possible to analyse this data still further by separating the difference current into the INCIDENT and REFLECTED components.

Figure 21 Longer Antenna

[Graph of incident and reflected currents]


  It can be seen from figure 21 that a linear change of current produces a vector difference curve almost identical to the measured curve and from this it can be assumed that if the INCIDENT and REFLECTED current drops are not exactly linear, they are almost so.

  It is now possible to use the above observations to provide some simple formulas. As the current drops in a linear manner along the length of the antenna in both directions, we can now state that the current at any point in a well matched quarter wave antenna is:-


[Vector difference current formula]

Where Iin is the input current

θ is the distance from the end of the antenna in degrees

This formula can be simplified:-  Since Sine - θ = - Sine θ

[Vector difference current formula sine derivation]


  So, after the measurements and the calculations it is proven that the vector difference current curve is in fact sinusoidal but is made from two separate linearly decreasing currents. This fact, that there are two currents, will be used as the basis in this series of articles to gain a full understanding of the real mechanisms involved in antenna radiation.



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