Antenna Wave Tilt page 9

2.24) Receive Results 120°

Figure 30

3.25) Receive Results 105°

Figure 31

4.0) Establishing Radiation Paths and Sources

  The analysis of the large amount of data collected starts with the establishment of the individual paths and sources of the separate radiations that arrive at the Rx Antenna at different times. This involves the careful selection of peaks in the received magnitude vs time graphs and constantly cross referencing them to establish probable paths and sources, an operation which is best performed in a Spreadsheet. The paths that are received at the Rx Antenna base are the easiest to establish as they are of near constant length. The paths received at the Rx Antenna tip are more difficult as the radiation tends to take shorter paths if they are available and this makes it necessary to calculate multiple possible paths and judiciously select the relevant path. Due to the fact that the longer the paths become the more difficult they are to establish, the analysis shown is restricted to the first six individual radiation paths. It is a process that sometimes requires some subjective judgment and is unlikely to end with highly accurate results but judicious selection of less obvious peaks will ensure that the individual radiation paths are generally correct and the overall path mapping revealed.

4.1) Radiation Path 1    Tx base to Rx base

  First the data points for the relevant measured peaks are used to construct a Transmission Delay vs Rx Antenna Angle graph which also displays the calculated paths values for comparison as shown below in figure 32. The shortest radiation was the first path and source to become obvious at an early stage of analysis because it is relatively free from interference from other radiations. The route taken is fundamentally from the base of the Tx Antenna to the base of the Rx Antenna.

  Note: The physical measurement of the two metre distance between the antennas suspended more than two metres above the ground was not easy to perform and it was planned to normalise this distance for calculation purposes at the time of data analysis, when it was taken as the shortest delay for any Rx Antenna position. This proved to be with the Rx Antenna at 0° and a delay of 6.58 ns, giving a maximum physical distance between antennas of 1974 mm, because the radiation travels at the speed of light. The difference in mechanical length and electrical length is most likely due to the removal of the 30dB inline attenuator after performing the through calibration.

  In order to establish whether the radiation at the Rx Antenna passes along the surface or is absorbed by the antenna at the tip and passed as an electrical current along the body, the Velocity Factor of a simple wire antenna found in an earlier experiment (0.82) can be used to determine the radiation path.

Calculation with the Rx Antenna conducting received signal

Delay from Tx base to Rx tip = (1974 - 500) / 300 = 1474 / 300 = 4.913 ns

Delay along Rx antenna as a conductor with velocity factor 0.82 = 500 / 8.2 / 300 = 2.033 ns

Total delay = 4.913 + 2.035 = 6.946 ns

Calculation with received signal radiating along the surface of the Rx Antenna

Delay from Tx base to Rx tip = (1974 - 500) / 300 = 1474 / 300 = 4.913 ns

Delay radiating along the Rx antenna surface = 500 / 300 = 1.666 ns

Total delay = 4.913 + 1.666 = 6.58 ns

Note that with the physically measured distance of 2 m these delays would become 7.033 ns and 6.667 ns respectively.

As the result of 6.58 ns obtained for the Rx Antenna at 0° is less than either of the delays calculated for a 2 m gap, it was taken that:-

1) The distance between the antennas is 1974 mm

2) The received radiation passes along the surface of the Rx Antenna at the speed of light.

  The path taken from the Tx base to the Rx tip was found to be true at Rx Antenna positions of 0°, 15°, 30°, 330° and 345° where the Rx tip is closer to the Tx base. It is interesting that the Rx tip tends to attract the radiation but does not produce an overall shorter path to the Rx base. It must also be considered that the Rx Antenna is within the Tx Antenna reactive near field and can therefore modify the Tx Antenna performance.

  Another deviation of path was found between 195° and 270° where the path deviates to a point approximately 87 mm from the base of the Rx antenna. This occurs when the Rx Antenna is in the III (third) quadrant and therefore pointing away from the Tx Antenna base.

  It can be seen from this analysis of the simplest path and source, that the interpretation of the data is complex and has to be approached in a logical manner in order to establish the basic rules that the radiation follows. These rules are then applied to the more complex and longer radiation paths.

Figure 32

  Once the delays are established, the related received signal magnitudes can be extracted from the original data files and entered into a graph as shown in figure 33. It can be seen from this graph that there is a deep null at 0° and a less well defined one at 180°, where the Rx Antenna faces away from the Tx antenna. This demonstrates that the polarisation, which is at right angles (90°) to the null position, is at 90° to the horizontal and the same orientation as the Tx Antenna ie 0° wave tilt. It should be noted that the measurements are performed with 15° steps and this is fundamentally the measurement resolution which gives a result of 0° ± 7.5°. The radiation polarisation is of course upwards because this is the first signal that originates from the Tx Antenna base and therefore has to be traveling in an upward direction as it leaves the Tx Antenna base.

Figure 33

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