Antenna Wave Tilt page 10

4.2) Radiation Path 2    Tx centre to Rx base

  The second path and source, as shown in figure 34, was found to be approximately constant in value and was therefore considered to be mainly received at the base of the Rx Antenna. From a calculated average time delay of 7.47 ns, it can be shown that the radiation originated from somewhere near the centre of the Tx antenna.

For path length 7.47 ns * 300 mm/ns = 2241 mm, compare this with:-

For radiation from centre of Tx Antenna, height above ground plane 500 / 2 = 250 mm

To find the distance between the Tx centre and Rx base, find the hypotenuse of a right angle triangle with sides of 1974 mm and 250 mm

Hypotenuse = (1974² + 250²) ^0.5 = 1989.8 mm

Total path length of hypotenuse + height above ground plane = 1989.8 + 250 = 2240 mm

  The radiation appears to take a path from around the Tx centre to the Rx tip with the Rx antenna at 0°, 15°, 30°, 330° and 345°, where the Rx tip is closer to the Tx base. For the rest of the revolution the radiation is to the Rx Antenna body/base. It is not possible to establish exactly where the launch point from the Tx Antenna and the receive point on the Rx antenna are because there are two variables and not enough formulas are available to make the calculation (it is mathematically underdetermined). However it is possible to use multiple calculations to demonstrate that both the radiation source on the Tx antenna and the receive point on the Rx antenna move up and down the antenna bodies slightly. This can be observed in the delay graph where the delay is increased in quadrants I and II but decreased in quadrants III and IV. It must again be considered that the Rx Antenna is within the Tx Antenna near field and can therefore modify the Tx Antenna performance.

Figure 34

  With the Radiation Path 2 peaks selected, the received signal magnitude is again extracted from the original data files and entered into a graph as shown in figure 35, which demonstrates that this radiation is of a low amplitude and not a significant contributor to overall performance. It can be seen from this graph that there is a sharp null at 0° and a flatter null at 180° and this demonstrates that the polarisation, which is at right angles (90°) to the null, is at 90° to the horizontal and the same orientation as the Tx Antenna ie 0° ± 7.5° wave tilt. It is very likely that the wave tilt of this second radiation is between 0° and 15° but measurements with smaller increments were not made because it was considered that the results achieved demonstrated that there were independent paths with different polarisations and that this is adequate to establish the antenna radiation mechanism in a later article. From the average path length of 7.47 ns, it can be observed that the radiation is derived from an INCIDENT current and the point of radiation from the Tx Antenna dictates that the radiation polarisation is in an upward direction.

Figure 35

4.3) Radiation Path 3    Tx base to Rx tip

  The third path and source is more difficult to locate because it often interferes with other paths and also takes short cuts when the Rx Antenna is pointing away from the Tx Antenna. It can be seen in figure 36 that the radiation tracks to the Rx Antenna tip from 0° to 120° but then changes to a path directly to the Rx base. The final path shown is constructed using considerable subjective judgement but the result achieved demonstrates a deep and reliable null at 0°. The 0° null with the Rx Antenna pointing at the Tx Antenna base has shown to be the sharpest null on the proceeding analysed paths and may be considered as the preferred point to observe the received voltage magnitude null.

Figure 36

  Once the path delay is selected and the received signal magnitude extracted from the original data files and entered into a graph as shown in figure 37 is constructed. It can be seen from this graph that there are nulls at 0° and 180° and this demonstrates that the polarisation, which is at right angles (90°) to the null, is at 90° to the horizontal and the same orientation as the Tx Antenna ie 0° ± 7.5° wave tilt. The radiation is of course upwards because the minimum measured time delay of 6.58 ns related to the antenna spacing of 1974 mm shows that this is the second path that originates from the Tx Antenna base and therefore has to be in an upward direction as it exits the Tx Antenna base. This confirms that radiation polarisation from the Tx base achieved in Radiation Path 1 which also has 0° ± 7.5° wave tilt.

Figure 37

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