4.6) Radiation Path 6
This is the last path and source to be analysed because it becomes very difficult and time consuming after this and the results become less reliable. The route taken is directly from the base of the Tx Antenna to the tip and back down to the centre and then radiated to the base of the Rx antenna. First the data points for the relevant peaks are used to construct a graph which also displays the calculated path for comparison as shown in figure 42.
With the path delay selected, the received signal magnitude is extracted from the original data files and entered into a graph as shown in figure 43. It can be seen from this graph that there is a null at 0° 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° tilt. The radiation is of course downwards because the measured time delay of 9.13 ns related to the antenna spacing of 1974 mm shows that this radiation originates from the reflected signal that has first passed in an upward direction within the Tx Antenna.
There are multiple radiations confirming the measurements made in the Near Field Radiation article, with the addition of the radiation from the centre of the Tx Antenna with the current in the INCIDENT direction. This is not detected in the Near Field Radiation article because the received magnitude for this source and path is only significant with the Rx Antenna at 15° to the horizontal and the Near Field Radiation measurements were made with the measurement plate vertical (90°).
The radiation from the base of the Tx Antenna has 0° ± 7.5° wave tilt (which is related to the orientation of the electrons in the base of the antenna as they emerge from the coaxial cable as will be revealed later in the Radiation Mechanism article).
The radiation from the centre of the Tx Antenna with the current in the INCIDENT direction was measured at 0° ± 7.5° wave tilt. Although a measurement with finer Rx Antenna orientation increments may reveal a result near to 7.5°.
The radiation from the tip of the Tx Antenna has 15° ± 7.5° wave tilt, which fits nicely with the Glasgow's statement of a radiation angle of 17.5° and the results obtained in Elevation of Antenna Radiation. Again, it would be interesting to use finer Rx Antenna angle increments to achieve a better resolution.
The radiation from the tip of the Tx Antenna has a greater magnitude than any other radiation.
The most efficient transmission is achieved in these tests with the Rx Antenna at between 15° and 45° to the horizontal, depending on which transmit radiation source is observed. This suggests that a different design of transmit antenna which radiates with a significantly lower polarisation angle may produce an improvement in overall gain when using a simple vertically mounted monopole receive antenna.
It was not possible to measure with this method over a much greater distance to confirm how the earth causes the polarisation to change due to effects from ground conditions as Terman stated because the VNA does not have the ability to process the long delays involved. Nor was there any attempt to measure at other frequencies because the antenna length was chosen for optimum resolution in the time domain within practical maximum physical dimensions. Measurements with relatively shorter antenna lengths may not have produced useful results because of the difficulty of extracting discrete paths from the large amount of data.
The monopole is an inefficient transmit antenna because it does not transmit at an optimum polarisation angle and has multiple radiations with different phases and polarisations that cause cancellations. Also, the time taken for the series of radiations to leave the antenna defines the minimum possible rise and fall times of digital transmissions and a singular radiation would be preferable.
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First Published by William J Highton on 20/9/2022
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