2.3) Measurement Test Setup
The test set up in figure 7 is very similar to that used for the near field measurements but the time domain is used without the gating function so that all radiation peaks can be compared, the required peaks selected and the known radiation reflections simply ignored. Transmission (Thru) calibration is performed with a (30 dB) VNA attenuator selected in order to protect the VNA input circuits from overloading and possible damage. The reflected measurement does not require this because of the attenuation of the RF Bridge and the 10 dB attenuator at the VNA input. A slow sweep is selected to allow for the electrical delay of the coaxial cables plus the Tx Antenna to Rx Antenna transmission distance which in the case of the outside field measurement was approximately 60ns in total. A narrow RBW is also required in order to reduce external radio interference. High quality cables are essential and RG214 with N type connectors have proven to be successful for all inter-connections related to this measurement.
3.1) Reflected Power Results
The reflected power of the transmit antenna was measured first to ensure a satisfactory return loss at the matched frequency of 161.6 MHz as shown in figure 8. It was found this model antenna displayed different characteristics from all of the previous models partly due to the fact that it was twice the length of previous models and partly because the wider frequency sweep revealed a further matched frequency at 434.2 MHz, but as this is very approximately a λ 3/4 mode (which would have many phase cancellations and longer more complex radiation paths) this characteristic was ignored by selecting only the shorter radiation paths in the analysis.
3.2) Receive Results 90°
Starting with the 0.5 m length Rx antenna upright (at 90° to the horizontal) and placed approximately 2 m distant from the 0.5 m length Tx antenna, it can be seen in figure 9 that there are eight discernible peaks in this particular Rx Antenna orientation. These peaks can be located in the result file by a simple mathematical function and then manually entered into a spreadsheet for further analysis. The four peaks of interest for the 90° position lay in range from 6.69 ns to 9.91 ns and these peaks will be used in the construction of each individual radiation path and source.
The first peak at 6.69 ns has to be the radiation from the the Tx base to the Rx base because it the first peak and extremely close to the distance between antennas. Note that it takes 6.58 ns to travel 1974 m at the speed of light from the Tx base to the Rx base, with the Rx Antenna in the 0° position where the Rx tip and base are aligned to the Tx base.
m/c = 1.974 / (3 x 108) = 6.58 ns
The second peak at 7.63 ns is the radiation from the approximate centre of the Tx Antenna to the bottom of the Rx antennas. The path taken was verified by placing temporary grounded screens between the antennas.
The third peak, which is at 8.51 ns, is a sum of the radiations from the Tx Antenna tip to the Rx Antenna base and from the Tx Antenna base to the Rx Antenna tip as these two paths lengths are identical when the Rx Antenna is at 90°.
The fourth peak at 9.9 ns, which is the last practical discernible peak, is the radiation from Tx Antenna tip to the Rx Antenna tip.
The following peaks are very difficult to analyse and have been ignored in this article because it is not possible to be fully confident that the correct peaks are being referenced. The shortest reflection possible from the ground with the dimensions used is 14.6 ns and most peaks from this are ignored by only sweeping to 15 ns. There are no objects other than cables that can cause reflections that are closer than the ground.
3.3) Receive Results 75°
With the receive antenna now at 75°, it can be seen in figure 10 that the peaks have moved considerably and that a 15° step in Rx Antenna orientation is too large and smaller steps would be preferential in order to be able to differentiate more easily between individual paths and sources.
To omit 5 pages of magnitude graphs continue to paths and sources
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