2.2) Low Capacitance Probes
The voltage distribution characteristics are measured with a high impedance very low capacitance voltage probe. The importance of high impedance and very low capacitance in the following measurements is that even the smallest of loads will severely affect the antenna performance and pull the fundamental matching frequency such that measurements can easily be highly inaccurate. The major problem being that with a probe of higher capacitance and lower real resistance component not only a considerable shift of phase can occur, causing both an error in phase measurements and affecting the antenna matching frequency, but it also causes the difficulty of not knowing what frequency to take voltage measurement results because there are multiple return loss troughs. An almost pure very small capacitance does still pull the antenna matching frequency but the S11 trace on the VNA shows a distinct trough at the matching frequency and it is easy to track.
Several different commercial types of probes were tried and various in house designs simulated and built. The circuit simulation for one probe input showed a load of 0.14 pF||84 KΩ and even this very low load became a considerable problem at the frequencies involved because the S11 trace showed multiple return loss troughs. Resistive probes (56 KΩ + 51 Ω) performed reasonably well as they have very little reactance but the problem with these is that the high value resistors required to avoid loading the antenna give a large attenuation and cause the output to be very noisy.
The final high frequency probe design was assembled in a carbon fibre tube and has thin cables and thus contains the minimum amount of metal, thereby minimising the disturbance caused to the antenna characteristics. This probe has an input of approximately 0.5 pF|| 40 MΩ at the measurement frequency and a pre-production model is available from Chemandy Electronics under the model name Probe 2 A prototype probe is shown in figure 4 near the tip of an antenna and attached with an improvised plastic clip which applies pressure at the contact point in order to obtain a reliable connection. The retort stand is made from plastic and the probe is held in place with a Gorilla camera tripod, which is also all plastic.
Figure 4 Low Capacitance Probe
The voltage measurement made on the VNA is a S21 with Linear Magnitude and the probe and test set up is calibrated using a 'Thru' calibration. During calibration the antenna is removed and replaced with an accurate 50 Ω termination. The length of the calibration circuit should ideally be the same length as the antenna connection but small phase offsets are mathematically corrected in the results analysis where the VNA reference 0° input is offset to +90° in order to make calculations. When properly calibrated and the VNA gating implemented, the input voltage to the antenna should be virtually identical to the calibrated voltage. This fundamental check proved to be essential in order to avoid making repeated lengthy erroneous measurements.
2.3) Measured Vector Sum Voltage
Measurements are taken at mechanical intervals of 1/9 or (λ/36) along the antenna length and this distance represents a phase shift of 10 degrees. The VNA time domain function of gating is used to ensure that there are no reflections back to the antenna and for this measurement it is only necessary to ensure that there are no radio reflective surfaces within about a wavelength of the antenna under test. It may be possible to achieve accurate measurements in an anechoic chamber using a spectrum analyser without gating but this method was not tried. The result of one set of measurements made with an Active Probe 2 is shown in figure 5 as a voltage versus phase/mechanical position curve and it can seen that the result is not a straight line because we are measuring a vector sum voltage and the results need mathematically processing in order to extract the separate INCIDENT and REFLECTED voltage.
During the processing of the data it is necessary to allow for the reduction in voltage along the antenna length. Unlike the current, which reduces to virtually zero after being reflected from the end of the antenna, there is still voltage remaining in the reflected direction right up to the antenna input. It was therefore decided to incorporate a voltage drop coefficient for the AUT into the following calculations and this was established as Cv = 0.0014/° for this particular antenna by using curve matching techniques.
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