Antenna Current Distribution Page 5

4.1) Antenna Current Distribution Measurements

An antenna is made from a  length of enamelled copper wire soldered into an SMA connector (in this case a 254 mm length of 1.25 mm) wire. Provisional tests on this antenna using a toroidal ferrite and measuring return loss with a vector network analyser showed that the ferrite affected the antenna return loss considerably pulling the centre frequency (from 255.5 MHz to 250.5 MHz). Therefore, a constant frequency measurement is likely to have significant error because of the ferrites effect on the antenna matching. As it is planned to use a diode pump detector circuit which is a wide band peak detector, it is preferable to use a narrow frequency sweep to find the antenna matched frequency, which will ensure that the results are always representative of an antenna used at the optimum (matched) frequency. The test circuit used for all of the current distribution measurements are as per figure 13. This arrangement with an in house version of the VHF 300 MHz amplifier ( giving an antenna input of up to +20 dBm) was more than adequate to drive all of the test probes used into their linear regions during all of the following measurements.

Figure 13 Antenna Current Measurement Test Circuit

[Drawing of antenna transmit set up]

4.2) Toroidal Ferrite Probe

A current probe is constructed using a toroidal ferrite, a diode pump circuit and a digital voltmeter (Lascar DPM1AS-BL snap-fit low power voltmeter is suitable) and is shown in figure 14 below. The 10 μF capacitor is a low inductance tantalum type and is a large value in order to be able to maintain its charge during the VNA frequency sweeps. The diode pump principle is explained in Principle of the Current Detection Circuit as is the linearising method which is used later in order to improve accuracy. The low inherent capacitance to ground of the probe ensures the minimum antenna perturbation and the best accuracy possible using a near field intrusive measurement device. As the digital voltmeter is battery powered, there is no disturbance from any power supply or earth wires. The digital display can be clearly seen at a 'far field' distance thus removing the need for personnel to be in the near field area which would affect antenna behaviour. A ground plane is provided under the antenna but is not connected to either the test equipment or to ground.

 

Figure 14 Current Probe

[Circuit of toroidal ferrite probe]

Measurements are then performed with a narrow band swept frequency with the VNA set to S11,  such that the antenna acts as a monopole of quarter wavelength, to find the antenna centre frequency.  The analyser is then set to carrier wave at the matched centre frequency for each for the individual measurement steps. The toroidal ferrite probe is moved along the length of the antenna and measurements taken at intervals of 10 degrees. The result of one set of measurements is shown in figure 15  as a probe voltage reading versus the physical position of the toroid on the antenna.  The voltage curve which represents the vector difference current distribution in the antenna is scaled but not linearised. The probe circuit gives a peak positive voltage reading and this occurs when the phase at the input to the antenna is at +90°. This makes the phase at the end of the antenna 0° and the reflected current is then negative phase. This convention is kept throughout these studies and has been found to be the simplest to use both mathematically and in the comprehension of the electrical behaviour within the antenna.

Figure 15 Quarter Wave Monopole Current Distribution

[Graph of raw current probe measurements]

The result may come as a bit of a surprise because it is nothing like the sinusoidal curve described in text books. Looking at the curve in figure 15 it is obvious that there is a problem with the measurement at the very end of the antenna as shown at the right hand side of the curve. We have already proven that this should indicate a vector difference current of zero in the previous measurements, regardless of the value of actual directional currents present. This is because the INCIDENT and REFLECTED currents at the end of the antenna are almost in phase due to the short distance travelled in relation to a wavelength. There is also very little difference in the amplitude of the currents because they have not travelled far and so very little has been radiated. The toroidal ferrite probe is obviously measuring more than just current at this higher frequency (1 MHz was used in the initial toroidal ferrite measurement!) and so there has to be a probe voltage contribution from electric field and/or radiation.

4.3) Flat Plate Probe

In order to remove the electric field/radiation voltage contribution from the toroidal ferrite probe current measurement, a flat plate probe is used which will only detect the electric field/radiation. A flat plate probe is constructed using a small plate of brass or copper, a diode pump circuit and the digital voltmeter (a Lascar DPM1AS-BL snap-fit low power voltmeter was again used as in the original measurement) and is shown in figure 16 below. The plate is orientated so that the flat surface faces the antenna, it does not encircle the antenna and therefore is insensitive to current.

 

Figure 16 Flat Plate Probe

[Circuit of flat plate probe]

 Measurements are performed at the same points as the toroidal ferrite probe measurements and the results of one such measurement sweep are shown in figure 17. As it has now become obvious that the current probe is measuring mainly current at the base of the antenna and the flat plate is measuring only electric field/radiation near the end of the antenna, the results in figure 17 have been linearised at these two points. The flat plate results have also been scaled so that the current difference at the end of the antenna is zero. The extrapolated results at the very extremes of the antenna (90° and 0°) have been added in order to give a complete picture.

Note: The flat plate results were repeated several times before it was possible to obtain a smooth curve and this is probably mainly due to the fact that the detector is a peak detector, which is the least accurate of detector types. According to Radio Engineers Handbook by F E Terman, McGraw-Hill, 1950 page 934, a peak detector can have from -25% to +50% error with a 50% second harmonic and 8 to +50% with a 50% third harmonic. It is therefore important to avoid compression in the driving amplifier which can generate unwanted harmonics.

Figure 17 Linearised Results

[Linearised probe measurement graph]

 

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