4.1 The Amplifier circuit

 

 

 

 

 

 

 

 

 

 

 

 

                                               Figure 4.1.2 - Amplification shown on Oscilloscope                                        Figure 4.1.2 shows that the amplifier circuit could successfully amplify the voltage when a 19 kHz signal was fed into it by the signal generator. The yellow wave (CH1) represents the output, the amplified sine wave with the Oscilloscope probes at T1. The blue wave (CH2) represents the input, the original signal feeded into the amplifier circuit, with the Oscilloscope probes being placed at J1. Initially, there was no amplification at CH1. However, when the variable resistors R2 and R4 were adjusted to the maximum, the amplification increased to its maximum as well. Using the voltage gain formula, the voltage gain was 10.

 

4.2 Tuning the Antenna to the appropriate resonance

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

                                       

                             Figure 4.2.1 - Results of test for tuning the antenna

 

Figure 4.2.1 depicts the tabulated process of tuning the antenna. After careful consideration, the most suitable VLF radio transmitter was found to be located at Harold E. Holt, Australia. It has a frequency of 19.80 kHz. It is approximately 2761.18 km from Singapore (Figure 4.2.2). Once tested, the natural resonance of the antenna was found to be too high (151.00 kHz). Thus, tuning capacitors were added to the loop, beginning with 1nF polypropylene capacitors. It was noticed that the larger the value of the tuning capacitor added, the greater the reduction in the resonance of the antenna. After a total capacitance of 7.28 nF was added using 14 tuning capacitors, to switch tactics, the number of loops was increased instead. This reduced the resonance of the antenna greatly, until it was within a few hundred Hz of the frequency of the VLF station (19.80kHz). The final resonance of the antenna was tuned to 19.50 kHz.

 

4.3 The Amplifier Circuit and Antenna

 

 

 

 

 

 

 

 

 

                                               

                                                      

Figure 4.3.1 Amplification with the Antenna

Figure 4.3.1. shows the amplificaiton of the current induced in the tuned antenna when it is connected to the amplifier circuit. The blue waveform (2mV/div) is the signal input into the amplifier circuit and the yellow waveform (10mV/div) is the amplified signal output from the circuit. This shows that the antenna is able to capture the VLF signal as the measured frequency, using the oscilloscope, is between 17kHz - 20kHz. Because of noise and disruption, this value continually changes but it is mostly at the above stated range. Initially, there was no voltage gain. As the resistance of the variable resistors was adjusted, the amplifier circuit amplified the signal by an optimal 7.5 times (Vgain = Vout (peak to peak) / Vin (peak to peak) = 30/4=7.5V).

 

4.4 Digitising of signal using Spectrum Lab

 

 

 

 

 

 

 

 

 

 

 

 

 

     

Figure 4.4.1 Spectrogram of recieved signal on

Spectrum Lab. 

       

 

                                                                                 Figure 4.4.2 Chart of signal recieved from 0530 to 0700

 

Once set up, Spectrum Lab was able display the signals recieved from the antenna in a spectrogram. The spectrogram is of time against frequency and the stronger frequencies are displayed as yellow lines. The stronger the frequency, the more intense the colouration and the strongest signals can be percieved as white. As seen, the spectrum in the range of 19.0 kHz and 20 kHz have the most intense colouration and the lines appear as white at the 19.80 kHz frequency. This frequency corresponds with the frequency of the VLF signal transmitted from Harold E.Holt. Other signals are also picked up and displayed on the spectrogram due to interfering electrical appliances. Also, increasing the gain of the amplifer circuit helps to enhance the signal strength. The log files exported from Spectrum Lab were charted onto a graph of the power against time of the 19.80 kHz radio signal recieved to display the changes in signal strength of that specific frequency. This allowed the observation that there are various spikes and falls in signal strength throughout the day. The signal strength was observed to peak at 6.30 am. When checked against the GOES x-ray flux charts, this spike was observed to be accurate. This could be due to a small flare as there may have been an increase in the ionisation of the ionosphere. Other than that, there were spike in singal strength in the morning and afternoon due to the proximity of the sun to Asia at those times. This also corresponds with the data on GOES x-ray flux charts.