10.12.2021 -- The purpose of this lab is to build a square loop antenna that will pick up the AM signal input for our radio. This is constructed by wrapping copper wire around a frame (in our case, a pizza box) and connecting the two leads to a load (again, this will be our radio).
6.1 Square Loop Antenna
The below circuit in Figure 6.1 shows the setup of the antenna to the radio. A trimming capacitor C is used to pick up the desired radio channel. We first used a load resistance to find the best capacitance to pick up the AM channel at 1230 kHz.
Figure 6.1 Square Loop Antenna setup
6.1a Preliminary Antenna Study
We used a MATLAB program to calculate the optimal capacitance and inductance neded for the antenna given the frequency needed and the antenna parameters. A table of the calculated square loop inductance data and the plot are shown below.
Table 6.1 Calculated Square Loop Inductance (in μH, B = 2 inches)
Figure 6.2 Plotted values from Table 6.1 – N=8 in blue, N=12 in red, N=16 in green
6.1b Practical Square Loop Antenna Design
To build the antenna, a pizza box was used as a frame to wrap the wire around it. The antenna was then connected to a tan circuit to trim the capacitor to pick up the desired frequency. We calculated the optimal number of loops needed for the antenna using the MATLAB code from earlier. The table below shows the calculated inductance and capacitance needed to recieve the different frequency channels.
C (pF) | L (μH) | N (# of loops) | frequency (kHz) |
82.35 | 263.2956 | 16 | 1230 |
60 | 215.39 | 16 | 1400 |
53.93 | 203.2936 | 16 | 1520 |
Table 6.2 Capacitances and Inductances needed to recieve different frequencies
Figure 6.3 Tank circuit to trim capacitor
One way we tuned the capacitor was by finding the maximum amplitude of the output signal. The output of the tank circuit at 1230 kHz s shown below in Figure 6.4.
Figure 6.4 Output signal shown in blue
The antenna was tuned using the spectrum analyzer on the oscilloscope. The signal generator was disconnected, and the same antenna orientation was used to maximize the power level of the incoming signal. Then we looked for a peak at out desired frequency and measured the power level and voltage amplitufe from there. The data from this is shown in Table 6.3 below.
frequency (kHz) | power level (dBm) | Vamp (μV) |
1230 | -65 | 125.74 |
Table 6.3 Power level and voltage amplitude at given frequency
Conclusion
Overall, our antenna worked well with the tank circuit. After replacing the resistor load with the radio, we discovered that the antenna will pick up one channel very strongly with anoter weaker channel underneath. When the radio was using the DC power supply in the lab, the radio was very loud and clear. However, the radio is much quieter whena 9V battery is connected to the radio circuit.
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