PART 6: Square Loop Antennas
INTRODUCTION
In this lab, the student will be working with a square loop antenna . The goals of this lab are to design and construct the antenna, as well as tune it for optimum reception. This is the final step for the radio that the student has spent the entire semester building and perfecting. The student expects to leave lab with a working radio hearing from one of the primary AM radio stations in the Auburn area.
Practical Square Loop Antenna Design
To begin this lab, to student will be determining the ideal number of turns needed for their antenna. Square loop antennas are often constructed on a square cross wooden frame with one end supported by a board, shown in Figure 6.1. Loops with a larger area provide a stronger or more sensitive received signal. For this lab, the student will be using a pizza box as its frame instead of a wooden one. The antenna has a primary coil defined by side length A, winding depth B, and number of turns N, which are assumed to be evenly distributed over the winding depth, as indicated in Figure 6.2. The received signal is sometimes transformer-coupled to a secondary loop, as shown in Figure 6.3.
FIGURE 6.1: Square loop antenna wooden frame
FIGURE 6.2: Primary portion of the square loop antenna
FIGURE 6.3: Square loop antenna after adding a pickup coil
The student’s pizza box has a side length of 14 inches(A), and a winding depth of 2 inches (B). Using the equations from Figure 6.4, the needed inductance and capacitance can be calculated. The range of the trimmer capacitor is between 8 pF – 120 pF must be considered. The best received AM radio stations in the Auburn area, 1230 kHz, 1400 kHz, and 1520 kHz, must also be considered. The student calculated the required inductance using the middle station (1400 kHz), and the middle trimmer value of 60pF. The maximum capacitance required to pick up the lowest frequency station (1230 kHz) was calculated with the inductance from the previous step. The lowest capacitance was also determined using the highest frequency station (1520 kHz). The trimmer capacitor will be able to support the range required. Using the MATLAB equation in Figure 6.5, the number of turns was varied to try to achieve a value close to the calculated inductance and capacitance. All the calculated results are given below in Table 6.1.
FIGURE 6.4: Equations to find inductance and capacitance of square loop antenna
FIGURE 6.5: MATLAB Code to calculate L and C
Next, the student then used a knife to cut notches into the corners of the pizza box to hold the wire in place and keep them separated. Carefully, the copper wire was wrapped around the box ensuring that it didn’t tangle, or break and electrical tape was used to secure the wires in place. The antenna was then adjusted for optimum settings using the antenna testing setup, shown in Figure 6.6. A trimmer capacitor will be added to the to the primary windings, creating a resonant structure and increasing the output voltage. This is because the gain of a loop antenna is less than a well-designed dipole antenna, or a given frequency. The antenna is adjusted using the spectrum analyzer on the oscilloscope with the carrier frequency set to 1230 kHz, span set to 50 kHz, reference level set to -20dBm, and bandwidth set to 200 Hz. Connected in parallel with a trimming capacitor and the oscilloscope, the antenna was repositioned until the largest spike was seen on the spectrum analyzer. The trimming capacitor was also adjusted until the highest voltage was seen, shown in Figure 6.7.
FIGURE 6.6: Antenna testing setup
FIGURE 6.7: Oscilloscope waveform at largest spike
To conclude this lab, the student connected the antenna to the radio and tested the system. Figures 6.8 and 6.9 show images of the full radio and antenna connected together. The working antenna with sound can be seen as well in the video shown in Figure 6.10.
FIGURE 6.8: Full Radio connected to antenna
FIGURE 6.9: Full Radio connected to antenna
FIGURE 6.10: Video of radio and square loop antenna
CONCLUSION
In conclusion to this lab, the student was able to successfully work with a square loop antennas. The goals to design and construct the antenna, as well as tune it for optimum reception were accomplished. The student feels in this lab a lot of beneficial knowledge was gained in building the ideal radio. Now that an antenna has been added to the radio, it is complete and the student feels accomplished and happy with the results. Overall this entire lab helped the student to learn a lot of interesting things and had a great time building and performing these experiments.