Lab 6 - Square Loop Antenna

Square Loop Antenna Construction

The purpose of this lab is to create and test a square loop antenna. A pizza box will be used as the frame for the loop. After the completion of this lab a full AM radio will have been built.

For the pre-lab a MATLAB script was created that would calculate the inductance of a square loop antenna given the depth, width and number of turns using Joe Carr's equations. f = 1/(2*pi*sqrt(LC)) was used to calculate the resonance frequency of an LC circuit so that the antenna could be tuned to a specific frequency. In order to determine the number of turns needed for our pizza box, we used the following steps. First, we calculated the inductance that is required for a resonance frequency of 1400 kHz using a 60 pF capacitor. This is the middle frequency that we are trying to detect, and the middle value of the tune-able capacitor. Then we used a MATLAB script to get as close to this value as possible with an integer number of turns around our pizza box.

We tried to get the biggest pizza box that we could find that was still square. We ended up finding a 16 inch pizza box from the pizza place in Terrell. Using the steps outlined above and the dimensions of our box, we determined that we needed to do 14 turns. We made notches on each corner of the box and wrapped copper wire around the antenna.

Now, we needed to measure the inductance of the antenna. We did this by using a tank circuit. Figure 1 shows the circuit that we created to measure the inductance. The capacitance was not tunable but fixed.

Figure 1: Circuit used to measure Inductance of the Antenna

We swept the frequency from the function generator to find which frequency had the largest output voltage. The known capacitance that we used was 93 pF. With this value the resonance frequency was 720.5 kHz. Using the equation L(uH) = 1e18/(4pi^2f^2*C(pF)), that gives an inductance of 524 uH. With this inductance, the capacitance that we need to tune to 1230 kHz is 31.91 pF. This is within the range of our tune-able capacitor which means we should be able to pick up this station.

Full Radio Test

With the addition of an antenna we should have a fully functioning radio. Stephen and I decided to use my radio because of it's reliability. We connected the antenna and tuning capacitor in parallel across the input to the RF amplifier and turned everything on. Inside the brick fortress, Broun hall, we were able to get a very weak, but still distinguishable signal. There was very noticeable noise that was present as well which was due to the power supplies.

To improve the sound quality of our radio we changed the power supply to two 9 v batteries and tested it outside. We were able to get a signal but it was extremely quiet. We changed the gain of our audio amplifier by increasing R2 on the op amp stage and tried again. We went outside at night away from other noise and pointed the antenna towards the closest AM radio tower. The radio was able to pick up a clear signal that was comprehend-able. We heard a nice commercial for MS research.

This link is to a video of the first try in Broun and then audio from the second try outdoors.

Conclusion

In this lab, we successfully built a square loop antenna that was capable of receiving AM signals. The square loop antenna design is easy to make and works relatively reliably. To receive a good signal, the position of the antenna is very important. The tunable capacitor was very sensitive to changes in capacitance. We actually found that we got the best radio signal when my hand was hovering a few inches above the tuning capacitor. It's unclear if this was truly affecting the signal or if it was superstition.

An interesting improvement to this radio would be to use an antenna that is not as dependent on orientation. All in all the radio works very well. Looking ahead to our project, we are going to try to create a live audio delay. This basically boils down to creating an audio sampling system and then using a DAC to play this sampled version. The goal, when completed, is to have a coherent audio signal through the system.