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An Improved
Antenna System
For our final design project, my lab partner and I decided to improved the antenna system of our radio by adding a current balun and building a better antenna. Throughout the project, I learned a lot about how a balun system works, and how to remove the common-mode current in the transmission line.
Through testing, we determined that our project did improve the radio overall, however we had difficulty with RF amplifier, which didn't allow us to hear any voices through the speaker. Since we couldn't hear our results clearly, we set up a series of tests to determine if the balun and antenna actually improved our system.
A Better Antenna
When designing the new antenna, we chose a diameter of 30 inches, and built the frame out of plywood. We had saved the Matlab script from the antenna lab and used our new values for the frame to find the ideal number of loops.
Choosing 16 loops, the script gave us an inductance of approximately 468.7 uH, which corresponds to a capacitance 35.7 pF. Since it is difficult to know the exact value of the trimming capacitor, we supplied a known voltage across a known resistance and our antenna to make sure the tuing capacitor was at the correct value.
Once this was finalized, we tested the antenna on the frequency analyzer. This test showed the antenna had a gain of about -66.4 dBm.
However, once we tested the antenna with our radio circuit, we did not hear any stations. Other antennas also did not work with our circuit, so we concluded that it was a fault within the circuitry. More on this problem is explained in the fault analysis section.
4:1 Current Balun
Artificial w/o Balun
Antenna w/o Balun
Artificial w/ Balun
Antenna w/ Balun
Project Conclusions
With the testing that was conducted, I believe that we did build a better antenna system. Our newly designed antenna gave us a better gain for 1230kHz over our previous antenna and the balun did provide a cleaner signal to the radio circuit.
In the last few lab meetings, our radio circuit failed to provide us with an adequate sound output. We extensively tried to mitigate this problem, as described in the fault analysis section.
While we did not manage to pick up a radio station, I was proud of our new design and learned a lot by picking the projects that we did. Had we more time, I would have liked to try and build a 1:1 current balun and compare it to our 4:1 balun. Also, I would have spent more time trying to find the exact cause of why the radio circuit wasn't performing.
New Antenna Design
Frequency Analyzer with New Antenna
Adding a Balun
To add to the complexity of our design project, and hopefully improve its performance, we also constructed and tested a 4:1 current balun. We chose a current balun over a voltage balun since voltage baluns are known for working only under ideal conditions. It was also suggested that a 4:1 current balun would perform better with a loop antenna over a 1:1 current balun.
Our design utilized a 240mm ferrite core toroid, wound 15 times with two sets of paired magnet wire, one on each side of the toroid.
Since we could not pick up stations with our radio, testing proved to be tricky! To test the performance and see if there was any improvement, I decided to analyze the output signal of the circuit with and without a balun attached on the oscilloscope and compare the results. I also tested an artificial signal supplied by the function generator and the actual signal received by our antenna.
Testing showed that the balun did in fact improve our antenna system. When the radio was supplied with the artificial signal, clipping was eliminated by adding the balun. With the antenna supplying a signal, the balun greatly cleaned up the signal, resulting in less distortion and a more defined waveform.
Fault Analysis
We had realized our radio circuit wasn't performing after we conducted the mixer oscillator labs. At first we thought it was the mixer circuit that wasn't working, so we decided to not use it in our final design. But the problem persisted, forcing to check and replace every single component within our circuit. After this we found that it was our MOSFET transistor on the RF amplifier that kept failing. We also tried to rebuild an entire second radio circuit, which did not work.
While we were receiving an output from our radio with very good gain, we determined that the quality of the components used was not sufficient with the signals.
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