Lab Six: Square Loop Antenna
This lab will conclude the creation of a working AM radio. By adding an antenna, a specific frequency can be tuned by using a trimming capacitor in paralell with the antenna. Changing the value of the capacitor with the static antenna changes the resonant frequency of the antenna and creates a strong input for the chain of amplifiers.
Antenna Design
Three equations are used to find a proper number of loops to wind around a pizza box to create an antenna. Equations 1-3 are used to determine that a suitable number of loops around the box is about 16. Using magnet wire and electrical tape, the antenna is then formed as shown in Figure 1. Using Equations 1-3, the plot shown in Figure 2 is created by changing the area enlosed, A, and the number of loops, N.
\[L(\mu H) = 0.008N^2Aln(\frac{1.4142AN}{(N+1)B})+0.37942+(\frac{0.3333(N+1)B}{AN})\]
\[f=\frac{1}{2\pi\sqrt{LC}}\]
\[C(pF)=\frac{1E18}{4\pi^2f^2L(\mu H)}\]
Analyzing the Antenna
The antenna is now connected to the spectrum analyzer. The spectrum analyzer shows the different frequencies and their associated dBm levels. There is a noticable spike at 1230MHz as shown in Figure 3. This spike can be tuned using the triming capacitor, and increased when the resonance frequency is 1230MHz. This particular frequency has a spike because there is an AM sports talk station located in Atlanta and this has the strongest signal to Auburn, Alabama.
Connecting the Antenna to the Amplifiers
And finally, the antenna is connected to both ground and the input capacitor of the common sourse RF amplifier. When the speaker is connected as the load of the end of the A-B push-pull amplifier, there is a slightly noisy but very much so distinguishable sound coming out of the speaker. And just like that, an AM radio is born! A sample of the audio can be found below in Video 1.
Conclusion
There is now an operating AM radio. This process brought forward many different topics including, transistor biasing networks, q point analysis, input resistance, output resistance, gain levels, further oscilloscope use, and many other. This has largely made the world of radios less "magic" and more science based to me, as I now understand what components are required as well as how they interact with each other.