Science Buddies: "Ask an Expert"

[Posted on behalf of student]

Hello! My name is Gracie i am in 8th grade, I'm doing my science fair project on a crystal radio. I've built my radio, got it to work, and tried different things to make it sound better. But my manipulated variable is either the antenna length or the amount of coils, and I'm not sure how to measure the output variable thingy, the thing that changes. Basically, my project would be, for example, "How does the length of the antenna of a crystal radio affect the sound quality of the radio", so the output variable would be how good the radio sounds. However, Since this is sort of a matter of opinion, I'm not sure how to measure how "good" the radio sounds. I've tried, like, counting how many word i can clearly understand in a certain amount of time, and there were too many to count properly. I've had some other lame ideas, like rating the sound quality 1-10 or something, but at my school, science fair is a HUGE deal, so basically this project has to be as perfect as possible. I've seen "measuring the output impedance of a crystal radio" or something like that but it was WAY to complicated for me. Anyway, enough of my blabbering, any ideas on how i can measure this? Thanks so much for your time!


-- Hello! Sorry i already e-mailed you but I'm gonna have to bother you again. So when you coil the wire around the "oatmeal box", you make "taps" or like strip off the insulation and twist the wire, every I don't know five or so coils? Well anyway, when the entire radio is completed and set up and you put the alligator clip on one of the taps/loops, I don't quite know how to word this, but whats the difference between putting it on each tap? Like what changes when you put the clip on the 1st tap vs the 5th? Is it just using different amounts of the coil? And also how do you change stations/frequencies? Just the height of the antenna, right? Anyway thanks for your time! (Again!)

Gracie - I'll try to answer your questions in reverse order. When you wound the coil on your cardboard form you were building a component known as an inductor. As you tapped the approximate 40 turns every 5 turns, you were creating connections that had a different value of inductance at each tap. I built this coil once and measured the overall inductance and I believe it was around 160 microhenries overall, and tapped at 20 microhenry intervals. The way a radio tunes to a particular station is to create a 'tuned' circuit that equates to the frequency of the station you want to receive. This is referred to as the resonant frequency. A tuned circuit is a combination of an inductor, your coil, and a capacitor. In the case of the crystal set project, there is no separate capacitor, so the circuit relies on 'stray' or 'parasitic' capacitance of the coil itself. Station frequencies above and below the desired frequency are partially rejected, so all the desirable energy is maximized (and the unwanted signals, interference, are rejected). In a traditional radio, stations could be tuned by either varying the value of the inductor (typical in very old automobile radios) or by varying the capacitance. In your case, we are basically changing the value of the inductor by using different taps across the coil.

The fact that you have a working radio is a good starting point to do introduce some variables. I agree that sound quality is pretty subjective, but it is certainly affected by signal strength and related to the audio volume you hear in the headset. While volume itself is subjective as well, you may still use it in your comparisons. You could find another way to measure the output of your radio using a simple digital multimeter, probably reading millivolt signals across a capacitor (that would help to average the audio energy). Unfortunately, radio signals are always varying, and the station is not providing you with a sinusoidal audio tone to test, so even that approach would be a little subjective.

Antenna length is a good variable to use. The center of the AM broadcast is about 1,000 kHz or 1 megahertz. The wavelength of a radio wave at that frequency is about 300 meters. Your antenna is much shorter, so you can assume that the longer you make it, the more signal energy you will capture, shorter, and you will see your signal diminish.

A second variable is to show the effect of tuning on the audio volume you perceive at the headset. As another part of the experiment you could add a fixed capacitor in the range of 50 to 500 nanofarads across the coil to see if that improves reception in conjunction with tuning. Good luck, let us know if you have any other issues or questions.

Rick Marz

Gracie - In my last line, I meant to say picofarads, not nanofarads.

Rick Marz