Radio waves and building a radio and stuff for science fair? ^_^?

Question

I'm building a radio for my science fair project but i'm not sure what to test about it. i was thinking about the wave interference, but not sure... Any ideas and details on execution of em? (oh, and the radio's bn made out of an oatmeal box^_^)
thanks in advance! ^_^

Answer 1

What you should test about it depends on what form of radio you are building. If you are building an AM radio receiver (e.g., crystal radio set) from a kit, then the parts should be provided for you, and the end result of building the kit should be a radio that covers the AM broadcast band, allowing you to receive signals from AM broadcast stations. If you are "homebrewing" a radio (i.e., not from a kit), then you need to pick values of parts and their circuit layout to make the radio receive the signals you want it to receive. If you are building a radio transmitter, then you can do a lot more testing, as you can check its range with various antennas on the transmitter (and receiver) and through various media (e.g., the wall of your house or school as compared to an open field) and over various terrain (e.g., an open field as compared to opposite sides of a hill).

You can test various things that are tested for commercially built radios, such as sensitivity and selectivity. Sensitivity measures how weak a signal you are able to receive. You could start with a long (e.g., 50 foot) antenna and shorten it a few feet at a time until you can't hear stations any more and make notes of your results. Selectivity is how well your radio blocks other strong signals and only receives the signal you want to hear. You can go to the fcc.gov website, click on Mass Media Bureau, and look for AM Query to search the FCC database for local AM stations and their transmitter locations. You can go to within a quarter mile or so of a local AM transmitter and use your radio to try to listen to other stations and see if it avoids letting the nearby station interfere with the signals from distant stations. If you get interference from the local station, you could move farther from the local AM transmitter until the interference goes away and keep notes of your results.

Antennas have a big impact on the performance of radios, so you could experiment with different sizes and shapes of antennas and compare your results. High-performance antennas are usually made to be resonant at their desired operating frequency, but, for AM radio, a resonant antenna would be about 150-900 feet (50-300 meters) long, so you will probably be using a shorter, non-resonant antenna.

If you want to get really ambitious, you can figure out what parts in your radio cause it to be tuned to the AM broadcast band and change them so they tune to other bands, such as shortwave, where you can hear stations from all over the world. For example, you could change the number of turns of wire around the oatmeal box to change the inductance or add a capacitor to a polyvaricon variable capacitor (that has a tuning knob on it) to change its capacitance. Either of these things will change the frequency range of the radio.

If you try changing the frequency range of your radio but get a lot interference that keeps you from hearing shortwave stations, check to see if broadband over powerlines (BPL) is being implemented in your area by an electric power company. If that's the source of your interference, write letters to your mayor, city council, state representatives and senators, and governor, as well as the FCC and U.S. representatives and senators, telling them that you can't even test your science fair project because of all the harm BPL is causing to the airwaves.

Answer 2

Are you building a receiver, transmitter, or both? If just a receiver, check its upper and lower frequency limits. Test the effectiveness of different kinds of antenna. Try to include a signal strength meter, so your measurements can be objective. Choose a relatively nearby broadcast station as your signal source. Keep a log of what stations you hear and how far away they are. Show them on a map. Have fun! That's how I got started in amateur radio and electrical engineering.

Answer 3

"i was wondering how hard would it be to build an am radio transmitter?" You mean how illegal? Pretty illegal unless you have a license to operate a transmitter AND the knowledge to operate it on just the right frequency. The AM band is commercial and therefor NOT the right frequency. You would need a commercial license to operate a radio station and unless you are Rupert Murdoch, it is very unlikely that you will get one. I would do something else. Incriminating yourself is not a great way to get good grades in science class.