cragar said:
You could try building an electron gun by using a heated cathode like maybe an electric oven element and heat this with current . then put this in a pvc tube , and then attract the electrons over a voltage and when the electrons travel from the cathode to the anode they will emit light , and you can vary the voltage . and then you could receive them with a household radio , or maybe try building a photomultiplier ,
And you would haft to evacuate the air out of the tube for best results .
This is an insanely complex, difficult, inefficient, and generally wrongheaded and dangerous way of approaching the problem of "producing radio waves". Some specific high-frequency, high-power, narrow-band applications use specialized vacuum tubes such as magnetrons and klystrons. The details of their operation are far more complex than you describe, and their design and construction is very non-trivial. And from the information given, unnecessary...free electrons are not required for the production of radio waves, the vast majority of radio transmitters simply use an alternating current coupled into an antenna of the appropriate length to efficiently radiate at the chosen frequency. Your suggested electric oven element in a PVC tube is only an electrical and fire hazard, you're almost certain not to observe any thermionic emission, let alone get useful radio wave emission. And a photomultiplier tube, aside from also being extremely difficult to design and construct, will require operating voltages of thousands of volts and would only receive radio waves as incidental interference.
jonathanplumb said:
My idea is that taking how visible light refracts and reflects into our eyes to create a visible image of our surroundings, I can do the same with radio waves. If I were to fire a spectrum of radio waves at various elements and measure its reflections, I could then fire a spectrum at a larger area and re-create the scene at an atomic level. At first it would start with little precision, showing us something similar to Atari or Nintendo graphics, but with the advances of the technology could potentially produce fully 3-dimensional representations of rooms.
The idea is better than visible light because radio waves pierce through tons of solid objects. This type of device could potentially be like a giant x-ray gun.
You are essentially talking about an imaging radar system...an extremely complex and difficult project, unfortunately. It's not just a matter of producing radio waves, you have to emit them in highly directional beams, detect and measure the strength of the weak reflections, time the echoes of something that moves at one foot per nanosecond (you need timing for ranging, intensity measurement alone won't do)...even a simple radar ranging system takes a lot of difficult engineering. Also, radio waves are far lower frequency than visible light, meaning larger wavelengths...tens of cm for the high frequency microwaves used by WiFi and microwave ovens, tens of meters to kilometers for AM/FM radio frequencies. This is a fundamental limiting factor in how sharply you can focus a beam with an antenna array of a given size, and in the sort of detail you can image with radar at that frequency. Imaging radar systems exist (http://www.planetary.org/explore/topics/saturn/titan_radar.html ), but you'd have a hard time finding one that could produce a recognizable image of a room.
Sonar is much more achievable. There are simple sonar ranging devices available as ready-made modules (
http://www.radioshack.com/product/index.jsp?productId=2909789), and it's far more reasonable to expect to home-build a complete sonar system (still a major project, and it sounds like you've got a lot to learn before you could make real progress on it, but achievable with hobbyist equipment and budget). Light travels about 30 cm per nanosecond, 300 km per millisecond. Sound travels about 34 cm per millisecond, and cheap microcontrollers can easily measure events with sub-millisecond resolution. Sound is also much easier to emit and detect.
