Sunday, November 12, 2017

Frequency spectrum and temperature, including RF and visible light

This is just a response to a recent thread on TRB involving telecom via light bulb. It may be interesting but is just a collections of facts that the reader can use to draw their own conclusion. First I'll travel back about 60 years and relate some personal history. My grandfather was a working blacksmith, he had a shop until I was in my middle teens. It would be interesting to watch and try to understand what he was doing. Among other things he made knifes, some of which are still being used. He could look at the color of a piece of steel in the forge and tell you within close tolerance what its temperature was. He knew from experience what this chart tell us.
As you can see the steps are in 100 degree F steps. He could tell just what shade he wanted before quenching the metal to make a spring or a bolt etc.
The interesting thing is the chart can be extended to include a much broader range of frequencies.
 Here we see radio and TV waves are the same electromagnetic wave as visible light but at different wavelengths. Here is the cool thing. Any body being heated will radiate the waves and the frequency can be defined by the temperature. What? Yes if I could devise a way to direct the waves to my radio I could tune it to a blank spot on the band and heat an object to produce a signal it could receive. I can hear the nay sayers! This is why I'm not posting on forum. I may be dumb as a box of rocks but not stupid.
So what is/was the application?
Here we see a filament in an electric lamp. Notice it is not just a piece of wire it is a coil which has been stretched to form a helix. Not a straight helix but a wiggly helix yes?

 Now we jump up on the roof and find an interesting antenna. Observe the helix is very similar to out bulb filament. Much larger and straight. It should be noted that helix as this one will radiate or receive from the end. Because it is designed to send or receive satcom. It could be used for omni directional com also but would be designed at different dimensions.

Yes the rubber ducky is a helix sized for broadside or omni directional com. So let's look closer at the pen light bulb.
Notice the filament is still a coil but now it is straight. (I show this one because you can see the filament)
This is the one being used for the transmitter. If you look you can see the filament. Might require a little imagination. Now apply the info and consider
1. As the wire heats it will radiate visible and invisible electromagnetic signals.
2. If the power to the light is varied at an audio rate it will cause the frequency to swing at an audio rate. This is what we call a Frequency Modulated wave.
3. Consider grandads chart, 100 degrees produces a different color. Less than 100 degrees will produce enough frequency swing to detect. BUT that's not all of the story.
4. There is a thing called thermal ionic emission. As electrons are emitted into the void in the bulb that can create EM radiation.
5. Another character at play here is Skin Effect. True as the filament heats it produce changes in the wire resistance. As the frequency changes it produces changes in AC impedance.
6. Remember the rubber ducky antenna? Looking at the filament as a rubber ducky can produce an image of it radiating RF.

You can see from this chart the RF spectrum is as broad as a football field and as small as an atom. The amazement here for me is not that the bulb can radiate a signal but that someone developed a receiver that was able to receive it!
The plan was to modulate a light beam but the bulb response was to slow because of its mass. The receiver was able to distinguish frequency changes in the RF spectrum.
That's my story.

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