Dr Skip wrote: > Johnny Carson (or was it Letterman?) suggested this on TV 30yrs ago > or so. I did it in the late 1970s, so probably 32 years ago. Mabye he got the idea from me ;-) >> Aha. It's The luminiferous aether! >> Warning Will Robinson! >> Here theyre bee Dragones. >> >> It's like the string but without anything like the string. > > Remember, I said not to use simple examples beyond their > simplicity... >> Ultimately all roads will lead to James Clark Maxwell. Guys, you are making this too complicated and confusing. Mentioning Maxwell at this point silly and possibly even damaging. Simplify, but don't make it inaccurate. Good presentation is all about keeping the context of the intended audience in mind. Now where were we... Ah yes, we last saw that any voltage change anywhere causes a little energy to be radiated into space. Every circuit that creates a changing voltage is a transmitter, although in most cases a very very inefficient one. Radio electronics is about doing this deliberately at useful efficiencies. Let's think about what transmitting looks like from the circuit's point of view. Go back to the example of the two pieces of aluminum foil one meter apart with a 1/2 meter wire from each to a small circuit board near the middle. What would those two connections look like to a driving circuit? Mostly like a very small capacitor. The two pieces of aluminum foil make a capacitor, although at 1m separation a very very small one. That's the first order effect. If you dig deeper, you consider that the wires going to the two plates have some finite inductance. This is not because the wires are imperfect, but because when current travels in a line, it will cause a circular magnetic field around that line. So now these aluminum foil pieces with wires to them look like a capacitor with some inductance in series. However, remember that neither capacitors nor inductors (at least perfect ones) can dissipate any energy. They can temporarily store energy, but in the long run this has to come out back into the circuit. But we saw earlier that this setup transmitted some energy into space. Obviously that's not ever coming back to the circuit, so how can it look like just a capacitor and inductor to the circuit? Something doesn't add up. It doesn't add up because we left out something. There is just a little resistive effect in there that accounts for the energy lost into space. So for now we'll add a little resistance in series with the capacitor. Only a very small amount of energy is transmitted, so this is a rather high resistance. I'm not getting into antenna theory here (that's a few books worth on its own), but antenna design is usually about ballancing the inherent inductances and capacitances so that they cancel each other out and what's left is the apparent resistance from the circuit's point of view. While it looks just like a real resistor to the circuit, it doesn't dissipate the power and turn it into heat, but rather radiates the power into space. Now we're going to skip over most of antenna theory and get to a simple result you can reproduce at home on your own. Take two pieces of wire the same length, and lay them out end to end in a straight line. Bring the two middle ends close but not touching. Think of them as connecting to two pads on a small circuit board if that helps. It so happens that those two points look like a resistor at a frequency such that the total end to end length of wire is 1/2 wavelength. The power that goes into that resistor at that frequency is quite efficiently radiated into space. Not only that, the resistance is well known and is about 75 ohms. I know this is unintuitive at the moment, so just believe me for now. There may have been too much hand waving, so here is a crude attempt at a picture: --------------------X-------------------- The X indicates the little circuit board and the dashes are the wires going out in opposite directions on either side. The speed of light is about 300Mm/s, so at 300MHz the wavelength is 1 meter, and 1/2 wavelength is 1/2 meter. So if the end to end length in the picture above is 1/2 meter, the two wires will look like a 75 ohm resistor at 300MHz. At 1m total length it would look like 75 ohms at 150MHz. Note the important stipulation of this 75 ohm phenomenon happening at a particular frequency. Clearly it doesn't happen at DC since there is looks like a open circuit. The inductive and capacitive effects are still there, but at the critical frequency they happen to cancel each other out such that the net current to voltage phase shift is 0, which makes it look like a pure resistor to the circuit. That means this thing draws real power from the circuit (at the critical frequency). It doesn't burn up this power as heat like a real resistor would, but instead radiates it into space. From the circuit's point of view, it's the same thing though. This is what a antenna does, and the type of antenna described above is probably the most basic of all and is called a "dipole". There are lots of different antenna types. They vary in their radiation pattern (which direction the energy gets radiated), the resistance at the critical frequency, how they behave over a larger range of frequencies, etc. For example, another type of antenna is called a "folded dipole". I don't want to get into that further, but it happens to look like a 300 ohm resistor at the critical frequency. So lets say you have a 1 meter dipole as shown above. We know it will look like a 75 ohm resistor at 150MHz. So how do you make a transmitter? You feed a 150MHz signal between the two pads on the little circuit board. Let's say you can produce a 5V peak to peak 150MHz sine wave on that board. That's about 1.8V RMS. Feeding 1.8V into a 75 ohm resistor dissipates 43mW, which would make a ordinary resitor just a little warm. Feeding the same 1.8V at 150MHz between the two pads on the little circuit board above will instead radiate the same 43mW into space. That's actually a fair amount of power. Off the shelf receivers could pick up that signal 100s to 1000s of meters away. Time to go again. Maybe more later. ******************************************************************** Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products (978) 742-9014. Gold level PIC consultants since 2000. -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist