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So at any given time, electromagnetic waves of many different frequencies are flooding the space around me. I have a little 2 meter radio. Are not all these electromagnetic waves exciting the electrons in the antenna at the same time?

Let's just go out on a limb and say the only RF reaching my radio is 2 meters. Let's just say frequency 146.120 and 146.805. If the radio is getting hit with both these frequencies at the same time, how is the antenna providing the voltages for just the one frequency that it is tuned to? It doesn't but then I'm not understanding how an antenna can receive multiple frequencies at the same time. Seems it would be just a jumble.

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Filters.

A filter is a circuit which is designed to react differently depending on the frequency. Every real (not mathematically ideal) circuit element does this to some degree, but filters are designed to have a very specific frequency response.

Here's a very loose and incomplete high-level schematic of a simple radio receiver:

schematic

simulate this circuit – Schematic created using CircuitLab

As you can see, the filter has four terminals: two for the input from the antenna, and two for the output to the demodulator. (In practice, they might more likely share a single ground bus for both input and output sides, for a total of three terminals.)

The effect of the filter is that the part of the signal coming from the antenna which has the desired frequency passes through — as if the wires on the left were connected to the wires on the right — and the rest of it does not — as if the wires on the left were either shorted together or disconnected.

Filters can be made out of various components, but most commonly inductors and capacitors. Introductions to electronic circuits often say something like “capacitors pass AC and block DC; inductors block AC and pass DC". This is true, but simplified; there is not just “AC” but the entire spectrum of possible frequencies, and the higher the frequency the more the capacitor will pass it and the inductor will block it. If you put an inductor and capacitor in parallel or in series, you get a circuit with a resonant frequency — a specific frequency (actually a small range of frequencies) which it will pass (or block) much more than other frequencies. That's the simplest filter. Additional complexity in filter design gets you "sharper" filters that are better at passing what you want and blocking what you don't want.

In addition, the antenna itself has frequency-varying characteristics and therefore acts as a filter. This is why there are different size antennas for different frequency ranges.


The above is a very simple picture. The main thing that I've omitted (besides e.g. amplifiers, which every radio but a “crystal radio” has in order to produce an audible output volume) is tuning.

One approach to tuning is to adjust the filter's resonant frequency (or passband) to the frequency you want to receive. Early receivers (see crystal radio and tuned radio frequency (TRF) receiver) did use this technique. However, it is difficult to make a filter that is both adjustable and sharp enough to be good at receiving one station and not also an adjacent one (it has poor selectivity).

Instead, modern radios (starting with the superheterodyne design) internally generate a signal at a particular offset from the desired frequency (the local oscillator (LO)) which is combined with the signal from the antenna (mixed) in such a way as to produce a copy of the radio signal at a fixed frequency (intermediate frequency (IF), equal to the offset between the LO and the input) which is then filtered using a sharp non-adjustable filter before being passed to the demodulator.

Originally, the local oscillator was an adjustable filter (usually containing a variable capacitor and a fixed inductor) and amplifier connected in a positive feedback loop — the filter selects the particular frequency and the amplifier keeps the signal oscillating. (Here, the filter doesn't need to be especially sharp because it's handling only the fed-back signal — which will quickly settle on only the resonant frequency.) Modern radios instead use digital circuits to generate the local oscillator signal, via digital-to-analog converters (DAC) and phase-lock loops (PLL); this allows very precise tuning under computer control.

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  • $\begingroup$ Thanks for the detailed response. I'm still unsure how if both frequencies are hitting the antenna at the same time it doesn't come out as some mix of the two. If a draw 146.120 on tracing paper. Overlay 146.805 on top of it - it's not either of the frequencies. Both frequencies are making electrons in the antenna move before filtering occurs. $\endgroup$ – mikew Jan 16 '15 at 21:34
  • $\begingroup$ They are not "overlaid", they are summed, and what carries them (empty space, antennas) is linear, meaning it preserves that sum — and it's possible to separate out the original components. Keyword: "superposition principle". I'll add more later. $\endgroup$ – Kevin Reid AG6YO Jan 16 '15 at 22:38
  • $\begingroup$ Ah ah!! I think the superposition principle got my research on the right track and was the fundamental error in my thinking. Thanks. Assuming deflection doesn't exceed bandwidth then it's possible to use the filters you describe to recover the component parts of the signals (i think) $\endgroup$ – mikew Jan 17 '15 at 6:40
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Ever blow in an empty bottle? Why is it that you hear just one tone, and not airy noise like you get when you blow on most other things?

Why, when you pluck a string on a guitar, do you get a clear tone?

The answer is resonance. An empty bottle is a resonant system, specifically a Helmholtz resonator. Resonant systems are everywhere. Consider musical instruments.

Radios operate on a similar principle, except the resonator is made of electrical components, not mechanical ones. A very simple electrical resonator is the LC circuit. When we make a resonant system with the intent of separating some frequencies from another, we call it a filter. Modern radios can have much more complex filters, but a simple LC circuit is good enough for a simple radio.

In fact, the antenna does receive a jumble of electromagnetic energy. However, the resonance of the receiver, like the empty bottle, makes the radio particularly sensitive to just a narrow range of frequencies. All the other frequencies are still there, but because the radio is not sensitive to those frequencies they are much, much quieter, like background noise.

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  • $\begingroup$ Thanks. I guess I'm wondering how it works when 10 people are blowing across the bottle at the same time. $\endgroup$ – mikew Jan 16 '15 at 21:35
  • $\begingroup$ @mikew Hopefully they are all transmitting on different frequencies. The receiver is resonant only for one. $\endgroup$ – Phil Frost - W8II Jan 17 '15 at 1:12

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