Signal interference in receiving antenna?

Question

I’ve been scouring the interweebs for some insight into how electrical signals travel in a wire. More specifically, I cannot wrap my head around how a receiving antenna works with hundreds if not thousands of electromagnetic waves interacting with it. How is one signal able to be usefully extracted? Surely the movement of electrons back and forth would be influenced by all the electromagnetic frequencies hitting the antenna that their back and forth movement would no longer reflect a single (useful) signal? My flawed understanding is based on thinking of the electrons in the receiving antenna as being analogous to a single conga line. Should I imagine the wire as many conga lines of electrons instead?

Answer 1

An antenna is best viewed as a transducer between in-circuit waves and free-space waves.

What is a wave, then? A field? A signal?

There is one field here, the electro-magnetic field. As the name suggests, this field can be separated into two electric and magnetic fields, intimately linked by exchange laws (Maxwell's equations). (We generally express the latter as a system of four equations, or it can be written as two or even one in more advanced systems, like the covariants used in relativity.)

A physical field is the representation of a property that varies with position. In this case, we have the electric and magnetic intensities at each point in space (at a given instant in time). Fields can be scalar (there is a single number at each point: like the temperature at any point in a room), or vector (having direction); the electric and magnetic fields are examples of the latter, and thus we have not just magnitude at each point but that magnitude points in a direction as well.

The electromagnetic field is a propagating medium. That is, if there exists a time-varying electric field, there is a complementary time-varying magnetic field, and so on and so forth; the rate at which energy exchanges between these fields, is defined as the speed of light (light is an EM wave). Typically we call solutions to these field equations "waves", a particular repeating, propagating distribution of energy through the medium. A wave might be described by its shape ("{plane|spherical|dipole} wave" etc.), or something it's doing ("soliton wave").

A wave might also be understood as energy having a specific frequency range; or radiation incident from a given source -- but notice that "source" is tricky, because diffraction around a boundary can be treated as a source along that boundary (and so, is it still the same wave, or is it myriad new waves coming from that boundary?).

Importantly, the electromagnetic field is a linear medium. That is, if we have two transmitters sending waves through space, the resulting field at a given point, is the simple sum of both waves. The result might point in all manner of directions, but the vector components it's composed of, can always be separated into these terms. So, whether we call it one wave or many... it kinda doesn't matter, it's just whatever we're looking at at the moment.

"Wave" is kind of a weird word, and to some extent you'll just have to pick up how it's used, I think.

So, that's about waves and fields. What's a signal?

A signal is a smooth, time-varying function, typically representing a physical quantity in a given location. Typically in electronics, we're concerned with voltage and current signals, but power or energy, frequency, resistance, etc. are all important. (The EM field can be thought of as a space filled with signals, i.e. each point has a time-varying quantity.) Wires, or more generally, transmission lines, admit at most two signals (V and I, or forward and reverse waves, or sin/cos, or..) at any given point.

A transmission line is the one-dimensional version of a field. Two axes have been blocked off by boundary conditions (such as shielding), and a single axis of propagation remains. The boundary conditions act to guide waves along the structure; it's a waveguide. (Technically, what we think of as a transmission line, or wire, is a waveguide supporting a TEM00 mode.)

An antenna is a transducer, in the sense that it mediates between the one-dimensional transmission line mode, and the three-dimensional free space mode; it selects some bandwidth, polarization and direction out of those additional degrees of freedom.

So, bringing that all together:

• There are myriad waves (in whatever way we mean "wave") in the air, from various sources: noise, atmospheric, transmitters both intentional and not, and outer space; travelling in every which direction (some more than others).
• An antenna allows some of those waves to be selected (based on direction, frequency and polarization); how much, is part of the transmitter/receiver system design, but understand antennas can range from narrow (highly selective) to ultrawideband (10x or more), so it depends entirely on the type chosen, transceiver operating range, etc.
• The signal in the feed line, at the transmitter output / receiver input port, or wherever, has a voltage or current that represents the superposition of all signals coupled by the antenna.

The conga line model isn't out and out wrong, in certain respects, but it can lead to some strange interpretations if taken too seriously. For example, what agency has an "electron" here, is it carrying its own tune? Is the whole line cooperating for one rhythm at a time, or what?

The most meaningful interpretation, I suppose, would be if we suppose each dancer can shake their booty in a whole superposition of beats -- a fast vibration here, a slow twerk there -- all at the same time. The combination of vibrations travels up and down the conga line at a fixed speed, with (essentially) no change in amplitude from point to point. But now, it doesn't matter that they're dancers, they have no agency, no personality, it could be marbles in a channel, or water molecules in a pipe; or more accurately still, particles in a gas (technically, a metal contains an electron gas).

To put a finer point on it: wires, transmission lines, like the [free] EM field they harness, are linear: superposition applies, and myriad signals can flow simultaneously, in either direction, without interacting. The signals are carried as fluctuations or vibrations in the EM field around the conductor, which can be analogized to the wave motion of a fluid.

Answer 2

An antenna has a specified length. This length filters out all waves with wavelengths that do not match the length of the antenna.

Think of a tunnel with a complex cross section. Many cars with different cross sections can drive towards that tunnel simultaneously, but only the cars with matching cross sections can enter the tunnel.

Answer 3

Surely the movement of electrons back and forth would be influenced by all the electromagnetic frequencies hitting the antenna that their back and forth movement would no longer reflect a single (useful) signal?

Yes. That's what happens. An antenna will pick up signals more strongly at frequencies at which it is resonant but in general an antenna will pick up lots of signals (to varying degrees) across a wide frequency range. Picking out a single wanted signal from the possibly vast range picked up by the antenna is the job of the receiver, not the antenna.

A tip: don't concentrate on what the electrons are doing. It seems to be a common beginners' trait to get tied up in knots trying to work out what electrons are up to. Learn to think in terms of currents and voltages first. You can worry about electron behaviour later, when necessary (which turns out not to be very often).

Answer 4

Electrons moving back and forth is OK. They move because the electromagnetic wave is a traveling, oscillating electric field (paired with a traveling oscillating magnetic field). Electric fields make charges want to move. Oscillating fields make them want to move back and forth. In the case of copper wire, the charges are able to move back and forth because copper is a good conductor.

But antennas are selective. They do not receive all the signals in the world. They receive signals in certain frequency ranges they have been tuned to receive. They are resonant. They only receive signals that excite their resonance (signals that are on the right frequency).

After the antenna you go to a radio receiver. The antenna is somewhat selective. A good receiver is like a world champion at selectivity. It can ONLY hear the specific signal it is tuned to hear. Everything else is suppressed to a truly phenomenal extent using the magic of superheterodyne. Like being in a room so soundproof that a jet engine outside is just a quiet hiss.

Answer 5

A moving element can hold in the same time numerous different motions which are all summed together. Think yourself having a bad hangover, every limb trembles uncontrollably say half an inch here and there. But you can still get up and walk to somewhere to get a remedy.

The trembling continues, but somehow you still succeed at the same time to walk to the wanted direction. Someone can notice from your complex combined motion only the trembling and think "Thanks God, I'm not in that condition!"

The barman in the place where you finally enter notices the familiar customer coming in and says "Welcome, Sir! Nice weather today! How can I help you? The usual?" He ignores your trembling totally, he is interested in only that part of your quite complex motion that brings the paying customer in. If he noticed the trembling he would add "With a straw, I presume!"

Different electromagnetic fields from different radio stations can cause to the same electrons in the same radio antenna several simultaneous movements (=currents) which all exist at the same time. The total current is the sum of the separate motion components. Cleverly designed circuits filter apart the wanted current generally based on the frequency. If you turn the tuning knob of a radio receiver you generally change the frequency that the receiver accepts. Rest is filtered out.

Answer 6

The antenna parameters have to match the wavelength of the signal it has to receive as Stefan Wyss had posted. Another thing is that any received signal might have very low signal strength. It has to be passed through a bandpass filter and amplified to get frequencies of the signal your electronic circuit uses This also removes any noise signals.

Answer 7

You are making things more complicated than they have to be. You are looking in the wrong place for an answer. Yes, there is an orgy of signals present on any antenna. You only care about one of them. It is easy to find; use filters built inside the radio receiver. The antenna has little to do with it. Most of the work happens inside the radio by signal processing.

Radios have a specification called selectivity, which measures the performance of a radio receiver to respond only to the radio signal it is tuned to and reject other signals nearby in frequency. Radios use several stages to filter a frequency of interest. The filter bandwidth and rejection ratio are so high the detector circuit cannot detect any other signal.

To a lesser extent, the antenna can act as a shallow filter by adjusting the resonant length and augmenting the radio’s selectivity. Resonance is important if transmitting, not so much receiving. Think of it like your spouse, and you are watching a game on TV. Your spouse blathers on, but you don’t hear a word of it; you filter it out and only hear the TV.

Answer 8

I think you ascribe too much meaning to electrons. It's like asking yourself how the same air molecules can convey all the instruments of a symphonic orchestra together. The air molecules don't really travel all that fast. And electrons are actually glacially slow. What either of them do is to react to pressure by passing it on to other particles, and that passing on happens quite fast and predictably according to wave propagation equations. And if different instruments create waves of different (or even equal) frequency, all those waves add up and pass through the same medium independent of one another.

At least in a very good approximation unless we are talking of signals so large that the signal propagation breaks down.