ArtemisGoldfish

I understand things like waveforms and frequencies quite well, but I can't find something that will adequately explain how we get the radio waves in the air and get it back out. In a receiver, does the antenna receive a tiny voltage that is amplified to be useful? And how do we make a line voltage or waveform come out of an antenna? My brain still only thinks that it would act like an open circuit, but it obviously doesn't. What part of a radio transmitter makes it able to turn voltage signals into radio waves?

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Leftyretro

Actually you have asked a very good question. The mechanism is not intuitive at all and I won't even attempt to try and make an understandable answer, just to say that the antenna does indeed act as a transducer to change electrical AC current into electromagnetic radiation on transmission and the reverse for receiving. Also keep in mind an antenna can just be a simple run of PC trace or the wire portion of a component, not always efficient but able to radiate or pick up RF (radio frequencies) none the less. That's why EMI (electromagnetic interference) generation and rejection can be such a tough problem to deal with on a design and construction basis.

Lefty

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k7elp60

To add to what Leftyretro has said. The transmitting antenna radiates both a electromagnetic wave.The transmitting antenna also produces an electrostatic wave. This wave is at a right angle from the electromagnetic wave.
The electromagnetic travels the greated distance.
Another thing is that the antenna matches the impedance of the transmission line to the impedance of air.
The antenna can be visualized like Leftyretro said as a transducer. In a audio speaker the speaker is also a transducer, it converts an AC wave to sound, and a microphne converts sound waves to AC waves.

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dknguyen

I do not know. It's one of the most mysterious things in circuits to me- both how a an antenna radiates and receives an EM wave (I'm sure I've been taught and know the physics of it, but I can't seem to put it all together), as well as how a circuit is supposed to pick up the "dead end" on a circuit that is a normal dipole antenna. Maybe this will help...

Antenna - How One Works

But since I do not know myself, I cannot vouche for any inaccuracies. I was hoping my antennas course next semester would shed some light.

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k7elp60

I just had some more thoughts on the subject. The electromagnetic wave produced by the transmitting antenna travels thru space to the receiving antenna. The receiving antenna is a conductor and because it is in the magnetic field a voltage is induced in the wire. Granted the voltage level depends on a lot of factors, but is generally in the uV level, or mV level. Several of the factors that determine the amplitude of the received signal are the transmitting power level and distance between the transmitting antenna and the receiving antenna.
I have personally measured received levels in the US AM broadcast band in the mV and V levels on a receiving antenna.

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Sceadwian

It's not all that complex to understand it intuitively at least on a course level. Changing electric fields cause changing magnetic fields and vice versa, that's why they're called electro magnetic waves, you can basically think of them as an electric field started at an antenna and collapsing causing a magnetic field to be induced in the air near it which collapses into an electric field etc.. etc.. if the shape of the antenna is correct for it's medium (free air usually) the energy is transmitted, some structures of antenna (Yaji uda, biquad) transmit/receive better in one direction and have nulls outside of that area.

You may notice if you've ever taken apart a cable box or an Ethernet card that there are usually metal shields around the main RF processing chips, this is because at high frequencies even short pieces of wire (like the PCB traces or the internal chip traces themselves) act as antenna radiating RF, so they use the metal/foil to keep the chips which use frequencies that aren't legal to broadcast over the air from reaching the world. In a close system with matched transmissions lines RF never leaves the system so you can use pretty much any frequency you want.

I don't understand the truly basic physics of it, mainly because the physics of electromagnetic wave interaction with physical matter is VERY poorly understood. A lot of people have trouble with the fact that the light we all see and the heat you can feel from a bon fire are actually exactly the same thing as the radio waves that antenna put out and pick up, just at drastically difference frequencies, different materials and structures of materials react differently with various frequencies of EM waves.

A good antenna to try to understand is a simple half wavelength dipole. Which is two elements heading in opposite direction to each 1/4 wavelength of the frequency of interest long (these are basic RF terms look them up on Wikipedia or google)

Something else people might not really relate RF in real life is that most energy transfer from light heat etc.. is from the molecular and atomic structures of matter acting as an antenna for the truly high frequencies that nature deals with. The highest RF frequency most people are familiar with is Wi-fi at around 2.4GHZ though some cordless phones use 5+ghz, the waves are 130-60 centimeters long. Just for relation visible light is around 380-750 nanometers, which is around 540 Terahertz. Gamma rays come in at 300Exahertz and the wavelength is only 1 picometer. At the other end of the spectrum, many amateur radio operators on earth use a band around 3-30mhz to communicate with which is between 10 and 100 meters. Read the above description of a half wave dipole and you'll see why HAM operators are sometimes seen as weirdos by their neighboors trying to put up antennas. But those frequencies get a lot of range because the bounce of ionized layers of the atmosphere allowing people to communicate over the horrion (even across the world)

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Papabravo

The basic understanding comes from the distributed parameter model of a transmission line and the associated solution to the partial differential equation with boundry conditions.

Reflection and absorption are obvious consequences of the termination of the transmission line. Terminations can be open, short or any complex impedance you care to name. A good antenna for a given frequency, with no termination, will match the output of the source(transmitter) to the impedance of free space so that there is little or no reflection back to the source.

A good transmitting antenna will also work wonders for receiving because it is resonant at the frequency of interest and has a response that drops off slowly enough to be useful for a band of frequencies.

The freeware program EZNEC can be used to model antennas and make pictures of what is happening. I recommend it.

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RadioRon

One way to look at the problem is simplified to this: in an antenna, an AC current is flowing. The current creates a magnetic field. As the current is increasing and decreasing (its AC remember), the magnetic field it is creating on each cycle pushes the field created by the previous cycle outwards to make room for the new field. So the magnetic fields are pushed off into the distance in the form of concentric rings.

This link has a pretty interesting explanation of this process:
ARRLWeb: Why an Antenna Radiates

So, the current creates an AC magnetic field that pushes itself away from the antenna. Along with the current, there is also a voltage in the antenna, and it creates an electric field that takes off with that magnetic field (magnetic fields and electric fields like to hang out together as they are very closely related). You may recall that when you have a voltage and a current in a wire, you can describe the wire as having a resistance and there is power flowing in the wire, measurable in watts. Well, its the same sort of thing in fields. An electric field (the voltage) and a magnetic field (the current) which push away from an antenna represent a flow of power (power = voltage x current in a wire, same with the fields in space). So what actually transfers through space is power, not voltage or current alone. In fact, we even know what the "resistance" of space is to those fields, it is about 377 ohms (link: Impedance of free space - Wikipedia, the free encyclopedia ).

Since the thing that transfers through space is power, antenna designers who are making receiving antennas focus a lot on how to couple both fields back into the receiving antenna and then piping that power that is generated in the wire into the coax that goes down to the receiver. So, we do our best to receive power, in watts, not just volts or amps. As a result, impedance matching for maximum power transfer is also a big deal for antenna designers ( link: Impedance matching - Wikipedia, the free encyclopedia ).

When you want to receive a radio wave, you put your antenna up into the sky where it can be bathed in the incoming radio waves. As the fields of the radio wave surround the receiving antenna, the magnetic field induces a current into the antenna wire and the electric field that is travelling along with the magnetic field induces a voltage on the antenna wire. Technically, the way this happens is exactly the reverse of the process i described in the explanation for the transmitting antenna that I gave above.

The amount of power that you get out of an antenna can be very low, for example, 0.000000000000001 watts (or, say, -120 dBm). To make things a bit easier for designers, we have standardized the input resistance of most receivers to be 50 ohms (although there are many exceptions to this), and knowing this and the power, you can then calculate what voltage and what current will appear at the input to the receiver from an antenna. The amounts are very very small. I mean really really really small. Voltages and currents this small are only a tiny bit larger than the natural ac noise that occurs as electrons vibrate randomly due to heat. It is this natural vibration of electrons that limits how good a receiver we can make, that is, how sensitive a receiver can be, because we typically need our desired signal to be a bit stronger than that "thermal noise" to understand the signal. This thermal noise is what we refer to as the "noise floor" of a receiver, a floor below which we cannot understand any signals.

As I implied earlier, the antenna designer that is building a transmitting antenna is focussed on how to get as much current as possible flowing in the antenna wire because the more intense the current in the wire, the stronger the radiating fields are. There is, again, a huge focus on impedance matching in the design process.

One more interesting fact. Antenna designers that are designing transmitting antennas and those that are designing receiving antennas are really one and the same person. It turns out that in most common antennas if they are good at transmitting, they are also good at receiving, as long as the frequency in each direction is about the same.

Well, that's the short version. Do you want the long version?

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Mikebits

I was thinking, with a name like RadioRon you ought to have a good explanation, and I must say you lived up to your name. Well done.

3iMaJ

You're referring to the electromagnetic property called reciprocity. The transmit pattern of an antenna is identical to the receive pattern. Assuming of course all else has remained the same. Which is clear from the description of the property.

Reciprocity (electromagnetism) - Wikipedia, the free encyclopedia

Space Varmint

Start with basics... A frequency has a wavelength. It is determined by the speed of light which is also the speed of an electromagnetic wave. The speed of light is 300,000,000 meters per second. This is what is used to cut an antenna to resonance (the transmit/receive) frequency. So if I want an antenna to resonate at 7 MHz, the frequency wave length will be 40 meters. 300 / 7 = 42.86 meters.

Normally we do not use a full wavelength but a half wavelength will also resonate at the given frequency. We call this a dipole for 1/2 wavelength. There are several reasons for using a dipole as opposed to a full wavelength. It has to do with the phase relationship between voltage and current along an antenna wire. I won't get too elaborate but you may want to source some diagrams showing the voltage/current relationships through various wavelengths. One very obvious reason for using a dipole is that if the dipole is center fed meaning the two transmission line wires are connected to two 1/4 wavelength pieces of wire not connected to each other, than we have a very high current at this point and a very low voltage which means it is a low impedance. In a transmitter with any significant power this is desirable because we do not want a high impedance at the output of the final amplifier or it will be highly prone to feeding back into one of the driver stages or even the oscillator. So quite often you will see transmission lines rated at about 50 ohms to match the antenna impedance.
On the receiver side the high current at the center feed point of the dipole antenna also has proven to be optimal for maximum sensitivity.

quixotron

Right matching 75 ohms, the impedance of a half-wave cennter fed dipole is much easier to match to a piece of 50 ohm coax. the wavelength is in freespace, on pcb's, the wavelength decrease as the inverse of the square root of the dielectric constant.

for a half-wave, the current is strongest at the center and zero at the ends, this makes sense since the antenna is open-ended and you get a reflection. a dipole antenna, is thus a standing-wave antenna.

Space Varmint

That's very interesting quixotron. I did not know that rule of thumb about PCBs. Also a wavelength is shorter in a piece of wire than in free space. There is another rule of thumb for calculating the length of a dipole. A half-wave in a wire can be calculated by dividing the frequency in MHz into 468 to get the length in feet. A half-wave in free space is calculated by dividing the frequency in MHz into 492.

Example: 7 MHz---------468 / 7 = 66.86 ft

Sceadwian

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Space Varmint

OK, come to think of it I did run across that as it pertains to transmission lines. I have dabbled in them a little bit. A transmission line is no more than two wires equally spaced throughout the length of the two wires to maintain a constant impedance. One wire of course being the signal and the other, ground. So insulating spacers can be used to maintain the distance between them. This is real latter line and uses air as the dielectric. If the two wires are plastic coated with the spacing maintained throughout the length of the wires, the plastic will have a higher dielectric constant so the spacing between the wires will be smaller for the same impedance. You can still get transmission line like this. It is called "twin lead" and it can be obtained from Belden. You might ask why would you want either of the two types mentioned above? Because they are balanced. A coaxial cable which has a center conductor with a braided shield conductor on the outside separated by I believe they use polyurethane, is unbalanced. For optimal transmission line through put the unbalance condition should be corrected to reduce SWR (Standing Wave Ratio). This can be done with "Balun" core transformers which is short for balanced-to-unbalanced.

An interesting thing to note about real latter line is that in the case say, where a dipole is center fed having an impedance of 75 ohms as stated above, the latter line spacing can be gradually increased to created a better match between the 50 ohm line and 75 ohm antenna. You usually want to start about 1/4 wavelength before the antenna. This brings up a whole other topic known as quarter wave matching stubs.

By the way...anyone interested in this topic can get a very firm grip on it by getting "The Radio Amateurs Hand Book" published every year by the ARRL (American Radio Relay League). I recommend trying to get an older manual because as they have kept up with modern day electronics as it pertains to radio, you will not find as much detail as before in radio theory basics.