How does a Radio Antenna Work?

In summary: When you say "efficient" do you mean, that one antenna design vs another may generate more photons, or that the photons shoot off in a direction that makes them more usable for the receiver?People don't really think of radio-frequency radiation in terms of discrete particles -- they typically use the wave model instead, as it's much easier to use. An efficient antenna produces a large-amplitude EM wave for a given feed power, and produces little heat. An inefficient antenna produces a small-amplitude EM wave for the same feed power, and converts most of the power into heat.
  • #1
darrink1
2
0
How does a Radio Antenna Work?
I'm trying to get a grasp of the physical mech.

You generate an AC signal on a conductor (i.e. at 30mhz)
The free electrons on the skin of the conductor osc. back/forth at 30mhz.
The acceleration / decleration of the free electrons causes photons to be released from the free electrons (Photons being the E field)?
The acc / decl will only be large enough to cause photons to be released, if the conductor is long enough to accommodate at least 1/4 wavelength?
 
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  • #2
Photons are "released" anytime an electron oscillates, regardless of the length of the antenna. The only difference is the efficiency with which electrical power is converted into EM radiation. A better antenna -- with the correct length for the frequency in use -- is a more efficient radiator.

- Warren
 
  • #3
I take it you're discussing transmitters, how does a receiving antenna work?
 
  • #4
chroot said:
Photons are "released" anytime an electron oscillates, regardless of the length of the antenna. The only difference is the efficiency with which electrical power is converted into EM radiation. A better antenna -- with the correct length for the frequency in use -- is a more efficient radiator.

- Warren

When you say "efficient" do you mean, that one antenna design vs another may generate more photons, or that the photons shoot off in a direction that makes them more usable for the receiver?
 
  • #5
People don't really think of radio-frequency radiation in terms of discrete particles -- they typically use the wave model instead, as it's much easier to use. An efficient antenna produces a large-amplitude EM wave for a given feed power, and produces little heat. An inefficient antenna produces a small-amplitude EM wave for the same feed power, and converts most of the power into heat.

- Warren
 
  • #6
Here's a reasonable intro to antennas:

http://en.wikipedia.org/wiki/Antenna_(radio)

By "efficiency", we mainly are referring to the "radiation efficiency" of the antenna, which is how much power is actually radiated into the far field and beyond, versus what the input power to the antenna is. If you plot the input impedance of an antenna versus frequency, you will see that it has a very reactive impedance at frequencies away from its resonance(s), which means that the power is not being launched off the antenna. At the resonance(s), you get a mostly real input impedance, which means that the power is going away somewhere, and that somewhere is mostly radiation away from the antenna. The main resonance for a "half-wave dipole" antenna is where each of the two dipole elements is 1/4 of a wavelength in free space. The main resonance for a "quarter-wave monopole" is where the single monopole element (usually vertical) is 1/4 of a wavelength, placed above a ground plane. The input impedance of a half-wave dipole antenna at resonance is about 75+j0 Ohms.

It is common to use a Standing Wave Ratio (SWR) meter when tuning up antennas. An SWR of 1.0 means that the antenna is perfectly matched to the feed amp and coax, and the radiation efficiency is 1.0. SWRs of >1.0 mean that there is a mismatch, and you are losing radiation efficiency.
 
  • #7
What decides in a material what frequenzy the light waves emitted after it absorbs a photon?

It doesn't send out the same light frequenzy as it absorbs, and some goes to produce heat. I wonder what in the material that decides it.
 
  • #8
Oh, and as far as receiving, the same resonance issues apply. It's just that when receiving, the currents induced on the antenna by the passing EM field cause a terminal voltage at the feedpoint of the antenna, which generates a propagating EM wave down the coax to the receiver's input amp circuit. Of course, these receive signals vary tremendously in amplitude, so the receive circuit will use Automatic Gain Control (AGC) to deal with the large dynamic range, and will generally use low-noise circuit design techniques (especially sharp bandpass filters) to be able to recover faint receive signals.
 
  • #9
billiards said:
I take it you're discussing transmitters, how does a receiving antenna work?
Take the same equations, but everywhere you see a "t" write "(-t)". :biggrin:
 

1. How does a radio antenna pick up signals?

A radio antenna works by converting electromagnetic waves, also known as radio waves, into electrical signals. These waves are created by the radio station's transmitter and travel through the air until they reach the antenna. The antenna then captures the waves and converts them into an electrical current, which is then amplified and converted into sound by the radio receiver.

2. Why are radio antennas typically long and thin?

The length and thickness of a radio antenna are directly related to the frequency of the radio waves it is designed to receive. In radio communication, the size of the antenna must match the wavelength of the radio waves in order to effectively capture and transmit the signals. This is why longer antennas are used for lower frequency signals, and shorter antennas for higher frequency signals.

3. How does the shape of a radio antenna affect its performance?

The shape of a radio antenna can greatly impact its performance. For example, a dipole antenna, which has two rods extending in opposite directions, is more effective at receiving signals from the sides and not from the ends. On the other hand, a loop antenna, which is a circular shape, is more efficient at picking up signals from all directions.

4. Can a radio antenna receive signals from multiple stations at the same time?

Yes, a radio antenna can pick up signals from multiple stations at the same time. This is because radio waves travel through the air and can be captured by the antenna simultaneously. However, the radio receiver must be able to differentiate between the different signals and tune into the desired station.

5. How does the location of a radio antenna affect its performance?

The location of a radio antenna can greatly affect its performance. Ideally, the antenna should be placed in an area with minimal obstructions, such as buildings or trees, in order to receive a strong and clear signal. Additionally, the height of the antenna can also impact its performance, as a higher antenna can receive signals from a greater distance.

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