What is the correlation between the frequency and the strenght of an EM wave?

[szputnyik]

I've been studying the electromagnetic spectrum, and I read that the frequency of an EM wave depends on its strenght. However if that is the case, how come there is an FM broadcasting station the broadcasts on 98 MHz that is strong enough to be received at a distance of 20km and there is another one broadcasting at 103 MHz that is weaker and can only be received at a distance of 5km? How come there are bright, high-energy red-colored lights, and weak UV lights?

[Nuke]

i assume by strength you mean amplitude.

amplitude in this case depends on transmit power, how much amplification the signal has before being broadcast. its entirely possible they are pumping more power into the 98mhz antennae than they are into the 103 mhz antenna. to use sound as an analog, the 98mhz station has its volume way up. those two frequencies really close on the em spectrum and the waveforms would have very similar properties.

im probibly not the best radio person, but i do know that when buying rf modules to look at both frequency band and tx power. more tx power more range. frequency is usually limited to a few bands, due to fcc regs, but can determine things like permeability of materials, data rate, antenna size, and other stuff.

Edited July 27, 2014 by Nuke

[WestAir]

I'd imagine the "strength" of an EMW and the "type" of wave, are two separate beasts. Frequency determines the type, and the amount of energy in the photon (or the number of them?) would determine the actual strength.

For instance, I'd bet any and every wave with a frequency measured in milometers will always be a radio wave or microwave (therefor non-ionizing, irregardless of strength), while throwing more energy into it will decrease the rate of its attenuation, thereby increasing its range.

I'm no EM expert, but this is my uneducated guess as to how it works. I imagine it's the same as having a G-String on a Violin. You can pluck it so softly you can barely hear it, or you can pluck it hard enough to be heard throughout an entire auditorium - in both cases the sound-wave heard is G.

[xcorps]

[maltesh]

I'd imagine the "strength" of an EMW and the "type" of wave, are two separate beasts. Frequency determines the type, and the amount of energy in the photon (or the number of them?) would determine the actual strength.

For instance, I'd bet any and every wave with a frequency measured in milometers will always be a radio wave or microwave (therefor non-ionizing, irregardless of strength), while throwing more energy into it will decrease the rate of its attenuation, thereby increasing its range.

I'm no EM expert, but this is my uneducated guess as to how it works. I imagine it's the same as having a G-String on a Violin. You can pluck it so softly you can barely hear it, or you can pluck it hard enough to be heard throughout an entire auditorium - in both cases the sound-wave heard is G.

The frequency and energy of photons are inextricably linked; the higher the energy of an individual photon, the higher its frequency and vice versa.

Increasing the power of your radio broadcast without increasing its frequency means that your transmitter is emitting radio photons at a higher rate, meaning it's more likely that at least some of them get through to the receiver.

[Nuke]

now were getting into the realm of particle physics.

[GoSlash27]

I've been studying the electromagnetic spectrum, and I read that the frequency of an EM wave depends on its strenght. However if that is the case, how come there is an FM broadcasting station the broadcasts on 98 MHz that is strong enough to be received at a distance of 20km and there is another one broadcasting at 103 MHz that is weaker and can only be received at a distance of 5km? How come there are bright, high-energy red-colored lights, and weak UV lights?

The energy contained within an EM wave is proportional to it's frequency, but energy isn't the same thing as power, which is what you're talking about.

Best,

-Slashy

[xcorps]

The frequency and energy of photons are inextricably linked; the higher the energy of an individual photon, the higher its frequency and vice versa.

Increasing the power of your radio broadcast without increasing its frequency means that your transmitter is emitting radio photons at a higher rate, meaning it's more likely that at least some of them get through to the receiver.

Radio broadcast is more than energy. You have gain, you have altitude of antenna, you have transmitter power. Transmitter power is NOT the same thing as the power of the wave.

A transmitter operating at 150kw on 108Mhz has the same wavelength as a transmitter operating at 1kw on 108Mhz. The amplifier determines the range, the oscillilator the freguency, the modulator the information.

[magnemoe]

I've been studying the electromagnetic spectrum, and I read that the frequency of an EM wave depends on its strenght. However if that is the case, how come there is an FM broadcasting station the broadcasts on 98 MHz that is strong enough to be received at a distance of 20km and there is another one broadcasting at 103 MHz that is weaker and can only be received at a distance of 5km? How come there are bright, high-energy red-colored lights, and weak UV lights?

if you talk about FM radio this is mostly about broadcast strength and antenna configuration. difference between 103 and 98 MHz is minimal. My guess is that the 103 is a local radio, in the old days FM was just up to 100 MHz and then expanded so many small stations use the high bands.

Now if you go up to the 1000 MHz bands you usually get shorter ranges. Again this is part policy, you want many small transmitters as it increases the total bandwidth a lot. The high frequenzies also has far worse penetration but higher bandwidth so they are used for wifi and cell phones

[lajoswinkler]

What is "strength" here?

[szputnyik]

By strenght, I mean how far can a radio station broadcast, how bright is a light, or how damaging is an X-ray or gamma ray.

[xcorps]

The distance a radio station can broadcast is determined by multiple factors.

The brightness of a light is determined by multiple factors.

[lajoswinkler]

By strenght, I mean how far can a radio station broadcast, how bright is a light, or how damaging is an X-ray or gamma ray.

As xcorps said, multiple factors, and they aren't the same for radiowaves and gamma rays, as they're really different.

[GoSlash27]

By strenght, I mean how far can a radio station broadcast, how bright is a light, or how damaging is an X-ray or gamma ray.

Broadcast distances and light brightness are both functions of power, while the damage from x-rays and gamma rays is a function of energy density. These are 2 different things.

For broadcast distances and light brightness, the power at the receiver is determined by transmitted power, antenna efficiency (at both ends), attenuation of the medium, and distance.

For damage from x- rays and gamma rays, the damage is ionization at the molecular level due to high energy density of the wave. The higher the frequency, the higher the energy density.

Best,

-Slashy

[GoSlash27]

Okay... somewhat bad analogy, but it should help get the point across.

The formula for kinetic energy is 1/2mv^2. That is, it is proportional to the mass of an object and the square of it's velocity.

Furthermore, every action has an equal and opposite reaction. If you fire a bullet from a rifle, the recoil into your shoulder is exactly the same kinetic energy as is stored in the moving bullet, yet the bullet is much more damaging to the target than the rifle is to you.

Higher energy density.

HtHs,

-Slashy

[SciMan]

Electromagnetic radiation is a bit of a funny beast when it comes to figuring out ranges because above a certain frequency, its less useful to use radio wave equations than it is to use light (photon) equations.

Brightness can actually be determined for both cases.

For radio transmitters, Effective Radiated Power is the specification to look for (IIRC it calculates the antenna's effects on the transmission as well).

And for light, I've seen many units used (candlepower, lux), but they all basically work out to how many photons/second over a specific area at a specific distance.

How much damage the radiation is capable of doing is directly proportional to its frequency and inversely proportional to its wavelength. Photons are usually measured by wavelength instead of energy, but the equation relating the two is actually quite simple.

For: E = energy, h = Planck's constant, c = speed of light in vacuum, λ = wavelength of photon

E = h * c / λ

There are 2 useful versions of Planck's constant out there. Depending on which one you use, the number you get out for energy will have units of either Joule seconds or Electron-Volts. For bulk energy transfer applications use the joule seconds version. If stuff involves emission or absorption spectra, the electron volt version is more convenient.

Here's a wikipedia link for more information about Planck's constant.

Also, here's a Wolfram Alpha link for more information on the specifics of the math involved in calculating the energy of a single photon.

Ultraviolet, X-ray, and Gamma ray photons all have a higher frequency and shorter wavelength than visible light. These photons are damaging because each one contains enough energy to make an atom lose one or more electrons if it absorbs one. That's why any EM radiation of ultraviolet or shorter wavelength is called "ionizing radiation". The ionization is a problem because it can break and change chemical bonds. The most obvious consequence of that to life is DNA damage. Usually that happens because an ionized a water molecule splits into OH- and H+ ions. These are called free radicals, and are what actually damage cellular mechanisms.

Here's an interesting application of the dual wave/particle nature of light.

If an antenna is small enough, it's resonant frequency will be in the range of visible light. This is called an Optical antenna.

Adding a diode across the terminals of that antenna creates an Optical Rectenna, and arrays of those could be more efficient than solar panels according to some predictions.

However, Nanotechnology would need to be used to be able to make small enough antennas and diodes.

If the antennas were able to be used as transmitters as well as receivers, you end up with what's called an Optical Phased Array.

The concept is the same as the AESA (Active Electronically Scanned Array) radars used in modern fighter jets, but applied to light frequencies. With enough computer processing power to do the math, mature Optical Phased Array technology would enable things like gigapixel displays and image sensors the size of a postage stamp, powerful lasers that can be aimed and focused with no moving parts, and optical camouflage.

Of course, the computations needed for the optical camouflage application take time, so there would be at least some delay between the image it displays and the actual background. It's not a "perfect" invisibility cloak, but it would work well for stationary or slowly moving things.

An Optical Phased Array should be capable of operating on any lower frequency as well.

It's amazing what electronics can do with small enough feature sizes.

Edited July 27, 2014 by SciMan

[lajoswinkler]

Broadcast distances and light brightness are both functions of power, while the damage from x-rays and gamma rays is a function of energy density. These are 2 different things.

For broadcast distances and light brightness, the power at the receiver is determined by transmitted power, antenna efficiency (at both ends), attenuation of the medium, and distance.

For damage from x- rays and gamma rays, the damage is ionization at the molecular level due to high energy density of the wave. The higher the frequency, the higher the energy density.

Best,

-Slashy

Not really. You can get hit by one gamma ray per second, or a million of them per second. We already established that we're talking about fixed frequencies for both gamma and radiowaves.

[ziogatto]

Well for radio waves it depends on what the medium you're propagating trough absorbs. In free space there is no absortion so all frequencies will have the same "strength" at a certain distance given the same power budget.

The thing you're asking is probably this: http://en.wikipedia.org/wiki/Free-space_path_loss

That formula has a dependency to frequency but only to compensate the implicit definition of the antenna. Basically you get more power at lower frequencies only because your antenna is bigger and absorbs more power. If you instead keep the same antenna then you'll find no dependency on frequency.

If you however propagate trough a medium then it depends on what you're propagating trough. The relationships are usually rather complex.

For example the losses due to propagating trough water are like so:

hence why submarines tend to use very low frequencies for communication.