# EM types

#### mentaaal

Joined Oct 17, 2005
451
hey guys, i have always been interested in the concept of electromagnetic radiation and have been trying to understand it. I havent as yet been able to get information which explains it in any kind of great detail. i have learned the basics like a changing electric field creates a changing magnetic field and all that but what has really mystified me is the differenct types of electromagnetic radiation in that a wave can have one of the fields in the direction of propagation? i saw that there were a few different types and was referenced in info i was looking at describing the use of waveguides.

could anyone explain this to me?

cheers

#### Salgat

Joined Dec 23, 2006
218
I suggest using Wikipedia, especially look into photons(my favorite particle).

#### Dave

Joined Nov 17, 2003
6,969
All types of EM waves (radio through to gamma) within the EM spectrum are radiation (energy that spreads within a medium). The essential difference between the different waves is the frequency of the EM wave, and consequently the wavelenth: wavelength = (1/permittivity)*(speed of light/frequency).

Due to the wave-particle duality, the EM wave can be considered as a wave (orthogonal E and H fields oscillating in space) or particles. The particle model is quite important in demonstrating a further difference between the differing types of EM waves, that being the energy of a photon (the elementary particle responsible for electromagnetic phenomena), where Energy = frequency*(Planck's constant). Therefore, the higher the frequency the more energy the particle the has.

Its a very crude overview, but explains the main differences. I would suggest you look at any specifics you may have and ask about them.

Dave

#### mentaaal

Joined Oct 17, 2005
451
well thats fair enough but what i am curious about is the different types of waveforms there are. like there are tm waves and te waves (the latter where the electric field is oscillating in the direction of propagation? That makes a f no sence to me at all!

Help!

#### Dave

Joined Nov 17, 2003
6,969
well thats fair enough but what i am curious about is the different types of waveforms there are. like there are tm waves and te waves (the latter where the electric field is oscillating in the direction of propagation? That makes a f no sence to me at all!

Help!
You wouldn't be the first confused by this!

What you are referring to are "Transverse Modes". These occur in waveguide structures (not exclusively, but I am most familiar with waveguides so will talk exclusively about that) because of the boundary conditions imposed on the travelling waves within the waveguide structure. As the waves propagate down the waveguide a sanding wave pattern builds up within the waveguide due to the effect of the boundary conditions imposed by the waveguide walls. This results in the establishment of a field (E or H) intensity pattern in the traverse plane (i.e. the plane perpendiular to the direction of wave travel).

This results in the TE, TM and TEM mode patterns that you may have come across.

Dave

#### mentaaal

Joined Oct 17, 2005
451
cheers dave, so then the wave, as it leaves the waveguide the wave would propagate as normal? both the magnetic and electric fields in a transverse fashion and in phase?

#### Dave

Joined Nov 17, 2003
6,969
cheers dave, so then the wave, as it leaves the waveguide the wave would propagate as normal? both the magnetic and electric fields in a transverse fashion and in phase?
Remember the EM wave (singular) is propagating "as normal", the transverse mode is referring to the establishment of a standing wave pattern - think of how a standing wave establishes on a TM line (ref. http://www.allaboutcircuits.com/vol_2/chpt_14/6.html). Once the wave leaves the waveguide into say a closed cavity a new standing wave pattern will establish within the cavity based on the boundary conditions imposed by the cavity walls, else the wave will scatter into the open space "as normal".

Dave

#### mentaaal

Joined Oct 17, 2005
451
makes loads more sence now! i am wandering this as i have seen people post tutorials on making waveguides for the wireless routers and such out of metal cans etc.

cheers!

#### Dave

Joined Nov 17, 2003
6,969
makes loads more sence now! i am wandering this as i have seen people post tutorials on making waveguides for the wireless routers and such out of metal cans etc.

cheers!
No problem, I know how confusing the world of electromagnetics can be!

Dave

#### mentaaal

Joined Oct 17, 2005
451
No problem, I know how confusing the world of electromagnetics can be!

Dave
ok if a standing wave gets set up in the waveguide... how is this not a bad thing? I mean if info is being bounced back as part of the standing wave... would that not interfere with other information being sent out? obviously it isnt a problem as this is happening in waveguides but why isnt it a problem?

#### Dave

Joined Nov 17, 2003
6,969
ok if a standing wave gets set up in the waveguide... how is this not a bad thing? I mean if info is being bounced back as part of the standing wave... would that not interfere with other information being sent out? obviously it isnt a problem as this is happening in waveguides but why isnt it a problem?
The standing wave is purely field intensity (energy if you will) - there is no information impressed on the wave. A waveguide does exact what it says - guides waves (EM, acoustic etc). You are thinking about modulation, where a message (information) is impressed on the wave for transmission. Whilst this exploits EM waves as a transmission mechanism, there are much wider issues at hand here.

Dave