EM wave / gas interference?

March 15, 2006 1:45 PM

Can a gas interfere with or block an electromagnetic wave?

A friend is convinced that the argon in the dual-pane windows in his solarium is responsible for his lack of XM Radio signal. When his antenna is in the solarium with all windows closed, he gets zero reception. When he opens windows, he gets full bars.

I'm of the opinon that the gas is blameless and it's more likely a coating on the glass that's at fault.

If it is possible, can you explain what actually occurs in the interaction btw the gas and wave to absorb or reflect the signal? I took three semesters ('95 - '96) of physics in college, so feel free to get technical. (I did switch majors, though....)

posted by pmbuko to Science & Nature (16 answers total)

The reflective coating on glass reflects RF, and attenuates the signal. Simple as that.

(FWIW, I'm a (software) engineer for a company making microwave radio products)
posted by kableh at 1:51 PM on March 15, 2006

Well, if you took three semesters of physics you should know more then me, but can't all atoms absorb and release photons at the right frequency for that element? If XM is broadcast at the right frequency then argon could do something too it. Of course, it's much less dense if it's a gas (Am I understanding you correctly that between the two panes of glass there is argon? Or is there argon fused into the glass itself?)

Obviously some gasses can reflect or obscure visible light, and visible light is RF.
posted by delmoi at 2:14 PM on March 15, 2006

The sky is blue, right? That's called diffraction.
posted by ikkyu2 at 2:14 PM on March 15, 2006

Shortwave and AM radio bounce off the gases in the ionosphere, so yes, gasses can interfere. Plus what ikkyu2 said.
posted by knave at 3:00 PM on March 15, 2006

Not what ikkyu said.

The gas doesn't make the sky blue, particles suspended in the atmosphere do.
posted by empath at 3:09 PM on March 15, 2006

I'm with kableh. Also, it's conceivable that they put a polarizing layer on the glass, which is going to filter out most of the signal.

Argon's not going to have electromagnetic properties that are drastically different than the air the waves are travelling through in the first place. It's only when you get up into the ionosphere that things start behaving differently.
posted by dsword at 3:12 PM on March 15, 2006

Yes, for instance, the atmosphere blocks most ultraviolet. But, the argon in that window is only 1/4" thick, the radio waves go through that much argon traveling 400 inches through the atmosphere where argon is 1%. Probably the metal frame. Radio waves can be blocked by a metal lattice work smaller than the wavelength (like the mesh in a microwave oven window blocks the ~10 cm wavelength microwaves). Not sure what wavelength XM uses.
posted by 445supermag at 3:15 PM on March 15, 2006

ikkyu2 writes "The sky is blue, right? That's called diffraction."

Actually, it's Rayleigh scattering. I guarantee you that argon atoms are not scattering radio waves.
posted by mr_roboto at 3:17 PM on March 15, 2006

The physics of this is going to require some pretty detailed knowledge of the material science of argon and EM signals. I doubt most of us are qualified. I bet kableh is correct.

(I took considerably more than 3 semesters of physics, and we never got anywhere near answering questions this specific).
posted by teece at 3:19 PM on March 15, 2006

empath: Air molecules do make the sky blue. Dust in the atmosphere does contribute somewhat (and is responsible for the red color of Mars' atmosphere).
posted by lukemeister at 3:56 PM on March 15, 2006

Best answer: The fact that the ionosphere causes medium to long wavelength radio waves to scatter and travel across long distances is due to the fact that the atmosphere gets ionized by the intense solar radiaton that is incident on the earth. This ionized gas layer and the surface of the earth effectively form a waveguide that allow waves of a particular frequency to travel long distances.

I highly doubt that the argon between the window panes is ionized to the degree that they are opaque to RF. More likely (as kableh mentioned) is if the windows are coated with a reflective metal layer (to reflect IR, I suppose), or if there's a electrically conductive bug screen, acting like a Faraday cage, blocking RF from coming in to the room.

EM theory states that if you are inside a conductive shell, then you will not experience any external electrical fields. So, it's gotta be something conductive that's enclosing your friend's solarium to keep the radio signal out.
posted by scalespace at 4:25 PM on March 15, 2006

Best answer: No, frequencies this low can only excite atoms and molecules by setting them in mechanical motion. Water vapour interacts with EM waves in that way, since the molecule is a dipole. Argon on the other hand is an inert gas, and one of the least polarizable atoms of all. It should not do anything significant to his radio signal.
posted by springload at 4:36 PM on March 15, 2006

It's not the argon gas, it's the glass itself or some coating on the surface of the glass.
posted by Rhomboid at 4:54 PM on March 15, 2006

The atmosphere is almost 1% argon. If argon absorbed XM signals so strongly a windowpaneful would make a difference, they could never reach the ground in the first place.
posted by jamjam at 5:08 PM on March 15, 2006

Here are some specifics on Argon : linky. You can now find out the wavelength of the XM signal and compare that to various transition. In order to do that take any two configurations given say :

Configuration Term LEVEL(cm-1)

3p5(2P°3/2)4s 2[3/2]° 2 93143.7653 M73

and

3p5(2P°3/2)4p 2[1/2] 1 104102.1043 M73

ignore most of the crap, you don't need it for what you want and look at the numbers italicized. Take the inverse of the smaller number and subract from it the inverse of the bigger one. Now take the inverse of the total. You now have the wavelength of the light (in cm ) needed to go from one state to the other. As i strongly suspect you'll see almost none of the numbers are close to the wavelength of the XM radio signal. You will find if you calculate the wavelength needed to go from one highly excited state (like a 10s to 12p maybe?) to another it is close to the XM radio signal. Unfortunately (or fortunately depending on how you look at it) this means you need highly excited Argon in you glass, which you don't. So long story short: the tighly bound Argon inside the window pane will not absorb (and thus reemit at various other frequencies or scatter if you will) XM radio signals. As an excerise for the student, what color light will excited Argon emit (from 4p to 4s, just choose any term its all for estimates any way)?
posted by ozomatli at 6:46 PM on March 15, 2006

Excited argon emits purple. But I cheated; it was in the illustrated Time-Life Science Library book, Matter, 1967.
posted by ikkyu2 at 9:41 PM on March 15, 2006