# Why do higher frequency waves have better penetration?

In free space, lower frequency signals seems to go farther because the signal is either diffracted by the ground or reflected by the upper atmospheric layers, making it actually go farther.

In urban condition, where we need to penetrate walls, does 2.4GHz travel further than 433MHz radio?

In the electromagnetic spectrum, do Gamma rays and X-rays have good penetration because they have high frequency?

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This question is probably better suited to the physics stackexchange. In general though, the penetration of an EM wave is determined by the absorption of whatever you're trying to penetrate. The very high frequency (high energy gamma rays) and very low frequency (ELF signalling) will penetrate almost anything, in between there's so many factors it's hard to write general rules. – Optimal Cynic Jun 9 '12 at 12:20
@OptimalCynic, This question should have a home on either site, in my opinion, but others may disagree. – Kortuk Jun 9 '12 at 12:56
@pstan, in an infinite dielectric with no boundaries a lower frequency will still travel further. As you move into something like Gamma and X rays you are reaching a completely unrelated phenomena. That is even higher frequency then light. Before you get to visible light you still go through the terahertz spectrum where both quasi-optical approaches are used and waveguides can be built easily with pieces of metal. After that you enter IR which like terahertz starts having interactions with chemicals heavily(terahertz hates water) and then you get to visible light. – Kortuk Jun 9 '12 at 13:01
I would suggest that the 2.4GHz propagation as apposed to 433MHz discussion be a single question and affects such as gamma and X-rays be kept separate. They are very different affects. – Kortuk Jun 9 '12 at 13:03
I've always thought of VHF/UHF/Gig as different types of balls. VHF as a meter big squash ball - very squishy so it doesn't bounce around very well before it dies -UHF as a basket ball, has more ability to bounce around and hit more surfaces - Gig as a small super ball, able to bounce around like crazy before it dies out and fit through smaller openings. In an urban environment buildings bounce the RF around so the more bounces you can get before it dies means the greater likely hood it will find it's way into a building via windows/doors etc. – Chef Flambe Jun 9 '12 at 17:11

It is not true that higher frequencies always penetrate further than lower ones. The graph of transparency of various materials as a function of wavelength can be quite lumpy. Think of colored filters, and those only apply to a narrow octave of wavelengths we call visible light.

What you are apparently thinking of is wavelengths so short that the energy is very high, like xrays and gamma rays. These go thru things solely because of their high energy. At lower energies (longer wavelengths), the waves interact with the material in various ways so that they can get absorbed, refracted, reflected, and re-emitted. These effect vary in non-monotonic ways as a function of wavelength, the depth of the material, it's resistivity, density, and other properties.

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The main advantage of higher frequencies is that they require shorter antennas for decent reception quality, and that's important for mobile devices. They also allow a wider band for modulating signals, so you can obtain higher frequency transmission.

But high frequencies are more sensitive to reflection, so they will have a harder time passing through walls and obstacles in general. At the same time, they will more easily leak through holes: a rule of thumb is that if you have a hole of the size of the wavelength, the signal can leak through it. But at the same time, you can't rely on it for a good transmission: so I'd say that the limit is quite fuzzy.

For further insights, look at line-of-sight propagation: microwave frequency can be refracted by smaller object than lower radio frequency, as it's strongly dependent on the wavelength. The comparison arises by the fact that microwaves have a spectrum that is more similar to the optical wavelengths, so they will suffer from some of the phenomena that hold for optics.

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In fact higher frequencies have worse penetration capabilities. If you consider a purely theoretical model, the so-called skin depth, which gives the thickness of the layer of a conductor to which an electromagnetic wave of a given frequency is able to penetrate it, you will see that the skin depth is inversely proportional with the square root of the frequency:

$\delta = \sqrt{\frac{2\rho}{\omega\mu}}$

($\rho$ is the resistivity, $\mu$ the magnetic permeability of the material).

This has also as a consequence that AC currents do not use the whole cross-section of a wire (and a properly designed hollow one would do the same job) and that's (partly) why a smaller antenna will do for proper transmission.

But in reality things are much more complicated than that. Wireless HD video is serious engineering challenge (partly) because the high frequency signals necessary to provide the appropriate bandwidth tend to bounce off the walls. At really high frequencies (i.e. ~60 GHz) necessary for such applications other absorption/reflection phenomena can compromise transmission: e.g. absorption by oxygen (in the air). This depends very much on the medium through which your wave needs to go through.

So, the short answer is no, higher frequencies aren't able to go better through walls than low frequencies.

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Skin depth as defined here is for good conductors only. In lossy materials it is $\delta_s = \frac{1}{Re\{j\omega \sqrt{\mu \epsilon}\}}$ – tyblu Jun 9 '12 at 14:44