# 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?

• 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.

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.

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.

• 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

"The laws of physics can be bent but never broken."

The way signals propagate through the atmosphere/space, hit and pass through, are absorbed, and bounce along a reflected path, as the discussion exposes, is complex. At lower frequencies a wavelength is longer, making it more difficult to design antennas to fit into small devices. The signals travel farther which makes coverage easier and less costly. However, that also causes signals to interfere unless signals that cross into a common area/space are differentiated in some fashion so that the interfering signals can be filtered by use of analog means or digital signal processing.

At higher frequencies, wavelengths become shorter, making the job of packing antennas into small devices less of a challenge and allowing capturing a higher level of the signal reaching the antenna. However, signals also are absorbed more in common building materials, foliage, and other objects. Signals tend to bounce more, causing multiple reflected signals to occur in areas where the signal is non-line-of-sight (NLOS). These are prominent design considerations among others.

Wireless technologies including signal processing and fractional-wavelength antenna design are being increasingly used to counter the negative impacts of signal propagation in order to become practical for communications. negative impacts, such as multiple-path propagation of signals is taken advantage of by signal processing so that signals are combined to raise the received signal to a higher SNR, signal to noise ratio, compared to analog methods that may try to filter out all but the stronger signal. Rather than use narrow-band antennas, for example, MIMO, multiple-input, multiple-output, signaling methods receive the multi-path signals and differentiate them in time-space, an analog function, digitize them and use signal processing to align for time differentiation caused by signal travel.

The issue of how signals travel is complex and must often be confined to a use-case in order to weigh the impacts or else it becomes unwieldy. However, a broad grounding in both the theoretical models and evolving methods to counter or take advantage of how signals travel, how absorption reduces interferences as well as impedes signal reception, and how reflection can multiply bandwidth by multiple frequencies reuse all must be considered.

Bringing this understanding into the world of applications requires practical considerations of component (antennas, chips, etc.), device and equipment availability and cost relative to alternatives. And, lastly, using multiple-frequency-carrier signaling methods to increase reliability and combined bandwidth of wireless communications and how that impacts the cost equations must be taken into account within a competitive applications environment.

• Nice first answer. – Oskar Skog Aug 8 '17 at 15:43

The way signals interact with obstacles is more complex than the baseline calculations: The way walls or other materials are formed can impede signals to a greater/lesser extent depending on the wavelength. At higher frequencies, wavelengths are reduced such that they may pass through openings or lattice type structures while lower frequency signals may be absorbed or reflected. On the other hand, molecules or component structure of materials can be resonant to particular frequencies: for example, water molecules are resonant at primary nodes near 2.4 GHz, 3.1 GHz. That is why microwave ovens typically operate around 2.4 GHz. That introduces a specific range of interference due to the presence of water in foliage, rain, and snowfall, etc. Some may have experience with this whether they know it or not: WiFi signals may travel outside of a building over a shorter range when it is raining because signals are absorbed in wet foliage, walls, and air space.

Several years ago, MIMO was emerging from prior use in defense and aerospace radars and communications into fabrication into semiconductors used in WiFi and mobile communications. Before that, many top design engineers were skeptical of its benefits versus costs and practicality. The sub-field of wireless has emerged to greatly benefit wireless communications, commercial radars and other applications. The higher frequency bands have stood to benefit the most because of less scattering, straighter line-of-sight affords better signal discrimination/isolation. That more can result in ease and better multi-path signaling properties compared to lower frequency bands.

However, the age we now live in is the age of multiple-frequency band communications in which the best band is the most opportunistic and suited to the needs of the application(s).

Three things happen to EM radiation when it encounters a barrier. It can bounce (reflectance or scattering), pass through (transmittance), or just plain stop (absorbance).

The intensity of radiation transmitted depends on several things: The wavelength of the radiation The intensity of the radiation hitting the barrier The chemical composition of the barrier The physical microstructure of the barrier The thickness of the barrier

For a variety of technical reasons, comparing lower (mid range 433MHz) and higher frequency 2.4GHz) compares like this: The lower frequency signals travel further than because the energy is higher and more concentrated in a single steady fashion that isn't absorbed as easily by air, which consists of a good deal of moisture. The higher frequency at 2.4 GHz is able to cut a path through the molecular structure of many materials but it's trade off is that moisture in free air tends to dampen the signal. Manu higher frequency transmitters are also designed with frequency hopping and encryption of some sort. It can find a path through partial obstructions more easily than lower frequency, large waves can.