Radio Frequency Penetration

Question

I've learned recently that lower frequency radio waves travel farther and have better object penetration than their higher frequency counterparts.

With this in mind, I was pondering about the two-way radios we use on the ship I work on.

Most everyone uses VHF radios for communication in the maritime industry, but we've always had a hard time sending or receiving transmissions when we are below deck with any crew above. We ended up ordering UHF radios and these radios fixed the problem, they work great.

Why is it that these higher frequency radios penetrate better than the lower frequency radios if lower frequencies are supposed to penetrate better?

Answer 1

I've learned recently that lower frequency radio waves travel farther and have better object penetration than their higher frequency counterparts.

Then you learned poorly. This is simply not correct. Different frequencies go thru different materials differently, but it is not true that lower frequencies (longer wavelengths) "travel farther" somehow.

Think of really high frequencies, like light. Here we can discern different wavelength with our bare eyes as colors. Surely you must realize that red light doesn't always "travel farther" than blue light.

What does happen is different wavelength react differently to different size objects that they can't go thru. There are three basic effects going on, reflection, absorption, and diffraction.

How much a certain material absorbs EM radiation is very material-dependent, and often not monotonic with wavelength. Think of color filters. A green filter blocks both red and blue light but lets green light thru, even though its wavelength is between red and blue.

Big things relative to the wavelength will block the radiation. However, waves also diffract along the edges of objects. This is sortof a wave bending along to follow the object. This happens only along a thin layer near the object, with the thickness of this layer proportional to the wavelength. Long waves, like 1 MHz commercial AM can bend around the edges of hills and the curvature of the earth better on a human scale than 100 MHz commercial FM, for example. This may give the impression that these longer waves "go further", but that's not what's going on.

Short wavelength don't bend around the same object as well as long wavelengths, but they can slip thru smaller holes in objects or between objects. Again, this is proportional to wavelength. A 10 m hole will easily let 3 m (100 MHz) signals thru, but mostly block 300 m (1 MHz) signals. This is probably why the shorter wavelengths work better between decks of a ship. They bounce around better and eventually make their way thru doors and the like, which longer wavelengths can't.

Answer 2

This has been observed before. A simple way of understanding this is that a cellphone will work a damn sight better in a car than a AM portable battery operated transistor radio with its internal ferrite rod. The main reason for this is the size of the holes in the structure compared to the wavelength of the radio transmission. Another reason is that the atmospheric noise, man-made or natural, is lower at UHF.

Nowadays with better transistors it's more than likely that on VHF there is more noise coming in through the antenna if your receiver is worth its salt and your location has a normal amount of man made noise. I prototyped a MF radio data system that went through walls only to find that it didn't like reinforcing steel. It was a handheld remote that never went into production.

Answer 3

I've learned recently that lower frequency radio waves travel farther and have better object penetration than their higher frequency counterparts.

Have you ever driven into a road tunnel with an AM (~ 1 MHz) radio on? Reception disappears fairly instantly. On the other hand FM (~ 100 MHz) keeps working for some considerable distance into the tunnel.

Have you noticed that satellite dishes have small holes all over to make them less susceptible to being blown away in strong winds? Thos holes don't affect satellite reception because they are much smaller than the wavelength of the transmission.

1 MHz has a wavelength of 300 metres and if the road tunnel opening were that sort of diameter then you could expect decent AM performance all the way through the tunnel.

Basically, what I'm saying is that the premise of your question is flawed.

Answer 4

to see it simple you can look at the radio wave as a set of 3d Spheres going outward. you have highs and low ones. you can treat those shares as power sources where the high ones are + and the low ones are -, but they are more like capacitors. this means that if you connect a resistor in between them they will discharge. if the resistance is too low it will short circuit and cancel either the entire sphere or a large part on it depending on the size of the sphere (radio wave), and how well the air or other materials in that sphere conduct. since it is not an actual capacitor, you are just using direct energy as a capacitor, this means that if a part of it is in the air and another part short circuits with another pole in a metal object then only the section in the metal object will be cancelled, and that around it slightly weakened.

so for a Low-frequency wavelength, the distance between + and - where the conductivity causes loss relative to the voltage is very large, for example, 100km, you won't likely encounter a 100km distance where the electrical conductivity is high(low resistance). when looking at an ultra-high frequency the wavelength can be smaller than 1mm so any small object will conduct. but even the air will be a great conductor at this small of a distance, especially when the air is moist.

then there is reflection, in general, low-frequency waves won't reflect as much and just pass through things, extremely high frequency will reflect off of many things very well, this also reduces its range(they lose a lot of energy in this process). however, it has one advantage, for example, if you have a transmitter and a receiver close to each other but the transmitter is put into a faraday cage then the small wavelength might be small enough to move through the holes when you have a completely closed metal box around the transmitter but the side on the opposite side of the receiver is open then the extremely high frequency one might give a better signal due to it reflecting a lot and so still going the other way with a lot of its energy. the low frequent one won't bounce nearly as much so there is almost no signal around the receiver.

Now one of the most important points, yet which everyone forgets is that a low-frequency radio signal uses an extreme amount less energy than the same amplitude high frequent radio signal. so if you power both with 20W of energy then a 2KHZ LF radio wave would have 0.001W of energy in each wave while for a high-frequency radio at 20W you will often have around one-millionth of that power, so at similar wattage low frequency has much more powerful waves, and in normal air or water it moves a lot further by default, even then the waves are of equal power.