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.