Different RF waves for different places(?)

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alexe100
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Posted by alexe100: Sun. Mar 13, 2011 - 02:09 AM
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Hi all

Do you know where can i find information about which radio frequency to use in some application scenarios?

For example, a coal mine. There is lots of rock to traverse... Which frequency should I choose to traverse the rock. 912Mhz? 433? 2.4 ghz?

Another example, a chemical plant which has lots of outdoor metalic pipes. Suppose you need to put some wireless sensor nodes to study some physical phenomenum. Which is the best frequency for those sensor nodes (more imune to reflection on the metalic pipes, etc)

Do you know any studies about this subject? Or studies about the behaviour of the different RF frequencies in the presence of different materials?

Thanks a lot

Alex

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Learning and Information, General Electronics
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valusoft
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Posted by valusoft: Sun. Mar 13, 2011 - 06:01 AM
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None of your quoted frequencies are suitable for rock penetration. You need VLF (very low frequencies: tens of kilohertz only) and very, very low baud rates.

Ross McKenzie ValuSoft Melbourne Australia

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ignoramus
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Posted by ignoramus: Sun. Mar 13, 2011 - 10:17 AM
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In some cases a leaky coaxial cable can be utilised.

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Bayho145
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Posted by Bayho145: Sun. Mar 13, 2011 - 01:19 PM
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Hi, Alex,
For example, a coal mine. There is lots of rock to traverse... Which frequency should I choose to traverse the rock. 912Mhz? 433? 2.4 ghz?

The rescue experts in your coal mine should already be equipped with ELF-Radio ("Extreme low frequency"), using frequencies as the radio to submerged submarines.

Ask them. I assume, they are already expecting your questions.

Another example, a chemical plant which has lots of outdoor metalic pipes. Suppose you need to put some wireless sensor nodes to study some physical phenomenum. Which is the best frequency for those sensor nodes (more imune to reflection on the metalic pipes, etc)

If you can't get rid of the reflections - use them:
a) Packet transmission
b) Diversity for the nodes, single antennae at the sensors. Use two or three antennas at the nodes. Let your nodes learn for each sensor, which antenna performs best.

Ciao
Wolfgang Horn

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Jokerman
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Posted by Jokerman: Sun. Mar 13, 2011 - 03:45 PM
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I remember years ago Wireless World had articles on thru-rock comms, something like this...

http://bcra.org.uk/creg/heyphone/index.html

-=mike=-

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alexe100
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Posted by alexe100: Sun. Mar 13, 2011 - 03:53 PM
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Thanks to all of you but i think you did not understood my question.
I do not have a coal mine. I do not have a plant.

I am searching for articles or books or web pages that discuss the various RF frequencies and the power of each them to be applied (traverse) different obstacles...

For instance, 812 MHz are good for this but bad for that. 2.4 ghz (f.e. bluetooth) is good for this and bad for that. this like these ones

Thanks again

Alex

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sparrow2
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Posted by sparrow2: Sun. Mar 13, 2011 - 06:04 PM
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Try to google something like "ssb best frequencies" for ham or marine.

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leon_heller
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Posted by leon_heller: Sun. Mar 13, 2011 - 06:18 PM
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http://www.hfradio.org/muf_basic...

Leon Heller G1HSM

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alexe100
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Posted by alexe100: Sun. Mar 13, 2011 - 07:19 PM
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"Try to google something like "ssb best frequencies" for ham or marine."

No hits...
I tried already google but i thing that i am missing the right words...

Alex

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Chris-Mouse
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Posted by Chris-Mouse: Sun. Mar 13, 2011 - 09:32 PM
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Perhaps the reason people have been having so much difficulty answering your question is because there isn't a simple answer to 'which frequency is best' for any give application. There are just too many variables to consider.

In general, lower frequencies are better at penetrating through and around obstacles than higher frequencies. At the same time, since higher frequencies tend to be line of sight, interference sources for higher frequencies also have to be line of sight, and therefore higher frequencies tend to suffer less interference than lower frequencies. There are also environmental concerns to worry about. 2.4GHz radio signals are very strongly absorbed by water in the air, making them useless for anything beyond short range communications. Some radio frequencies, like 2.4GHz, are also absorbed strongly by the human body, making high power transmitters a health hazard. (think microwave ovens)
Metal building walls will also completely block radio signals, leaving openings in the metal such as windows as the only way in. Radio signals won't go through an opening less than 1/4 wavelength across, so that's another limit that may need to be considered.

You may be able to do a few tricks to help you. For example, in your coal mine, it's hard to get radio signals to penetrate rock, especially if the rock is damp enough to be conductive. It might be possible to choose a frequency such that that mine shaft functions as a wave guide, and carries the signal throughout the mine.

By far the biggest restrictions on finding the frequency to use will be legal. Most radio frequencies are already assigned to specific uses, and you're almost certainly going to be restricted to what radio frequencies are already in use in the area, and what is available for licensing.

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ka7ehk
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Posted by ka7ehk: Mon. Mar 14, 2011 - 05:21 AM
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Not only is the best frequency hard to specify, but it also depends on the modulation, chosen. In some cases, FM is better or worse than AM. If the signal is digital, the "best" frequency can also depend on the baud rate.

Here is a factor you probably have NOT considered. If you are trying to use RF in a forest, some frequencies are highly attenuated if the dominant foliage length is a half wavelength of the signal. Some pine trees have short needles and some have long, and each will be bad at a different frequency. It will also be different if the trees are wet or have snow on them. Clearly, no one "best" frequency!

This has been painfully "discovered" in the United States where new unified emergency services communication system were to go into the high UHF TV channels that were vacated as part of the conversion to digital TV (and, all of the post-9-11 hysteria). Formerly, most of these were in the 450-500MHz band but now, being shifted to 800+MHz. Short-needle(about 70cm) pine trees that allowed easy communication with 450MHz (wavelength about 60cm) almost completely block 800MHz (wavelength about 35 cm).

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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sparrow2
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Posted by sparrow2: Mon. Mar 14, 2011 - 09:54 AM
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And I will add to Jims info.
2.4 GHz is bad near a lot of water (moist).
That is why the freq. is used in a microwave oven.

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alexe100
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Posted by alexe100: Mon. Mar 14, 2011 - 10:11 AM
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Jim, it was that kind of observations i am lookig for: forests, mines, concret, etc... real world experiences...
But know i see that there is lack of written information about this matter.

Thanks

Alex

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ka7ehk
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Posted by ka7ehk: Mon. Mar 14, 2011 - 03:29 PM
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Little is written. Much of what is written is in books, where it may be a small paragraph among many others.

I first became aware of these effects when reading an article in a very old IEEE technical journal about radio propagation. The article had graphs of attenuation in a tropical rain forest jungle. At the end, the authors made some estimates of why the attenuation was much higher at certain frequencies and how those frequencies related to the physical dimensions of the foliage. There were several subsequent articles describing propagation at various UHF frequencies in urban and suburban situations. Those authors also tied some of the unexpected high losses to lengths of leaves and needles.

But, this information is very hard to find, and I do not know of any one place, or even several, where it is all brought together. Further, much of it is anecdotal, based on observation rather than solid research.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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ams0178
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Posted by ams0178: Mon. Mar 14, 2011 - 03:44 PM
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Jim, when will you be publishing your real world experiences in a technical paper/book for all of us to learn? :)

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ka7ehk
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Posted by ka7ehk: Mon. Mar 14, 2011 - 03:52 PM
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I have presented this at several workshops on environmental telemetry, but probably will never publish. Much more research is needed to establish cause and effect, and I am not going to do that. Now, if some grad student wants to take this on, great!

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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ka7ehk
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Posted by ka7ehk: Mon. Mar 14, 2011 - 04:14 PM
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In complex "built" environments, "multipath" is also a significant issue. While this is often approached as simple constructive and destructive interference, it is often very challenging because there may be many paths, of varying lengths and losses, that all combine together. It is really better addressed with statistics rather than by trying to compute what is happening.

Now, as data rates go higher and higher, we are seeing "symbol multipath" in which the carriers may not interfere, but delay in the various signal paths cause the digital symbols to arrive at various times, and causing interference among themselves.

Some of this is being solved by so-called "MIMO" techniques (multiple input, multiple output, with more than one receive antenna and more than one transmit antenna).

Many times, it is a wonder than things work, at all!

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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alexe100
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Posted by alexe100: Mon. Mar 14, 2011 - 04:16 PM
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Yes, that could be my objective (make research) but it is beeing hard to get the state of the art...

Alex

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stevech
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Posted by stevech: Mon. Mar 14, 2011 - 08:54 PM
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Mines: are using very low freq. for earth/rock penetration. RF along the lengths of tunnels is used, but not for mission critical- as when a cave-in occurs, the low freq. must work. You can read about all this in the trade journals/on-line for that world.

Propagation in urban, hilly terrain, forests, etc. is all based on empirical data measured over the years, esp. for cellular, but also military. There are huge volumes of info on all of this. There is some theory here, such as non-line-of-sight multipath and delay spread affects on different kinds of modulation methods (OFDM, et al). But in highly non-line-of-sight (NLOS), it's mostly above validated emperical data. The TIA has many standards used by land mobile radio (public safety) and by cellular RF engineers, on what to assume vs. frequency.

But before frequency, one has to postulate what channel bandwidth is needed, what modulation method(s), and what coding schemes. This is what modern digital communications is about. It's heavily statistics-driven. The x dB disadvantage of a higher frequency can be offset by different modulation and coding methods. And vice-versa, in some NLOS situations, a counter-intuitive situation arises, e.g., in certain conditions such as warehouses and container stacks, 2.4GHz is better than 900MHz, due to refraction. Also, it's easier to get higher antenna gain as frequency increases, largely offsetting the fact that free space (LOS) attenuation increases with frequency.

So RF engineering is about a light dose of theory and a heavy meal of experience and review of prior art.

It begins with the information rates needed. Nyquist-Shannon got it right.

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valusoft
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Posted by valusoft: Tue. Mar 15, 2011 - 12:44 AM
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ka7ehk wrote:
If you are trying to use RF in a forest, some frequencies are highly attenuated if the dominant foliage length is a half wavelength of the signal. Some pine trees have short needles and some have long, and each will be bad at a different frequency. It will also be different if the trees are wet or have snow on them. Clearly, no one "best" frequency!
Jim, I recall reading a paper in the mid 70s about work done by some Indian engineers who were investigating the use of a particular tree genus in place of a commercial television antenna for village use. Wish I had kept the copy because my memory is rusty on the details. But it must have made an impression on me.

Cheers,

Ross

Ross McKenzie ValuSoft Melbourne Australia

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ka7ehk
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Posted by ka7ehk: Tue. Mar 15, 2011 - 05:23 AM
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There was research, I believe near or shortly after the end of WWII about "driving" a tree as an antenna. There was some sort of coupling loop (around the trunk?) with a matching network. As I vaguely recall, the trees were not very good, but better than nothing. This was for HF, I am pretty sure. But, like you, my memory about the details is rather sketchy.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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sparrow2
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Posted by sparrow2: Tue. Mar 15, 2011 - 04:00 PM
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For SSB radios there a some tables for how good different freq. are at the different time of the day, but they are all based on vessels, in freq 1-20MHz.

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ka7ehk
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Posted by ka7ehk: Tue. Mar 15, 2011 - 04:18 PM
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That has to do with long-range propagation as a result of ionospheric reflection of radio waves. The OP suggested, in the examples offered, that the emphasis of the question was VHF/UHF.

You are generally not concerned about pipes in an industrial plant at 3MHz. Nor 30MHz. Only a little at 300MHz. You certainly are at 3GHz.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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sparrow2
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Posted by sparrow2: Tue. Mar 15, 2011 - 04:20 PM
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sorry it for me it sounded more like OP was looking for general information.

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