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Propagation of different frequencies

ratsept

Hello everyone,

 

This is my first post here and I really don't know if this is the right place to ask this but...

I am developing a project where we need to track mining equipment in underground mine shafts. The problem is GPS obviously doesn't work underground so our solution is to use small stationary transmitters embedded in the walls of the shafts and with each trackeble piece of equipment carring a receiver we record the time and ID of a transmitter. The transmitters have to be cheap and very low power as we are planning to make them disposable. The receivers are usually mounted on trucks and such and don't need to be very efficient. We considered RFID but that could become very expensive if every piece of equipment would need a several thousand Euro standalone RFID reader. So we went with low powered active beacons and receivers. Well all is well when testing in the office - we get ample range (upto some 100m in open space) and reliable detection but when we take the equipment outdoors in the rain or snow we get problems. We tried 2.4 GHz and it was very cheap and efficient, but we had trouble when the receiver or transmitter were near the ground or a big mass like a truck. Snow and moisture had very little effect on range. Next we tried RFM12B modules at 868 MHz that were unbelievably cheap but also performed well below levels advertised in datasheets. We could get the range but power usage was unacceptable because of low transmission speed and high packet loss. Now we have new modules and are just about to begin testing them but as these are 868 MHz again I have my doubts.

 

I would like to know if anyone has any experience with anything like this and any helpful pointers would be greatly appreciated. Especially any insight into what does and underground environment do to different frequencies and is this even feasible.

  • 0

    Hi Tiit

     

    I would recommend that you you a lower frequency in your situation. A narrow band VHF module (like 173MHz made by Radiometrix) or a 433MHz one may work better tah the 868s.

     

    I woud recommed "Radiometrix" and "Easy Radio" .

     

    Alternatively you can use the 1 Watt digi XTEND moduels, but they are not going to be cheap.

  • 0

    If you treat the tunnel like a waveguide, then the higher frequencies would perform better, going on the reports I have ready regarding communications tests in tunnels. 173MHz would in this respect not be as good as 433MHz. Obviously switching to a narrow band application improves reception, but puts the costs up.

     

    I have previous experience with microwave applications (10GHz) and these work very well in enclosed places, so long as the receiver antenna or waveguide has the source somewhere in its footprint. The reflections do tend to be somewhat unpredictable, but in a non uniform environment (ie not smooth wall tunnels) they seem to have worked fine.

  • 0

    Tiit,

     

    Extremely low frequencies is the best way to go.

     

    Although I can not suggest a quick fix for cave communication, I do recommend starting from the following links.

     

    A student created the ability to send a txt message from 1000 feet deep, using a low frequency signal of course. Read more here.

     

    And the Wiki on low frequency communication. Wiki link here.

     

    Hope this helps.

     

    Cabe

  • 0 in reply to M0DCD

    Hello,

     

    I am not sure if frequency is the only thing you need to consider.

     

    You will need to design your system for a multipath environment. As wave propagates through space it will bounce off any obstacle that is between the transmitter and receiver.  I can completely understand that you are getting 100 meters in an open field but not getting that in your tunnel.  By the time the signal has reached your receiver it has changed.

     

    This problem will not necessarily be solved looking at the frequency.  You need to look at the way you are transmitting and how the wave changes as it bounces around.   We here, at Pinter Electronics Consultants have done several tests in this area of radio frequency research and have found a number of key characteristics that have helped.

     

    1. Multipath has been known to change the frequency of the high frequency (>1 GHz) propagating wave.  Frequency hopping spread spectrum is a good way to to deal with this. The receive needs to tune into the frequency so you will need to create some type of real time calibration so the radio can change the correction factor for each location.

    2. Multipath also creates minima and maxima power levels so a MIMO antenna system is also very helpful in order to get the most powerful signal.

     

    If you are having more problems we should be able to help you with your project, as we have done similar projects for military applications.

     

    Hope this helps,

     

    Chris Pinter

     

    http://www.pinterec.ca/services/wireless-development

     

  • 0

    Besides the equipment tracking, communications in and out, whether cell or public safety frequencies will also be required. There are a variety of "in-building" communications systems on the market, many have been used in subways and underground applications. My suggestion is to use a common VHF system/frequencies for tracking assets. BDAs or bi-directional amps are key to systems like this. Here is a vendor of portable systems: http://www.modtech-corp.com/?page=apps4

     

    Good luck!

  • 0 in reply to Chris_Pinter

    Hello and I would first like to thank everyone for such fast feedback. We have a lot to process from all this information and will most definitely try to use some of the ideas from here.

     

    With lower frequencies we would probably get better results yes but also our power budget would be broken and antenna placement would be next to impossible considering mine environment.

     

    Spread spectrum is definitely a thing to try but how will that affect power usage. Range alone is not the main problem as more powerful transmitters would fix that but for this system to be feasible we would need the beacons (transmitters) to be cheap enough to be disposable. The batteries would have to last a few years. That is possible with the beacons being dormant for most of the time and only becoming active in the presence of receivers (using WOR and transceivers for both beacons and receivers). With higher frequency we can wake up for a very short time and send the ID packet (ID can be as small as 16 bits). With spread spectrum this time would probably have to be longer to negotiate a link.

     

    I looked up MIMO antennas and it seems they are pretty complex. Would the same benefits be possible by simply using more receivers on the same piece of equipment and then use data filtering in the server to correlate the data from all receivers that were supposed to be at the same place at some time.

     

    Cost and power needs seem to be growing at alarming rates for this project.

     

    We used RFID at UHF in that same mine to track mining trucks above ground and even there we had problems with the tags being in close proximity to huge masses of steel and stone. With RFID this could be because tags need very good reception but with a semi active system where the beacons would be something like RFID tags - only responding when addressed could the system be reliable? 

  • 0

    Hi

    I did two products with radiomodule from Radiocraft RC1140. One hase been used for sport events (indoors/outdoors also under ground), the other product has been used in life jacket. I got good result with two different antennas, PCB and multiloop. I'm using PCB antenna when I have short distance and no snow or water on the antenna. Multiloop I have been used in snow and over sea. The output power has been 9dBm and the frequncy 434MHz. My devices is working both as point to point or point to multipoint. This is an easy configuration in the radiomodule. Please also have a look at company shortlink, they have good modules.

    Best regards

    Torbjörn

  • 0

    Hi

     

    There are wireless solution from TESSCO co. you can refer the doc http://resources.tessco.com/media/pdfs/apps/10_01.pdf for underground wireless communication.

    and also refer below article which gives more information

     

    http://www.miningweekly.com/article/wireless-instruments-improve-mining-communication-2007-11-02

     

    BR
    Bheema