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Thread: Need help to prove interference in 2,4GHz (howto measure e.i.r.p)

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  1. #1

    Default Need help to prove interference in 2,4GHz (howto measure e.i.r.p)

    Hi,

    I am fighting against a noise transmitter that interferes destructively with our otherwise well functioning WiFi access on a camping site in Denmark.
    A known source started transmitting in the ISM band using non 802.11 signals to reach clients miles away.

    Turning off the offending transmitter immediately solves any and all problems so there is no doubt of the source.

    We tried to get the regulator on site to measure if the signal is to strong but in my opinion his method of measuring seem wrong. I know a lot about WiFi and how to build short and (even illegal) long distance links but I am unaware of the specifics used to prove legality of a transmitter.

    I posted a lengthy explanation of the values measured here: http://forum.mikrotik.com/viewtopic.php?f=7&t=98137

    When we do a spectral analysis of the signal, we see at least -62dBm in approx 500m (1/3 mile) - a signal that would allow full WiFi bandwidth. In my opinion that is way to much. I refer the ETSI EN 300 328 harmonized standard (http://www.ietf.org/mail-archive/web...d3d1acPkgu.pdf) that is the foundation for danish legislation regarding allowable transmission in the 2,4GHz ISM band.

    Here I find:

    "Maximum allowable transmit power: 100 mW e.i.r.p. and 100 mW / 100 kHz e.i.r.p. density when using frequency hopping modulation, and 10 mW / MHz EIRP density when using other types of modulation"

    In my experience this would definately not allow any links of this distance with that bandwidth.

    As reference, the engineer from the government pointed his spectral analyzer towards 2 of my transmitters that was turned up to 500mW and gained by 6dB omni antennas - he found them to be way within limits.

    He also explained that Denmark has no official education requirements for his position and that he may well be wrong.

    Some years ago, I read that e.i.r.p should be measured as "energy absorbed on a well defined surface in a well defined distance" - typically 1 meter away from the transmitting antenna
    gine me setting up a para
    Can anyone explain / supply links / help in any other way.

    If this is "blue stamped" it would render the 2,4GHz ISM band useless in Denmark when idiots like me would flash it as proof of being allowed to transmit with Watts instead of miliWatts. Imagine me setting up a 32dB parabola antenna with a 1W radio and direct it towards a busy office building. I guess that I should start at the government office and see what they would do about that - it being legal and all ;-)

    Any and all help is highly appreciated.

    Thanks in advance

    /Niels

  2. #2

    Join Date
    Dec 2008
    Location
    Elgin, Illinois
    Posts
    1,655

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    I found a website that explains the E.I.R.P. measurements and how they are accomplished

    http://www.networkworld.com/article/...-nutshell.html

    Now from my understanding of the offending transmitter affecting your WiFi unit, it may be perfectly legal for the so-called offending transmitter to operate at a higher RF power level. You are screaming about some transmitter without providing a clue who manufactured the product (Model Number & Any FCC / ETSI Type Acceptance Numbers), what the inspector used to test the unit, i.e. Commercial RF Wattmeter and/or Spectrum Analyzer, what type of RF Coaxial Cable, what type of an Antenna Model Number & dBi Gain Figure and how high above the ground in meters, in other words you haven't provided any solid technical information on product "X" to even try to analyze the problem or your complaint.

    Dan
    WA9WVX
    F.C.C. Commercial General Radiotelephone Operators License since August, 1971

  3. #3

    Default

    Hi Dan, Thanks a bundle for your reply.

    The web site you refer is a perfect explanation to ensure that the output from a known installation will comply to federal regulations. However my problem is the reverse.

    And sorry if I seem to "bitch" - it is not intended, only we really suffer here and I am getting a bit desperate.

    Background short:
    We have a site operating 20 transmitters allowing access for 1.000+ guests at our camping site. This has ben expanded to current state during the last many years and all was in mint condition. Channel plan minutiously calculated and measured all over the camping site, anywhere we found it possible to achieve 20/20Mbps access to the internet through the 2,4GHz accessnet. Roaming worked like a charm and all was good.
    We use Mikrotik equipment all over the site.

    Enter "the offender":
    When they powered up their devices, our upload bandwidth completely disappeared - in best case we may send with a bandwidth of 0,1Mbps.
    As reference, we got them to power down the site for a short while - the result was an immediate improvement of our net access "back to normal".
    This simply to ensure that the "noise" originates from "the offender".

    The transmitter should be Ubiqity AirMax equipment attached (build-in) to a Panel antenna with a pigtail only. The antenna's gain is unknown but looks like a 20-25dB.
    The signal transmitted is non-802.11 - a proprietary protocol used by Ubiquity and the frequency is "between channels" - measured to 2434MHz.

    We tried every possibility available in our equipment to adjust our radios to comply with the "noise" - "Radar Detection (DFS mode)" = on, "Adaptive Noise immunity" = AP, Client+AP and Client only. All with absolutely no results whatsoever.

    We then did a measurement using a build-in Spectral analyser facility in a test unit (Mikrotik Equipment SXT 2) with a 1600mW radio and 10dB antenna gain (60deg.) (http://i.mt.lv/routerboard/files/sxt_2-150129090106.pdf).
    With this we measured a signal of -10 to -20dBm

    I have attached a file to this post with some of our results:

    Problem explanation and measurement results.pdf

    We then called the federal "noise police" that send an engineer to do some measurements. He explained that he was not specifically educated to "do the math" and that we would get an answer later from an expert. He had been there previously and gave us a copy of his findings at that time. To our big surprise, his first conclusion was that all was fine in his first observations.

    The maths may well be correct - I am not an expert on that. This is where my question to you all come in.

    The law here states that we should follow the ETSI EN 300 328 harmonized standard.

    In my experience, the signal is VERY high compared to what I normally see as "within legal boundaries".

    Also a quick reference measurement with his equipment (and math's) to one of our units with 500mW transmit power and 6dB omni antenna directly attached (minimal cable loss) should in his opinion be of less than 1/4 of the allowable effect.

    IF his maths prove right, I will be able to legally transmit any kind of noise (or signal) with a 2W radio attached to a 6dB antenna, thus providing a 8W powered signal spreading 360 degrees.

    Imagine what that would do to your local WiFi installation in the house. Or should I say - the lack of ability to get any signal to your iPad and laptop anywhere. All "blue stamped" by federal regulations.

    Interesting aspects!

    Thanks

    /Niels

  4. #4

    Join Date
    Dec 2008
    Location
    Elgin, Illinois
    Posts
    1,655

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    Niels,

    I've been doing some research on the Ubiqity AirMax equipment, antennas and proposed uses But I could NOT find what type of Digital format is being used nor the RF Output level in Watts or Milliwatts. There system states that the RF Bandwidth is 20 MHz and I had to assume it's +/- 10 MHz either side of the Carrier Frequency that was measured by you or the Field Engineer. After looking at your PDF document, I see that the other transmitter is off site, apparently mounted on a structure (tower) a couple of blocks away and from Ubiqity AirMax's Systems Design, they are using Microwave Parabolic Dish antennas which might provide +25 dBi Gain or 320 times the RF Wattage into this type of an antenna. The other thing I made note of is they are using these Microwave transmitters in a Point to Point (PtP) configuration which could be used for Television or Public Safety (as a Systems RF Backbone Linking) rather than using TELCO T1 Wirelines for better reliability. I'm only making a wild guess that they maybe using Frequency Hopping which is known to bring up more noise raising the noise floor itself but remember I'm just guessing not proof positive.

    When Digital transmissions were first developed 15 years ago, engineers first thought that there wouldn't be any interference between the different RF transmitted modes and RF Power Output levels could drop down -3 to -6 dB and then be able to provide reliable communication signals and more frequency reuse. In the practical real world, it was discovered that theory did not work out as they envisioned it due to many problems, i.e. atmospheric conditions, broad (frequency) bandwidth, the inability to develop RF Filters wide enough to knock down adjacent RF Channels without destroying their own equipment's performance and man made noise.

    Motorola had developed several Digital formats and the sold their Time Domain Multiplex Access (TDMA) to a company known as NEXTEL and I can say that system really worked quite well. Motorola also developed the Frequency Domain Multiplex Access (FDMA) for their Public Safety Markets and this system worked real well. NEXTEL needed more tower sites so they approached the Public Safety Markets and appropriate personnel and informed the local Public Safety Departments that they would install whatever height towers that the Public Safety Markets required for solid communications. The one item NEXTEL over looked was to go to Motorola to determine whether these two systems would experience any problems between the different Digital formats. Everything was changed out and all of the sudden the Public Safety Markets developed unwanted interference on their Digital systems but they did not have any method developed why or where the interference was being transmitted from. After months of complaints, the Public Safety's personnel got in touch with Motorola since all of the equipment was manufactured by the same company and they wanted Motorola's Field Engineering Department to track down the interference.

    Motorola send Field Engineering Teams to all of the Major Public Safety Markets. The teams readily discovered the offending interference sources and when those teams discovered the RF Input & Output Frequencies for both the Public Safety Markets and NEXTEL as both systems were on the 800 MHz Band but the RF Channel assignments were adjacent to the Public Safety Markets Receiver Input Channels and NEXTEL's Transmitters were up all the time using 25 KHz RF Channels. The antennas on the towers were separated only by 70' (20 m) in height, not enough vertical separation. It was a disaster waiting to happen and it did. Motorola couldn't be sued because they were NOT involved with the original idea from the starting point. NEXTEL had to pay for the Public Safety Markets RF Channels to be reassigned throughout the United States which cost NEXTEL dearly in USD, the time to have every tower site, mobiles & handhelds reprogrammed and realigned bringing the equipment back up to factory specifications.

    In essence, if you ever hear some one state that Digital signals cannot be interfered with ... tell them they're full of hot air! I can take an older Analog Service Monitor, go out and jam a Digital Receiver's signal & most Service Monitors normally max out at 0 dB!
    A normal receiver sensitivity is measured at -115 dBM and if you are measuring -107 dBM that's enough to Desense your Wideband 802.11 Receivers.

    BTW, your inquiry is geared more towards the Commercial Business Band & Computer WiFi problems as this website is for Amateur Radio problems.

    Dan
    WA9WVX

  5. #5

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    The measurement of field strength and referring this back to the EIRP of a transmitter is well documented and isn't terribly hard, given that you have a receive antenna of accurately known gain.

    My understanding of the Danish licensing requirements (I'm used to be an engineer for the UK RF regulator) is that the ISM bands are treated in the same way there as they are in the UK so, as long as the user of the ISM band either uses type-approved equipment for that band, or has an inspected and permitted system, the interference caused to other ISM users may well have to be accepted. You will need to have your local regulator consider the situation and rule on the issue.

    The problem is that the band in question is not a WiFi band, it is an Industrial, Scientific and Medical band so you may have difficulties getting the regulator to rule in your favour.

    Sent from my SM-G900F using Tapatalk

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