Mobile Radio Modem Surge Protection

Discussion in 'Wireless & RF Design' started by Dollarday, Oct 29, 2012.

  1. Dollarday

    Thread Starter Member

    Jan 25, 2012
    32
    1
    Hi everyone :cool:,

    I have a question...(Obviously :D )

    I use the following radio modem (Link: http://www.rfdatatech.co.uk/products/radiomodems_zrt.html) with an antenna for remote communication betweem a mobile weather monitoring station and a base station.

    The problem is, the modems keep breaking, which I believe is due to surges that the antenna pick up from lightning that travels through the modem.

    I want to use a surge protector, but they state you need a good ground. Unfortunately I cannot use a ground spike, because this would impare mobility. (It's a MOBILE station :rolleyes: )

    Is there another way to protect the modem without a decent ground connection?
     
  2. westom

    Member

    Nov 25, 2009
    52
    5
    If lightning is causing that damage, then a current from lightning is getting to earth destructively via the modem. Long before asking for a solution, first the actual problem must be defined. So, what is the conductor carrying a destructive current out of the modem to earth?

    Every wire (including the two 12 volt wires) can be either an incoming current path or the outgoing path to earth. If a surge is doing damage, then one wire is the incoming current path. Another is the outgoing path. Only then can a solution be defined.

    Is there another way to protect a modem? Plenty. But each is uniquely defined by the still undefined problem. Helpful is to identify what part inside each modem is damaged.
     
  3. Dollarday

    Thread Starter Member

    Jan 25, 2012
    32
    1
    Mmm, I agree with your approach Westom... but sending it back to the supplier to find the fault is going to take ages...

    The modems break in the following condition: They still work, and their output power remains the same, but they only work at short distances, which leads me to believe that the "receive" side of the modem is damaged. (Which leads me to believe it's the antenna inducing the current)

    The 12V wires are connected to a battery inside a enclosure and their quite short, so I don't think they would be the cause. (Plus, then the damage would present itself at the power source of the modem)

    Assuming it's the "receive" circuit of the modem that get's damaged, what protection methods can I use that does not require a spike being driven into the ground?
     
  4. westom

    Member

    Nov 25, 2009
    52
    5
    Assume a surge was incoming to the modem. Then an outgoing path is via either one or both 12 volt wires, through the battery's case, into the enclosure, and out to earth. What might be damaged? The receiever's RF amplifier. One example of how damage might occur.

    A surge that has no problem crosslying three kilometers of sky will also conduct across wire insulation or a battery case.

    A manufacturer is unlikely to say what is wrong. The point was about fewer who better know this stuff by doing that analysis. And how you might better learn. Without maybe a friend who can identify the defective part (and then make that modem functional), then you are unlikely to learn what specifically was damaged. But again, the point. Learned by literally tracing an incoming and outgoing surge path. Both must exist to have damage. Others who never did this would only recommend popular hearsay. Such as a protector that somehow and magically makes a surge disappear.

    I believe your modem has options such as an RS-232 connection. If using RS-232, well, RS-232 is an easiest reason for damage. RS-232 is common mode signalling - one reason why damage is so routine on those type interfaces.

    Damage is due to a path to earth. Protection is about making a better path to earth so that protection already inside the modem is not in that path. Then internal protection not overwhelmed by a surge current. Damage means it found a path to earth. Protection means chaning that current path. Connecting every incoming wire to earth either via a direct connection or via a protector. Worry most about that path to earth.

    No protector does protection. A protector only does what a wire might otherwise do. For example, if the antenna lead is connected to earth, then the antenna fails. One solution was to earth that antenna wire via an NE-2 neon glow lamp. The lamp is an open circuit when no surge exists. And is a short circuit to earth during a surge.

    Bottom line conclusion remains. Damage would be from a current that found earth destructively via (maybe) the RF amp. So reception is now weaker. Protection means every incoming wire first connects short to a common ground. So that every potentially destrutive current gets to earth without going inside that modem.

    Put a hypothetical sphere around the modem. Any wire that crosses that sphere must first have a short as possible connect to a common ground. Even all three wires of an RS-232 connection must connect to earth either directly or via a protector. Then superior protection already inside the modem is not overwhelmed.

    Surge current is finding a path to earth. You must identify that path.
     
    Dollarday likes this.
  5. Dollarday

    Thread Starter Member

    Jan 25, 2012
    32
    1
    Awesome explanation... It's all so obviuous in retrospect, thanks!
     
  6. Abigail1

    New Member

    Nov 25, 2012
    1
    0
    Nice informative Discussion.
     
  7. tinamishra

    New Member

    Dec 1, 2012
    39
    1
    Nice stuff of information is provided here regarding radio and communication and it specially based on signals and frequency and many of factors depends on the earth Establish a low impedance path to earth ground from the PKG's aluminum enclosure. Using metal screws on the end brackets should help you do this. The PKG's serial data reference ground and power supply ground are internally connected to the RF connector ground, and the RF connector is grounded to the aluminum case by the washer and nut on the exterior of the case. Ground the voltage supply to a path to earth.
     
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