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VHF Units

Discussion in 'General Electronics Discussion' started by Puno, Mar 4, 2011.

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  1. Puno

    Puno

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    Feb 25, 2011
    is it possible to hardwire 2 VHF radio units provided the cable between them is long enough to reduce the amount of power transmitted.
     
    Last edited: Mar 4, 2011
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    In theory you could wind down the power and connect the antennae together. Practically speaking, you would almost certainly still need to use an attenuator, or you risk damaging (or at the very least grossly overloading) the receivers.
     
  3. Puno

    Puno

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    Feb 25, 2011
    thanks Steve, I have been thinking maybe connected some sort of load in between the two units I have tried putting ant. dummies on each of the radio but very distorted the 2 units are only close within a 2m at least separated by board partition in between.
     
  4. davenn

    davenn Moderator

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    Sep 5, 2009
    one has to ask ... why you would want to do that ? what are you trying to achieve ?

    ok so what is the lowest power setting that the radios can produce ?
    1Watt ... 0.5Watt ? more / less??
    To avoid damaging the receiver front ends you would have to attenuate the signal to at least -50dBm preferably lower (-60 to -80 dBm)

    so let us assume you can lower the transmitter to 0.5 W (500mW) = + 27dBm so you would need a minimum of 80dB of attenuation to get down to ~ the -50dBm level, that would be an extremely long bit of coax. Fixed attenuators are the way to go.

    just for a comparison, a decent receiver would have a sensitivity of ~ 0.2uV = -122dBm

    Dave
     
    Last edited: Mar 4, 2011
  5. Puno

    Puno

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    Feb 25, 2011
    Dave, this is for training purposes only in a classroom so as not to interfere with local radios well radios were installed by contracted tech and I was called in to see one of radio stop receiving anymore only txing. I checked it out and thats how the contractor wired them up and I thought too the same as Steve said so I thought the same you are saying there is no way of wiring them directly and being only 2 or 3 m apart. I tried with dummy loads but too distorted so I thought thats the way to go about install an attenuator in between, thanks Dave, Steve.
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    The distortion is probably caused by the input stage of the receiver being overloaded.

    I started my reply with "in theory". In practice, the output stage of the transmitter will leak some RF, and even with a perfect dummy load connected to the output, and even with very good shielding, *some* RF will get through.

    In your case, you may well find that leakage (possibly even from the dummy load itself) is causing your problems.
     
  7. Puno

    Puno

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    Feb 25, 2011
    Steve
    I tried it with the dummy loads in case it works and that eliminates the direct wiring and the antenna. It worked fine in one room and as soon I went to the other room where the other radios are the received signal is weakening and that maybe is the received signal is not good at all. I was hoping to get away with it using only dummies.
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    OK, so your dummy loads work fine with limited range. You could make yourself a dummy load. It's almost certain to be nowhere near as good as a bought one and will probably act as a slightly better antenna.

    A dummy load is typically a connector that replaces the antenna with a (again typically) 50 ohm resistor of appropriate power soldered across it. Make sure you use non-inductive resistor(s). Wirewound resistors are not suitable.
     
  9. Puno

    Puno

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    Feb 25, 2011
    thats correct Steve what if i fitted the dummy loads from each unit and fitted in a shielded diecast case therefore all dummy loads are next to each other, and this box is mounted right between them and from there i could just run coax to each unit.
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    You might get away with that. But there are many variables (e.g. it may be the radio that's radiating RF, not the dummy load(s)). But give it a try. No need for a diecast box. Just run the coax to some point and have the dummy loads near each other.

    If it works, great.

    Just remember that this is more of a bodgie solution than one that smells at all of any actual "engineering".
     
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