Measuring electricity passed to ground from an antenna

  1. First of all, excuse me if this question is too basic: I feel like it shouldn't be a struggle to understand, but the fact is it is.

    We put up an antenna to play with radio a little, and someone we were working with showed us that there's a reading on the voltmeter without any of the equipment hooked up.

    We are in a city, so there is wifi and power lines, and we're doing old-fashion analog testing to try to reduce the voltage before connecting everything.


    The interesting part to us is that when we isolate the antenna by laying it on the shingles, then connect it to ground with the voltmeter plugged in in between, we get a reading of ~4Volts AC (it changes as we move it in relation to the power lines or wifi antennas).
    [​IMG]

    However, if we lay it bare on the ground while still connected to the voltmeter, the voltage drops right off.
    [​IMG]

    The question is: Is our test flawed in some way?
    And; though it sounds ridiculous as I pose it: should we consider installing the antenna on the ground permanently?


    Thanks in advance
     
  2. jcsd
  3. Baluncore

    Baluncore 2,713
    Science Advisor

    Welcome to the PF.
    In this case I think you have been distracted by a crude AC multimeter sensing the stray fields radiated by the power grid.

    If you use a high impedance AC voltmeter you will get a reading. That is because no matter where you are on this planet there will be stray AC fields from 50 Hz and 60Hz power distribution systems.

    To give a better reply we would need to know what type of meter you were using? Was it tuned to a particular frequency?

    Also, what type of antenna, in what orientation for what frequency ?

    80 years ago people were building underground antennas to attenuate static noise. Then it was discovered that they also attenuate the radio signals, since then, getting antennas away from the ground has always been the aim.
     
  4. AlephZero

    AlephZero 7,300
    Science Advisor
    Homework Helper

    If you are measuring several volts, you should be able to see what the frequency content is with an oscilloscope. If most of it is at mains frequency, your radio receiver will just ignore it. If it is coming from a cellphone mast or something similar, you might want to think about filtering it out. Either way, putting the antenna on the ground is probably not the best way to fix it.
     
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