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Metal as an antenna

Discussion in 'Ask Jack' started by Jamie Ward, Feb 9, 2019.

  1. Jamie Ward

    Jamie Ward Gold

    Hi Jack,

    I was hoping to clarify a question I asked at the open house.

    'When does metal become an antenna?'

    All materials will have reflection, transmission and absorption qualities, with the ratio of these being dependent on the material.

    For high-frequency radiation, metals generally have an extremely high reflective property and less transmission and absorption. Earthing/Grounding metals actually do very little to its shielding properties, implying that it is essentially reflective to high frequencies.

    A satellite dish for television has to be accurately positioned to receive a line of sight signal, i.e. it is not able to 'attract/draw in' a tv signal if it is out by a few degrees. My understanding is that that the curvature of the dish then reflects it to the receiver that sits in the middle of the dish.

    An analogue tv aerial also needs to be positioned in quite a specific way.

    This leads me to believe that metals aren't 'drawing' in a signal from another location but rather capturing a signal that is passing through that direction anyway.

    When I asked you this at the open farm you mentioned the ability for metal to act as an antenna depends on the local ionosphere. i.e. if the air is highly charged and conductive and the metal is sat in that environment and you are touching it will conduct.

    I can understand this being true for low-frequency electric fields, but would this be true for high-frequency signals when metals are generally reflecting them?

    Could it be that the higher frequency carrier wave reflects but the modulated lower frequency signal is absorbed/conducted?

    Many Thanks,

    Sergio Valadez, Pebbles and JanSz like this.
  2. Jamie Ward

    Jamie Ward Gold

  3. drezy

    drezy Gold

    I have a few I used to have to push 8Gb down transmission lines... Once your metal gets hit by a signal it's fair enough to start to look at it like a transmission line. I'll delete the post if it's unwanted.

    This article is alright:

    Check out the "Dielectric loss tangent or tanĪ“" section for higher frequencies.

    Every material will have it's own Dissipation Factor(a.k.a. loss tangent,tanĪ“ )
  4. Jack Kruse

    Jack Kruse Administrator

    Metal has to resonate with the wave it connects too. This is why all tuning forks have a frequency they respond too......and that response then works via harmonics of the original resonant frequency.
    drezy likes this.
  5. Jamie Ward

    Jamie Ward Gold

    OK, this makes sense. I have seen research, I think Pavlik, where he shows certain building materials amplify the power density for a specific frequency, which he wrote as being caused by resonance.

    So, it's material and geometry that can cause resonance...

    Thank you
  6. Jack Kruse

    Jack Kruse Administrator

    I believe it is about topology more than geometry, but the idea is equivalent.

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