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Curious Radio Frequency Blocked with Lead?

  1. Nov 6, 2007 #1
    Hi all,

    I'm now very busy trying to findout how to block external RF at my apartment room. I'm on 23rd floor. A huge radio boardcasting tower (714KHz, 90MHz) is just 20 meters away running along with my building. It's higher than 25 floors.

    I know that many GROUNDED conductive surface can shield the room. unfortunatly It's imposible to find a ground on 23rd floor (Except building electric ground)

    So my question is
    1. If I shield the room with, say, aluminium plate, without ground. The protection will be less but how much?

    2. For no ground, I think of finding other material. LEAD!. Lead can block gamma, X-ray. It should more effective in shielding RF compare to other material, is it?

    3. What material can I use for the windows (Glass)?


    Thanks so much
     
  2. jcsd
  3. Nov 6, 2007 #2

    f95toli

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    Gold Member

    1) It depends, if you somehow manage to make a "cage" with a continous conductor covering walls, floor and ceiling the shielding is very nearly perfect.

    2) Lead is useless for this. How well a material "blocks" high freuqyency EM radiation depends only on the conductivity, this is why copper is good. Of course, if you can afford it silver or gold would be even better than copper.

    3) Thin metal foil. You can actually buy "films" that are especially made for this purpose. They are often used in "top secret" buildings (some goverment agencies, research labs etc) .
     
  4. Nov 6, 2007 #3

    Dale

    Staff: Mentor

    f95toli is right, it is all about the conductivity. Lead would be much worse than copper.

    For 3 you can use a thin metal foil or you can use a wire mesh. You don't have to have every spot covered as long as the holes are much smaller than the wavelength of the radiation you are trying to block. At 90 MHz that is about 3m, so as long as your wire mesh is no further than about 3cm you will be well shielded.

    Whether you use mesh or foil, it must be electrically well connected with the remainder of your shielding.
     
  5. Nov 6, 2007 #4
    If in that case

    Thanks guys,

    1. All about conductivity.. This means thickness of metal/shield is irrelevant ? I thought the more thick/dense of the shield, the better of reflection.
    2. Will aluminium Foil used in kitchens works for room shielding?
    3. Do building electric ground work for grounding? or better to unground?
    4. If I just cover all ceiling and floor just as f95toli suggested, but cover walls only 2 sides which facing the tower. Will that work?
     
  6. Nov 6, 2007 #5

    marcusl

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    1. Thin foil or screen will suffice.
    2. Yes, more or less, but you'll have trouble making conductive seams. The rule of thumb in the EE profession is your metal box must be water tight if it's to be RF tight. RF will leak through small gaps at the seams (if you have a 1/32" gap that's 2 feet long, the small gap size is irrelevant--it will radiate very efficiently like a 2 foot long antenna).
    3. Grounding is important. Ground to cold water pipes and AC electrical ground.
    4. Not very well. It is very difficult to fully shield RF even if you have full coverage because radio waves will travel along wires, water pipes, heater vents into the shielded space. Big gaps--doors and windows-- are big problems. Check out the doors on RF screened rooms for MRI machines at your local hospital, for instance--heavy metal, with interlocking spring fingers all around the jamb to make unbroken contact, and a big rotating latch like the one on submarine hatches. ("..must be watertight to be rf tight..")

    I assume your complaint is with electronic appliances. You probably hear the radio on your telephone, TV, VCR, etc. Before wasting time with foil or screens which you'll never get to work, try filtering each affected device. Ham radio stores (check out Ham Radio Outlet, for instance) will sell filters for your power cords, filters that plug into your phone lines, etc.

    You may also have some legal recourse. Ham radio operators are not allowed by law to interfere with their neighbors. Don't know if the same applies to commercial operators but it's worth looking into.

    If the problem is really bad, the ultimate cure is to move.
     
  7. Nov 6, 2007 #6
    Thanks Marcusl,

    I don't expect to perfectly shield the room just reduce most field strength. In that case, small and difficult detail can be skiped ? Or it won't work at all?

    This is a new apartment I bought. It will finished and ready for me to move in within a couple months. Since the apartment is quite near the tower, I think it will effect the electronic device and more or less effect health.
     
  8. Feb 22, 2010 #7
    >>> 3) Thin metal foil. You can actually buy "films" that are especially made for this purpose. They are often used in "top secret" buildings (some goverment agencies, research labs etc) .
    where can i buy those? are there any on eBay or Amazon? Do I have to ground such foils?

    And I am not quite sure how to ground. Do I just make sure the metal foil is connected to a ground by, say a wire? (i.e. clipping the foil on one end and clipping the ground on another). Can I use home appliances, say a fridge, as a ground?

    Can RF up to 10GHz or 100GHz be blocked using this method?



    actually i am hearing a high frequency noise at my home. i am suspecting it comes from my neighbors. or maybe from communication equipments/stations, i'm not sure...

    i know EM waves are not mechanical waves. but i guess they produce sounds indirectly, don't they? for example some high frequency sound could be heard by some when a CRT TV is on.

    is grounding a must? is connecting sheets of aluminium foil to a bronze statue a way of grounding? is the answer of this related to the strength of EM waves i am trying to block? and the word "grounding" makes me think of charges, is it because the aluminium foil charged up in anyway when it blocks EM waves, so that grounding is needed??

    i did a little search on the web to find out the reflectivity of aluminium against wavelengths. i get this: http://www.measurement.sk/2004/S3/Bartl.pdf but this paper examines only the range 0.1micrometer to 10micrometer.

    also, i suspect waves are from the west side of the home (since i can the noise most intensively there). not sure if this guess is valid, would it be possible that the waves enter from the east and somehow high frequency sounds are produced when the waves hit something in my home on the west side?

    and i fear if it would be harmful if i sheild every side of my home. in case an equipment at my home generates some waves, are these waves trapped inside such a sheilded house (back and forth) ??

    also, i remember i learn from school that radio signals sneaks through holes. but if my goal is simply to reduce the harmful effect, if any, does covering 80% of my house's wall sufficient in achieving this? (since ~80% of the energy is reduced)

    can you think of any better alternatives that i can do about the high frequency sounds?

    do radio waves have "critical angles", beyond which total reflection occurs, like optical waves?
     
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