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Name: Tomas
Status: student
Grade: 6-8
Location: FL



Question:
I am doing an experiment on what materials can block radio waves, and I am having some trouble attempting to quantify the materials I will be testing. I need to know if the amount of material I test will have anything to do with if the radio wave will be blocked or not. Also: what common, easily attainable objects are emitters of radio waves (excluding things like cell phone towers)?


Replies:
Tomas,

A bit of advice on the first part of your question, radio waves usually tend to bend around many objects quite well, reducing their apparent ability to block them. The more fully encased your receiver/detector can be, the more reliable your results. In most cases, the thickness of the material can have some effect, but there are many materials which if they are thick enough to work with, will be completely effective against the power levels you will likely be working with.

As for a good test source, if you can find a walkie talkie, or other two way radio that you can transmit on fairly continuously, you will have a decent source. However, it is important for your measurements that its distance from your receptor be kept at a constant level.

Ryan Belscamper


Hi Tomas,

I think you may not be clear on how radio waves are shielded. Radio frequencies pass though any non conductive material, regardless of thickness. Radio frequencies are affected by conductive materials such as metals, but you cannot effectively "block" radio frequencies merely by putting a metal plate (for example) between the transmitter and the receiver. In fact in order to have any significant reduction in signal, such a plate must have a width and height of at least several wavelengths, and even then the difference would only be small. What is more of a problem, is that radio wavelengths are not small: FM radio waves for example have a wavelength of about 10 feet, so you would need a huge metal plate in order to notice any effect at all. At cell phone frequencies, the wavelength is smaller, but still in the range of a foot or more.

The only effective way to shield an object from radio frequencies, is to place it in a shielded box. That is, a metal box that completely encloses the object. This is why radio reception in a concrete building is often poor: The steel rebar in that reinforces the concrete, acts as a metal box. The concrete itself does nothing.

Generally, shielded enclosures made of higher conductivity metals like copper or aluminum work better than those made of lower conductivity metals such as brass, steel, or stainless steel. The higher a metal's conductivity, the thinner it can be and still be effective.

Regards,
Bob Wilson.



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