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Name: Marcelo
Status: educator
Grade: 9-12
Location: OR 
Country: N/A
Date: 4/29/2005


Question:
How we can measure radio wave radiation in a building?


Replies:
This depends on what you want to do, and of course on the frequency range of the radiation. I will assume this is for a school type educational project. Then perhaps a good thing to do is to have just an oscilloscope (maybe you have one in the physics area?, maybe $500, try ebay etc, maybe a portable, hand held one?). I say this because you want to see a result, its more exciting than just having a meter move, or a light come on. You can plug into the scope various antennas. Antennas in this frequency range really are nothing more than wire, though they can get fancier, think of your TV antenna on your roof. But even touching the input of the scope with your finger, you yourself are a radio antenna. The frequency range would be up to whatever the scope can run, perhaps 200 MHz. But this covers the TV band, the FM band (88 MHZ), and easily to AM band (1 MHZ). You can see, and indeed often be swamped by the 60 cycles of wall current radiation. You cannot see cell phones until you get to >1000 MHz. You can see "noise", for example sparks generate a lot of junk RF. A telephone pick up coil is also a good radio receiver, to plug into the scope, then walk around the room with the coil and the scope, snooping for some ranges of broadcasting.

But you would see all of this mixed together, no rejection of one frequency for another. Would this be enough?

Beyond this, you need basically a radio receiver. And again people will want to know "for what range of frequency"?

If you have a signal source, a generator, then this is fun too, as you can establish a radio link.

Steve Ross


Sorry, Marcelo, I cannot quite answer your question today. This should be easy, but our equipment has gotten so specialized and expensive, that simple general needs can be a little difficult to fill.

Part of the problem is the really wide range of frequencies and strengths a person might reasonably want to survey. A billion to one in frequencies, and another billion to one in strengths.

You need to know the frequencies and power-densities you wish to cover. You need an antenna suitable for that frequency, and an electronic instrument (detector) suited to the frequency range and power range. And all of it must fit in your cost range.

Even give the word "radio", there is more than 1000:1 range of frequencies to cover, and even the most general antenna rarely works over more than 30:1. An antenna that works well for 1MHz AM radio stations is usually useless for microwave, and v/v. To cover all reasonable frequencies (i.e, from 60Hz to 60GHz), requires perhaps 3 or 4 completely different "broad-band" antennas or probes.

There is more than 10^10 range of power-densities to cover, depending on your concern. The larger the power density you are looking to measure, the simpler and cheaper the electronic instrument can be.

Power-densities above 0.01w/cm2 are presumed harmful; they can be felt directly or measured as heating in a simple little resistor, provided the antenna is efficient.

Intensities down to 1e-4 W/cm2 might reasonably have biological effects, down to 1e-6 if there could be freaky sensitivities (none yet proven, if you ask me), and those can usually be detected with an antenna and a simple diode detector.

That's what those small hand held "microwave oven leakage detectors" are. Below 1e-6 W/cm2 it is unreasonable to worry for biological reasons. But it is reasonable to have an abstract interest in what electromagnetic "sounds" are invisibly all around you, or have a practical interest in how radio-noisy your local wireless communication space is. Then you may want to detect signals a many orders of magnitude below 1e-6. For this we use fancy sensitive radio receivers, and cheaper radio scanners with signal-strength outputs (Radio Shack has some now; be sure to check for that signal-strength output, so you can connect it to a voltmeter), and expensive spectrum analyzers with X-Y plot-screens that look like oscilloscopes. (Those can be rented.) It can be a bit difficult to convert the dB numbers you get from scanners and spectrum analyzers into power levels or field strengths that relate to biological concerns. And if they are too sensitive, if you find a signal that saturates them, you'll need to put attenuators in between the antenna and the instrument to get a number measurement. It is not very hard to build these yourself in a little metal box, using resistors and a soldering iron.

Which is pretty long-winded.

So if you have one particular frequency in mind, such as the big radio station next door, or one particular band, such as radar or cell-phone, it would simplify our selection of equipment to know that.

At the lowest frequencies, below about 1MHz or 10 MHz, we do not bother to use an efficient antenna; it is quite adequate to directly measure the local electric or magnetic field with a wide-band field probe that can work from perhaps 10KHz to 10MHz, or maybe wider.

Perhaps if you write again and tell me about at least 2 of these:

- what kind of source do you think it comes from (maybe you have none in mind, but if you do...)

- what range of frequencies do you want to check?

- what effect are you concerned about? (your body? your TV reception? spy bugs?)

- what minimum field strength in V/meter or W/cm2 (or dB-something) do you want to check for?

Then I could try to find a more specific solution for you.

Alternatively, you can educate yourself further by surfing the web for field-strength meters, electric field probes, spectrum analyzers, and other such things, even if they are too expensive. Electronic hobbyists do sometimes invent their own meters and probes. Or study and get the minimum amateur radio license, just because it teaches you enough radio waves to understand all this for yourself.

I think I have had this question before, and although I like the issue, you can see why I did not quite manage to answer.

Jim Swenson


The tool I have used in the past for that function was called an "RF Sniffer".

Basically, it is just a radio receiver which only gives an output of how much signal it detects.

Ryan Belscamper



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