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Sherlock Ohms

The Case of the RF Disturbances

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bdcst
User Rank
Platinum
Re: RFI on the Rise
bdcst   11/29/2011 1:39:37 PM
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As the RF spectrum becomes more and more crowded proper pre-selector filtation becomes more important, but it also becomes expensive.  Very steep skirts are required to separate out services that abutt each other.  And filters introduce their own amomlies, ripple, group delay, etc.  Multi-pole cavity filters of the type used for microwave systems such as C-band satellite, are labor intensive as each pole has to be hand tuned during assembly.  They are not cheap.  The waveguide flanges, the semi rigid inter cavity coaxial cable links and the pickups on either end add to the cost.

You are protecting a wide band low noise amplifier and down converter.  In the past 25 years the cost has dropped from $100,000 for a nitrogen cooled parametric amplifier to under $300 for very quiet room temperature LNA or LNB of better specs!  Adding $600 for for a front end filter to a $300 LNB will not make your product competative in the marketplace even if it is better.

Ann R. Thryft
User Rank
Blogger
Re: RF Ignorance
Ann R. Thryft   11/29/2011 3:38:09 PM
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I think one of the main problems in trying to have a meaningful discussion about the possible biological effects of RF or EMF is that there are so many different kinds of radiation/electromagnetic frequencies and they are apparently not at all the same in their potential for affecting living beings. Another problem, partially dependent on the first one, is that not all studies are created the same, making it difficult to compare their results. A third is that biological systems are extraordinarily complex, especially compared to machines, and are subject to huge amounts of variables.


ab3a
User Rank
Platinum
Re: RF Ignorance
ab3a   11/29/2011 4:04:38 PM
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And yet another problem is that the researchers who do these studies often don't understand the engineering side of the problem and the engineers don't understand the research side of the problem. I can remember reading about one study where the researchers ended up cooking their lab rats with relatively small levels of RF because they used far-field assumptions instead of near field equations for the antenna radiation patterns. 

One approach worth considering is to assume antenna reciprocity with MRI images. This would give people some idea of how much of what frequencies of energy the human body would absorb most highly. Right now the radiation standards are pretty broad and are based upon some crude thermal assumptions, with a fudge factor thrown in for good measure.

We might do well to consider limiting exposure to those frequencies that are most highly absorbed by the human tissues. And while we're at it, examine whether it is feasible to allow exposure to greater amounts of radiation as long as those wavelengths are not easily absorbed human tissues.

Also, we make the assumption that pulsed power averaged over time won't hurt anything. However some parts of the body may not conduct heat away all that well (making the assumption that the heat conductivity of human tissues can be approximated by saline water may not be all that well founded)

I agree that there is still much that we can do to improve RF safety --but in the scheme of things that can hurt or kill people, I think other research deserves priority. If we studied everyday household chemicals with half as much fervor as people have about this stuff, why, I'm sure we could lengthen the average human life span by a whole year.  (OK, I'm being a bit sarcastic here, but only a little).

Ann R. Thryft
User Rank
Blogger
Re: RF Ignorance
Ann R. Thryft   11/30/2011 11:29:50 AM
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Thanks, ab3a, for some good summaries of these issues.

However, there's a big difference between chemicals you can choose to buy or not to buy and bring into your home, and RF signals bathing your house, your car, and everywhere you go in a city without your consent. I think that's a large part of why those people are making so much noise.


RFI-EMI-GUY
User Rank
Silver
Re: blackberry blues...
RFI-EMI-GUY   12/12/2012 1:19:42 AM
NO RATINGS
Methinks your group should have an intervention with the tambourine player and his "crackberry" habit.

Or, perhaps an "accident" could happen involving the device and a pitcher of brew?

 

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