On Mar 15, 2005, at 1:04 AM, David Kirkby wrote:
> RICHARD GEORGE wrote:
>
>> Put some in your microwave oven and see what happens.
>>
> I see this advice given out a lot, but unless I am mistaken it is
> flawed.
>
> Microwave ovens work by emitting energy at a frequency (I believe
> around
> 2.4GHz) where there is a water absorption peak (due I think to the
> resonance of the two oxygen atoms).
Water has two oxygen atoms?
> So if a plastic had a lot of water
> in it, it would get hot if fed with 2.4GHz.
I have melted nylon with 7MHz RF, but I never tried it at 2.4GHz.
However, D-factor typically increases as freq. increases.
> There's a graph below
> showing hpw the loss of water changes by a factor of 10000000000
> depending on frequency!! (This viable to very short microwave
> frequencies, but the same applies elsewhere Ignore all the text - just
> look at the graph, and not both axes are logs).
>
> http://www.lsbu.ac.uk/water/vibrat.html
>
> But other materials loss peaks will not necessarily cause heating at
> 2.4GHz.
>
> Light travels through water relatavily easily - you can see through
> light pretty well. 600 nm (5 10^14 Hz) or so would be a green light
> (if
> I recall correctly). Yet lower the frequency by a factor of 5 or so to
> get to 3um (10^14 Hz) and water is *highly* absorbing. So water absorbs
> the low frequency less than the high one. In fact, water has a lot
> absorption peaks, where it is highly absorbing, as that graph shows.
>
> Put a transmitter at one of those frequencies and any water will get
> very hot. Yet it is very low loss at about 400um (ultra violet).
>
> You can see that the absorption of water varies from 10^6 cm-1 to 10^-4
> cm-1. Those two are a factor of 10000000000 apart!!!
>
> Other materials are the same - they will have absorptions that depend
> on
> frequency. Something that is poor at 2.4GHz where the microwave works
> is
> not necessarily poor at some other frequency.
>
> An example is PTFE. Most people know that as a good dielectric with low
> loss at RF. But move to light and it is pretty absorbing - you can't
> see
> trough more than a couple of mm of it.
>
> The hemoglobin in blood is another example. This fact is used by the
> pulse oximiters that clip on your finger and are routinely used to
> measure blood oxygenation in hospitals. The absorption changes
> depending
> on whether it is oxygenated or not.
>
> Perspex is another. Clear in the optical region (around 450-700nm), but
> highly absorbing at 10.7um (where a C02 laser works). I know a
> colleague
> who worked on C02 lasers said they used perspex goggles, as they
> perspex
> started melting if the laser was hitting them, and you would know this
> before it melted enough to damage your eyes. (You can't see 10.7um with
> your eyes).
>
> So I don't think the fact a material does or does not absorb at the
> specific frequency of a microwave oven tells you that much about how it
> will work at a very different frequency.
>
> YOU NEED TO KNOW THE LOSS AT THE FREQUENCY WHERE YOU INTEND YOUR
> TRANMITTER TO WORK - NOT AT SOME FREQUENCY THAT IS FAR REMOVED FROM IT.
>
>
> Just my opinion,
>
> Dr. David Kirkby, G8WRB.
>
>
>> K6KWQ Dick "12 Volt radios are for wimps, real radios can kill you"
>> Still can't make it go QRO with "More Power"
>> ----- Original Message -----
>> From: John Irwin<mailto:crazytvjohn@yahoo.com>
>> To: amps@contesting.com<mailto:amps@contesting.com>
>> Sent: Monday, March 14, 2005 3:54 PM
>> Subject: [Amps] nylons
>>
>>
>> dudes
>>
>> a large ace hardware store opened here last week , One block away.
>> I have noticed a lot of nylon items that look a lot like the ceramic
>> standoff insulators, they come with a hole in the center,and some are
>> taped for 6-32 or other simular sizes. lenght every 1/8 inch or
>> so,diameter 1/4 to 1/2 inch
>>
>> How does nylon act in rf? Like in a amplifier as used to support
>> variable capacitors or inductors? Next to the plate choke.
>>
>> Anyone put some in the pi coil and see if it holds up. or does it go
>> up in smoke?
>>
>> thanks
>> john kb9tc
>>
>>
>>
>>
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>
>
> --
> Dr. David Kirkby,
> G8WRB
>
> Please check out http://www.g8wrb.org/
> of if you live in Essex http://www.southminster-branch-line.org.uk/
>
>
>
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Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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