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). So if a plastic had a lot of water
in it, it would get hot if fed with 2.4GHz. 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
>
>
>
>
> ---------------------------------
> Do you Yahoo!?
> Yahoo! Small Business - Try our new resources site!
> _______________________________________________
> Amps mailing list
> Amps@contesting.com<mailto:Amps@contesting.com>
>
> http://lists.contesting.com/mailman/listinfo/amps<http://lists.contestingcom/mailman/listinfo/amps>
>_______________________________________________
>Amps mailing list
>Amps@contesting.com
>http://lists.contesting.com/mailman/listinfo/amps
>
>
>
>
--
Dr. David Kirkby,
G8WRB
Please check out http://www.g8wrb.org/
of if you live in Essex http://www.southminster-branch-line.org.uk/
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|