R. Measures wrote:
>
> 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?
Sorry, two hydrogen ones.
BTW, here's a joke someone told me recently, which that comment reminded
me of.
There are two hydrogens atoms in a water molecule.
One hydrogen atom says to the other "I have lost an electron".
The second hydrogen atom, says "Don't be silly, you have not lost an
electron.".
The first one then says "Yes, I have lost an electron - I'm positive."
>> 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.
I make no claim about the losses of nylon at RF, since I do not know its
properties. I'm only saying that testing at one frequency does not tell
you much about losses at another frequency.
In any case, there are different types of nylon. Perhaps with a search
of the internet, one might find some properly measured data, but unless
you know the exact source of the nylon, testing it yourself at the
frequency of interest is probably the best way.
I think you are right in saying loss does tend to increase with
frequency for solids at RF, although that graph for water clearly shows
the relationship is far from simple - for water at least. It may be
simpler for plastics at radio frequencies. But I think this idea of
relying on a microwave oven in unlikely to be a very valid test,
especially given it is around 100 to 1000x higher in frequency than your
HF amp.
G8WRB
>> 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
>
>
>
--
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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