RF 'echoes'

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Dustinit
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Re: RF 'echoes'

Post by Dustinit » Thu Jan 28, 2010 10:46 pm

To get a better handle on the reflected power due to mismatch
I found this program that may help and interactively explains it beautifully.
It can be found here

http://www2.fh-rosenheim.de/diegelmann/ ... nglish.htm
Look for the title
Reflection and Transmission at an Impedance Mismatch.

At the bottom z1 is source impedance z2 is load.
green line is voltage into load or termination
red is source voltage blue is reflected
black is the sum of source and reflected and is what is seen at the source.
echo terminology grates because the wave is continuous but would be relevant
for pulses shorter than the transmission line length.
Hope this helps .
Dustin
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Doug Coulter
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Re: RF 'echoes'

Post by Doug Coulter » Thu Jan 28, 2010 11:06 pm

Guys, I know some of you are "RF pros" for sure, as am I. I am also expert in fields where the term echos is more appropriate as well, when for example digital switching is involved where lines are a fraction of a wavelength for some frequency components, and many waves long for others. This term is used in the ISO PCI bus spec and many other places in digital land, as it makes way more sense than return loss or SWR there. Lyn wasn't all wrong to use it.

SWR and return loss are fine for narrow band, steady state where the results of a lot of cycles are added and smoothed out into the nice peaks and nodes. Now handle the case of a single step function that way -- echo makes *far* more sense in that context, not to be all that pedantic about it.

Obviously, it's the same root phenomenon either way, but one way talks far more intuitively about single events, the other for narrowband steady state situations. Step outside your narrow specialty! There are no "standing" waves in a single step or impulse event, it all flies back and forth on the transmission lines precisely as an acoustic echo goes back and forth between reflecting walls, so it makes sense to think of that way, and the RF concepts don't work as well for that at all.

Jargon and overspecialization, don't get me started! Both obfuscate and make things harder to understand when getting a little outside the box. And this forum is about just that --
Why guess when you can know? Measure!

John Futter
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Re: RF 'echoes'

Post by John Futter » Fri Jan 29, 2010 12:52 am

Doug
Iwas under the impression that lyn was going to drive his ICP/ CCP with CW RF not pulses.
I agree Echo is acceptable for pulse work whether it is RF, audio, electronic but not for CW
Try a web search for eliminating RF echos and it will not give the info Lyn wants --unless he is really trying to build an NMR which in this case is using pulsed RF.

Use the correct terms and then searching will provide the appropriate answers
From memory I think Lyn has a solid state ENr RF amp around the kilowatt level his ICP should only require 20 -80 watts so a 6 to 10 dB attenuator should keep the amp more than happy ie a return loss of 12 -20 dB presented to the amp output.
A cheap wideband attenuator is a 100m roll of RG58 5dB attenuation @11MHz, 10dB attenuation @ 22MHz etc. At this power level I would keep the roll of cable in a bucket of water for extended runs.

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Doug Coulter
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Re: RF 'echoes'

Post by Doug Coulter » Fri Jan 29, 2010 2:10 am

Oh agreed. I was just saying the term isn't categorically in error. Even standing waves don't stand, it's just a superposition of echoes, well explained in say, the ECL handbook.
Why guess when you can know? Measure!

Linda Haile
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Re: RF 'echoes'

Post by Linda Haile » Fri Jan 29, 2010 5:34 pm

John, I don't have an amp at present, maybe you have mistaken me for someone else. I'm planning to try different circuits including some built from scratch starting in the 20-80 watt range as you suggest. I'm starting from the pretext that an ICP source is a transformer with the plasma as the secondary as I believe this description is the simplest definition of an ICP source. My previous experience in this field is with air core transformers in Tesla tank circuits and I'm planning to build on this experience to develop an ICP source from first principles. I'm planning to experiment with tank circuits, linear amps and ferrites initially, rather than magnetrons. I've no idea which of these approaches will work (or work best), hence the experimental approach.

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Carl Willis
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Re: RF 'echoes'

Post by Carl Willis » Fri Jan 29, 2010 5:52 pm

Steve Robinson is the fellow with the big ENI amp.
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