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Re: Discharge impedance questions. (RG-8, matching)
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> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Re: Discharge impedance questions. (RG-8, matching)
> Date: Sunday, March 12, 2000 4:47 PM
>
> Original Poster: Terry Fritz <twftesla-at-uswest-dot-net>
>
> Hi Jim,
>
> The wavelength is 2980 feet at 330kHz. RG-8 has a velocity factor of
66%
> and is 26.4pF/foot. So the cable is 0.00508 of a wavelength and 264pF.
> However, I was always under the impression that if you have a 50 ohm
source
> and 50 ohm cable and the forward power is say 400 watts and there is no
> reflected power, that you must be running into a 50 ohm real load. In
that
> case, the cable length makes no difference other than phase delay.
True.. but is the source truly 50 ohms? Most high power sources have some
sort of matching network inside the generator to transform the impedance it
sees into the cable into whatever the power amp puts out. At that point
the reflected power (at the generator end of the cable) will be zero, even
if the impedance isn't 50 ohms. [ The whole match vs reflected power vs
impedance issue is a source of many misconceptions in the ham radio
business where someone thinks that an antenna tuner at the transmitter end
fixes the mismatch loss in the cable. ]
In your case, the cable is SOOOO short (compared to a wavelength) that the
effect is negligible, though. If you were looking for gnat's eyelash
precision, I'd worry more about the connector mismatch.
>
> What may be more of a factor is that I can adjust the frequency. Perhaps
> the coil's impedance will always have a 50 ohm point at some frequency.
I
> turn up the power and then adjust the frequency for the lowest reflected
> power possible. I assume if 400 watts is going in, and none is coming
> back, that all is well.
This is true.. all the power IS going to the coil, it just might not be a
50 ohm load at the coil end.. but then, you might not care? Measure the
magnitude (you don't care about phase, since, because it is matched, you
know they are the same) of the voltage and current (actually, either one,
because you know power) and you'll know for sure.
>
> I just ran the impedance and phase in MicroSim, There is a 50 ohm point
> that is at an angle of 179 degrees (almost zero). The impedance really
> peaks at 92 ohms but this other point gives an almost zero reflected load
> match. Wonder if that is good, bad,...
>
> MicroSim says I should be using 50 turns on the primary instead of 10 to
> get about 50% more power to the load (resistive part). Perhaps that
would
> match the resonant frequency to the impedance match points better...
> Really, all we are doing is matching the 50 ohm real source impedance of
> the generator to the impedance of the arc.
Exactly.. and since the arc isn't a nice impedance, it will be tough to
keep a stable match (as your designers of automatic matching networks for
plasma etchers have found, no doubt).
>
> I am not a genius at all this load match stuff but I'll see what I can
come
> up with. I'll dig up a manual to the power supply too. I think it spent
a
> lot of time talking of such things.
Impedance matching is not something that is trivial or initially
intuitive.. Well it is in a general sense.. you turn the knob until the
reflected power is zero. However, if you want to know what power is really
getting transferred, and so forth, it is a bit tougher. You can have a
great match, but have surprising losses in the cable. A 20 dB resistive pad
makes a fine matching device... the source sees 50 ohms almost regardless
of what's on the the other end. Not much power gets to the load though...
The other real problem is that most signal generators that put out any sort
of power (more than a few mW) are often not very close to a 50 ohm source.
I think my 100 watt ham transmitter is 50 ohm resistive output at only 2 or
3 frequencies. That 1:1.5 VSWR spec allows for a lot of variation!
>
> Cheers,
>