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Re: power....Re: Tesla Receiver Coil ..........success?

Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

At 08:59 PM 10/29/2005, Tesla list wrote: 
Original poster: <davep@xxxxxxxx>


>>   _power_ determination, in ac circuits, requirwa
>>   wattmeter, or similar, capable fo handling the
>>   waveforms/frequncies involved.  Ammeter indications
>>   tell little...

> What is the procedure for determining the RF power input to an SSTC
> operating from a pure DC power supply?
     Complicated.
     8)>>
> Can this be done using a calibrated RF current probe and oscilloscope
> plus an electronic voltmeter?
     Not riogorously, not, in my view, even approximately:
      V*I does not work with AC....
     Especially with complex wave forms, tho I'd think SSTC
     waveforms are less complex than others.
     A digitizing scope is one (the only?) thing that comes to
     mind...



Precision measurement of RF power is somewhat of an art.

The tricky thing is not waveforms: you can use any of the bolometric (i.e. using the energy to heat something up and measuring the energy that way) techniques to accurately measure the power.

It's figuring out an accurate way to couple out a small repeatable fraction of the power to measure, which usually requires measuring both forward and reflected powers, and that is somewhat of a challenge.

For a tesla coil, a current probe and a voltage probe and a digitizing oscilloscope are probably the easiest way. It would be straightforward to calibrate and make the measurements.

Another approach is to use some sort of reflectometer bridge with a pair of power detectors, but then calibration of the bridge is tricky, and I've not though about what building a reflectometer that works at 100 kHz would entail. Probably easier and cheaper to do the digitizing oscilloscope thing: especially since Velleman sells an inexpensive module that does it all, or, one could use the Analog Devices high speed A/D evaluation modules, which digitize into a fast FIFO that is then unloaded with a USB interface.

However, if these schemes don't appeal, you can use what's called the "six port measurement" technique, developed by Glenn Engen a few decades ago. It basically says that you can do the measurements with an arbitrary 6 port device, with power meters hooked on 4 of the ports.


     RF input, while interesting, ignores the losses in getting
     from DC, or power line AC, to 'rf'.

Indeed... and that's probably where most of the efficiencies (or lack thereof) lie.

     best
      dwp