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Re: Random TC Questions- streamer current

Original poster: Terry Fritz <teslalist@xxxxxxxxxxxxxxxxxxxxxxx>


I have also used light bulbs for RF loads. They seem to work very well and present a very pure (low VSWR) resistance up to a few MHz. The resistance does very with power, but running 100W at 60HZ or 350KHz seems to give the exact same results. John C. compares the brightness of bulbs in the circuits to use as a very simple power meter.

The filaments are just so small and compact, they don't have much L or C to mess with the higher frequencies. The 250 Watt IR types present very close to a 50 ohm load. Use RG-8 coax right up to the bulb socket though.



At 10:41 AM 3/19/2005, you wrote:
"Is it really valid to assume the current in a light globe is the same
50Hz as 100kHz?  I would suspect that the "coiled coil" tungsten
may alter this.  It might be interesting to compare this to another
of current readings such as with a high speed bridge rectifier feeding a
capacitor and a DC ammeter.

        Almost certainly, because the Q of the filament is so low that the
reactance probably doesn't count below 1 MHz or so.  There are tubes
specifically made for power measurement which consist of two identical
straight filaments mounted in an evacuated envelope.  The power to be
measured is fed to one filament and the current in the other adjusted
from a separate source to give equal brightness (the eye is very
sensitive to small brightness differences), at which point the two
powers can be considered equal.  I've used a similar method to measure
powers at frequencies as high as 1200 MHz and the accuracy, compared to
a VHF wattmeter, seemed to be within 10% or better.