On Fri, 5 Apr 2002 22:36:07 +0200 "Roger Blain" <rblain@iafrica.com>
writes:
> Hi List,
> I have been following the thread on the Bird power measurement with
interest.
> My dilema is I am trying to build a directional coupler, to measure
> the power going into a plasma tube.
> The oscilator is 812A based Hartley with unknown impedances (I guess
> they could be calculated, but my valve circuit theory is virtuly non
> existent!) , certailnly the load is nowhere ideal either, nor near
> 50ohm.
>
> After revewing a few articles etc, I managed to build a basic Bruene
> Bridge directional coupler to the point I could scope out the V & I
> curves of my 3.5MHz carrier, I am also able to measure the HV
> voltage waveforms across the plasma tube using HV scope probes.
>
> Now where can I go to find some internet based references that will
> give me a good overview of directional coupler theory not related to
> standard 50Ohm impedances, and how I can relate what I am seeing to
> power both forward and reflected as well as SWR. I understand the
> practical use of these terms, but am battling for information that
> will allow me to design/build a fairly accurate power measurment
> device.
>
> Regards
>
> Roger Blain
>
Hi Roger,
I can't point to any references on the net but, if you found them they
would say that (constant) impedance is a factor in the design of a
directional coupler.
Since you have an essentially resistive, albeit wandering, load you
might just use an old (thermocouple style) RF ammeter to calculate your
power output. I'm assuming that the plasma tube exhibits a fairly
constant voltage drop when ionized.
73 & Good evening,
Marv WC6W
*
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