Re: QRP Pen circuit


Date: Fri Feb 10 1995 - 15:49:09 EST

Dana, you wrote, in part:

> No, there is no VSWR at the collector, at least not as you described.
> If the load presented to the collector is other than the 60 ohms or so,
> the output power will be different. If you have a 52 ohm load in the
> above example, you need to re-arrange the power formula and plug the
> new collector load in. Ex:
> Power Output = Po = (Vcc - Vce(sat))^2 / 2*RL
> With a 52 ohm load, you get (6-.5)^2/2*52 = .291W.
> If you increase the power supply to 12V, you get 1.272W.
> The output power is determined by the collector voltage,
> impedance matching from the collector to the antenna, and
> the impedance of the antenna. If any of these three factors
> change, so does the output power.
> On a side note, the reason the notion of collector VSWR is not
> correct is that the transistor, assuming class-C operation, always
> presents either a very high impedance or a very low impedance. In
> either case, the "SWR" calculated as a ratio of load impedance
> to source (transistor) impedance will be high. I don't really
> want to open the can of worms that continues to rage on on the
> newsgroup about SWR and Maxwell; suffice
> it to say, there really is no notion of collector VSWR.
> Any comments?

Good discussion! And ditto not wanting to spread the
discussion here ;-).

I gave a similar discussion of output networks, impedance and power level in
the Technical Tidbits column that Mike, WA8MCQ edits in the latest QRP
Quarterly. It's my latest "Joe's Quickie."

The only thing I have to add to the above, is that the formula predicts the
MAXIMUM power available. Just using a static saturation drop for the
transistor doesn't give a full appreciation of the drop in the transistor
under class c rf conditions. It is a good formula for estimating the
maximimum power expected under ideal, low-loss conditions. That is, you can
approach this power level, but not exceed it.


Joe E. N2CX

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