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Re: SUITSAT forensics...



--- Mike Lemons <sputnik_ki6adn@fastmail.fm> wrote:

> A short circuit across the final output transistors would behave like
> this.  Assuming that the short is not from the power supply rail to
> ground.  I wonder if broadcasting into an open circuit, as you
> suggested, would also lower the current drain on the power supply
> batteries?

Maybe.  But I'd say that *A LOT* depends on the particulars of the final power
amplifier circuitry, and the impedance matching that takes place between the PA
transistor and the antenna connector.  The current might go up, or it might go
down.

If the transmitter happens to have high VSWR power foldback circuitry (not sure
how common that is for QRP handhelds), then the current drawn from the
batteries would almost certainly be less than it is when operating into a
properly matched load.

So, you'd get less heating, and longer battery life with the lower current draw
in this scenario.

Which brings to mind another thought...

I'd heard of situations (and I have never verified this personally, so don't
shoot me) that the final amplifiers of some handhelds are tuned at the factory
to match the impedance of the supplied antenna (rubber duck), rather than a
true 50-ohm load!

It would be interesting (to me, at least) to measure SUITSAT's output power
into a 50-ohm load to see how close it comes to the 500mW advertised into such
a load.

Another thing: Since it sounds like the handheld in question can be made to
operate at various power levels, the design is probably such that the highest
transmitter efficiency probably occurs at the highest power level setting.  If
this is true, then it might be advantageous (if time allows when SUITSAT-2
comes around) to re-tune the transmitter's final for maximum efficiency at the
500 mW level, and probably get a few hundred milliwatts of extra power out for
the same DC power in.

(Incidentially, UoSAT-OSCAR-9 and UoSAT-OSCAR-11 both operated on 145.825 MHz
FM with around 300mW out with plenty of signal.)


73, de John, KD2BD


Visit John on the Web at:

	http://kd2bd.ham.org/
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