[R-390] Measuring Sensitivity

[Tim Shoppa]

Barry asks:

> What I'm still a bit confused about is the fact
> that the receiver is connected across the 1/2-ohm
> resistor.  Isn't the radio seeing a pretty bad 
> impedence mismatch here?  I realize this makes the
> generator immune to impedence changes
> brought on by the receiver's input, but doesn't this
> mismatch have a detrimental affect on the
> radio's ability to "see" the voltage across the 1/2-ohm resistor?

Low-impedance voltage generators driving high-impedance loads
are an easy case: the voltage at the load doesn't depend much
on the load or its impedance bumps/dips/peaks.

Think about it: a 120VAC 100-watt light bulb has a DC
impedance like 144 ohms or so, but that doesn't mean that
you want 144 ohms of wire in series with it! In real life
your wall socket has a souce impedance in the fraction
of a ohm range (drawing 15 amps will drop the voltage
by a few volts).

Now, for maximum POWER TRANSFER you want the
load to match the source impedance. But you don't care
about power transfer in this case, just the voltage at
the antenna terminals, all the extra power you're
generating in the signal generator you want to be
dumped.

Now if you were sending complex (non-sine) waveforms
you might be concerned about matching impedances
to reduce reflections, but that's not a concern either
because you'll either be using CW or audio-modulated
RF modes, right?

Tim.