[ARC5] Why an UNUN?

[mac]

On Oct 24, 2012, at 6:45 PM, J. Forster wrote:

>
> Not really.


Yes really


>
> A transmitter, like a battery or other power supply, can be modelled  
> as a
> Thevinin Equivalenrt, an ideal Voltage Source in series with an Output
> Impedance.


The equivalent circuit of a class C amplifier would look like an AC  
(RF) voltage source with a very low internal resistance.



>
> There is a circuit theory theorem that, for maximum real power  
> tranfer,
> the load impedance should be equal to Complex Conjugate of the source
> impedance.

We're not talking about maximum power transfer here.  A Class C amp is  
designed to provide a particular power output at a high efficiency,  
not maximum power transfer which results in an efficiency of 50% .   
You''re thinking of class A.


>
> This means equal to, if the source and load are purely resistive.

>
> This means if one is inductive, the other must be capacitive, and the


We keep the load resistance seen by the PA tube purely resistive here  
with our resonant tank circuit.   In practice we strive to present the  
amplifier with a purely resistive load if possible. Small values of  
reactance reflected into the tank circuit can be accommodated by  
tuning adjustments.  In those cases where the reactive components are  
too large external antenna 'tuners" are employed.


> inductive reactance equals the capacitive reactance.
>
> This means if the source is complex (resistive + inductive), the  
> load must
> have a load where the resistive componants are equal and the  
> capacitive
> reactance is equal, and opposite, to the source inductive reactance.
>
> ie:  The L & C are resonant at the operating frequency.

See above

Dennis D.  W7QHO
Glendale, CA