[ARC5] Why an UNUN?

[mac]

A transmitter does not present a "load" as such to the input end of a  
transmission line.  The transmitter delivers power to the transmission  
line, i.e., "drives" the line.

The commonly used term "output Impedance" is misleading, a transmitter  
does not "output" an impedance.  What we are actually referring to  
here is that impedance value which when connected to the output  
terminals of the transmitter  will reflect a value of impedance  back  
to the plate circuit of the PA tube that will, in turn, cause the tube  
to provide a desired power transfer to an external load.    
Approximately 12,500 ohms reflected to the plate circuit of the 250TH  
PA would be correct in the case of a BC-610.  (About 5K for a command  
xmtr.) Most transmitters include some form of   impedance translation  
network to allow connection to commonly  encountered load impedance  
values, 50 or 75 ohms for ex.  Variable link coupling to the  
transmitter's tank coil is a simple and common example of such a  
network.  The tank coil and the link form a transformer circuit and  
varying the mutual coupling between the tank and the link has the  
effect of varying the turns ratio of the transformer windings and  
hence the impedance transformation ratio.

" Loading" is the descriptive term commonly applied to the process  
whereby the impedance reflected back to the plate of the PA is  
adjusted so as to deliver a desired power output value to an antenna  
or other load.

Note that we normally talk in terms of purely resistive impedance  
values here.  We are not necessarily limited in doing so, however.   
The calculations are just more complex when reactance must also be  
taken into account.

Dennis D.  W7QHO
Glendale, CA

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On Oct 24, 2012, at 12:42 PM, Mike Hanz wrote:

> /at its feedpoint/ to the characteristic
> impedance of the coax (like 50 ohms resistive), you minimize losses in
> the coax significantly by having a similar pure 50 ohm resistive  
> load at
> the distant transmitter end.