[ARC5] Why an "UNUN?"

[David Stinson]

What's an "UNUN" and why do we need one on
the output of a Command Set transmitter?

Three people have asked me about this, so I'm 
sticking my stupid neck out and copying my reply
to the list.   I think it's "close enough" to get the concept
and I depend upon the list people smarter  than me-
and there's a lot of them- to iron-out any wrinkles
in this over-simplification, and yes I know I'm 
leaving out reactances and the tuning of them,
since that's a different subject:

An "UNbalanced to UNbalanced (UNUN)" transformer
is actually an "autotransformer,"  
designed to raise the Voltage appearing across the 
12-Ohm "source impedance" 4 times higher,
so the same power can be developed across 
the 50 Ohm "load impedance."

The transmitter's output being, say "12 Watts into 12 Ohms,"
means the output is designed to source a certain Voltage
at a certain current across something like a 12-Ohm resistor,
and the product of this Voltage and current equals 12 watts.
We call this 12-Ohms our "source impedance."

Our antenna, or "load impedance" wants to develop 
that same power- 12 Watts- but across 50 Ohms.
To get the same power across a higher impedance,
the simplist solution is to boost the voltage 
while reducing the current. 

So ...(yes; I know this is DC Ohm's Law and 
we're talking AC, but I'm after the concept)...
Our "source" is delivering 12 Volts at 1 Amp 
for 12 Watts, but our antenna wants 
48 Volts at 240 milliAmps to develop 
that same 12 Watts across 50 Ohms.
So we wind an "autotransformer," which 
boosts the Voltage 4 times for us, allowing
the full power that the transmitter can "source"
to be developed across the "load impedance."

That's probably clear as mud, but I tried. ;-)

If one is just using a non-resonant short wire,
like the 20-foot whip someone mentioned,
the UNUN is not needed, since the transmitter
output network can "tweek" to match that.

73 DE Dave AB5S