[ARC5] Why an UNUN?

[Mike Hanz]

On 10/24/2012 2:52 PM, David Stinson wrote:

> Couple of reasons for using the series cap
> and broadband UNUN:
>
> Assuming a resonant antenna, does not require
> one to build and/or add an external tuning network.
> More important:  It's simple, easy and cheap.

I can't disagree with anything that has been said, Dave.   One other 
consideration I might add is related to the distance from the antenna 
feedpoint to the transmitter.  if you have a longish run of feedline and 
the antenna can be conditioned /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.  Depending on the antenna, this may mean 
having a remote antenna tuner (at the base of a vertical, for example) 
to make minor adjustments across bands.  That's the approach all the 
later HF aircraft transmitters followed when it was no longer possible 
to have a transmitter feed the liaison antenna directly through the 
fuselage - it began with the AN/ARC-21, IIRC.  Using the unun with the 
series capacitor between it and the transmitter provides the high 
capacitance low resistive conjugate match that the transmitter is 
looking for on the one hand, and the pure resistive load that the coax 
is looking for on the other.  Win-win!  There is a small power loss in 
the unun, but they are pretty tolerant of small mismatches either way 
from perfect.  And that RF ammeter in the antenna relay really stands up 
and takes notice! :-)

73,
Mike  KC4TOS