[ARC5] Why an "UNUN?"

[Dennis Monticelli]

Well, I kind of look at it this way, Dave.

There are any number of unbalanced networks that can be used to
transform impedances (the pi, the T, and many others) both upward and
downward.  These networks consist of L's and C's and may or may not
incorporate transformers.  What tends to tie most of them together is
that they are tuned.  Some are high Q and some are low Q.  They are
not what one would call broadband.  A transformer wound properly can
serve as a broadband impedance transformer and thus serve us well over
the spread of a wide ham band or for multiple bands.  It can be
constructed to be balanced or unbalanced.  Because the ARC-5 has an
unbalanced tuned output network anyway and serves a single ham band, I
don't see a big advantage to using the so-called UNUN concept except
for one reason.  If one were to build a good broadband version it
could be swapped (externally) from one TX to the other without any
adjustments.  Very convenient.

I used an UNUN years ago to match a 40/80M wire antenna.  It was
located at the feedpoint and worked just dandy.

Dennis AE6C


On 10/24/12, David Stinson <arc5 at ix.netcom.com> wrote:
> 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
> ______________________________________________________________
> ARC5 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/arc5
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:ARC5 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>