There's a simple way to look at this that requires no high-level math or
complicated analysis.
_________ _________
I1 --> | | | | I3 -->
--------| |--| |-------
Transmitter | BALUN | | TUNER | Antenna
--------| |--| |-------
I2 --> |_________| |_________| I4 -->
The purpose of the balun is to eliminate the common-mode current. The
CM current is the NET current, that is, I1 + I2 at the input and I3 + I4
at the output. The balun does not store electrons, so it must be that:
Input common-mode current = output common-mode current.
That is true for both the balun and the tuner. No matter whether the
balun is on the transmitter or antenna side of the tuner it does an
equally good (or bad) job of choking the common-mode currents.
The advantage of putting the balun at the input is that the
differential-mode voltages and currents (the ones you want) are
well-controlled because of the constant 50-ohm impedance. The DM
current or voltage at the output can be much higher, depending on the
load impedance. Of course, the common-mode current and voltage are the
same at the input and output, but even so the worst-case stress on the
balun should be less when it is at the input.
The disadvantage of putting the balun at the input is that none of the
tuner circuitry can be grounded. For example, the control shafts of the
variable capacitors have RF voltage on them, so the knobs must be
isolated. If it is an automatic antenna tuner, that is less of a
problem.
Bottom line? The ARRL is not necessarily "wrong" to put the balun at
the tuner input. It's just a matter of whether you think the lower
worst-case current/voltage on the balun is worth the hassle of floating
the tuner ground.
Alan N1AL
On Thu, 2011-12-08 at 18:59 -0500, Don Wilhelm wrote:
> This is of interest to me, because in my first experiences with antenna
> forums (or was it newsgroups at the time), I recall a lot of exchanges
> with Tom W8JI on exactly the "balun at the tuner input and isolated
> unbalanced tuner". I concur with Tom - it does not work -- both from a
> theoretical basis, and also from Tom's measurements.
>
> This was "way back when" - as I recall my situation when all this was
> going on, I was running Windows 95 and the year was between 1997 and 1998.
>
> In the timeframe of this discussion, Zack Lau (ARRL engineer) who had
> first published the "balun at the tuner input" concept as a QRP tuner,
> had retracted that design because it did not maintain balance, but Dean
> Straw (ARRL engineer, editor, etc.) published his design of a high power
> tuner using the same concepts, and that design can still be seen in the
> ARRL publications.
>
> Apologies for the comments into the politics of the ARRL decisions on
> what is to be published, but that is both a bit of the history as I know
> it as well as my view of the technical side of this issue.
>
> If anyone can tell me how you can run a signal through a balun - and
> have equal and opposite currents at its output, and then run it through
> an unbalanced network with unequal elements in the two series legs and
> still maintain equal and opposite currents and phase, and I will then
> concede that an isolated unbalanced tuner with a balun at the input will
> work, but until that is presented to me along with detailed engineering
> level test data (not just "it works"), I will continue to believe that
> using a balun on the input of an isolated unbalanced tuner is a "pipe
> dream" that does not mesh with reality.
>
> 73,
> Don W3FPR
>
> aOn 12/8/2011 6:28 PM, Eric Swartz - WA6HHQ, Elecraft wrote:
> > Hi Ignacy,
> >
> > This is a common misconception. (One which I held until recently. :-) It
> > turns out there is no advantage to placing the balun at the input of the
> > L-Network tuner. Since one end of the balun is grounded by the input to
> > the tuner, it is still presented with the same stresses under high SWR
> > situations. Baluns at the input and output both drive balanced loads
> > equally well.
> >
> > We've now put together a web page describing the impact of placing the
> > balun at the input or at the output of a L-Network tuner. See:
> >
> > http://www.elecraft.com/KAT500/input_versus_output_balun.htm
> >
> > At the bottom of that page are several links providing detailed
> > technical analysis of these configurations. The first two, by W8JI and
> > W7EL are very clear discussions of this issue.
> >
> > 73, Eric WA6HHQ
> >
> > www.elecraft.com
> >
> >
> > On 12/8/2011 3:17 PM, Ignacy wrote:
> >> It seems to me that the story is more complex than it sounds.
> >>
> >> The input balun always works at low SWR and at 50 Ohm. It is very easy to
> >> have such balun. A small balun would easily handle a KW without heating.
> > ______________________________________________________________
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:Elecraft@mailman.qth.net
This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
