Why need a balanced line tuner?

[Karsten Eppert (DK4AS)]

I am happily using a doublett-antenna that I feed with a 600 Ohm
ladder-feedline.
When I came to decide, what kind of tuner to use, I could not find
anybody to give me an answer to my questions that would satisfy me:
-   use a balanced tuner, like the MFJ 974 H (which I finally purchased)
-   or use a standard-unbalanced tuner with a toroid-balun between the
tuner and the feedline.

In fact, the balanced tuner has a balun at it´s front end, looking
towards the transmitter. Other tuners have the balun at their backdoor,
looking towards the feedline.

What is now the advantage/disadvantage of which concept? Can anybody
explain that to me?

Yes, I am happy with my MFJ, but being an Elecraft-maniac in the
meantime, I would like to have the autotuner, which would need the extra
balun.

Thanks to everybody undertaking the effort to answer me.

Karsten
DK4AS
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[Don Ehrlich-2]

A balun at the input of a tuner is looking at a reasonably low SWR.  A balun
at the output is looking into the feedline which will typically be running
at a high SWR.
Baluns only work well when looking into a low SWR.  They can experience very
high losses otherwise as well as failing to function as a balun.


Don K7FJ

>
> In fact, the balanced tuner has a balun at it´s front end, looking towards
> the transmitter. Other tuners have the balun at their backdoor, looking
> towards the feedline.
>
> What is now the advantage/disadvantage of which concept? Can anybody
> explain that to me?
>
> Yes, I am happy with my MFJ, but being an Elecraft-maniac in the meantime,
> I would like to have the autotuner, which would need the extra balun.
>
> Thanks to everybody undertaking the effort to answer me.
>
> Karsten
> DK4AS

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[Ron D'Eau Claire-2]

Karsten, DK4AS wrote:
I am happily using a doublett-antenna that I feed with a 600 Ohm
ladder-feedline.

When I came to decide, what kind of tuner to use, I could not find anybody
to give me an answer to my questions that would satisfy me:
-   use a balanced tuner, like the MFJ 974 H (which I finally purchased)
-   or use a standard-unbalanced tuner with a toroid-balun between the
tuner and the feedline.

In fact, the balanced tuner has a balun at it´s front end, looking
towards the transmitter. Other tuners have the balun at their backdoor,
looking towards the feedline.

What is now the advantage/disadvantage of which concept? Can anybody
explain that to me?

---------------------------

By "doublet-antenna" I presume you mean that it is not cut to self-resonate
on any particular band. That means that it will show inductive or capacitive
reactance on any band. The tuner must compensate for this reactance was well
as transform the remaining resistive part of the impedance to 50 ohms for
the transmitter.

Any type of balun becomes unpredictable when in a circuit with a lot of
reactance. Some are better than others, but they all have problems under
some conditions. So putting a balun between the tuner and the feedline in
such a system many work well on some bands and work very poorly on other
bands.

Most commercial tuners sold over the past 30 years were designed to correct
*small* SWR errors that are typically found in practical antennas fed with
50-ohm coax, even when they were cut to length for the band desired. These
tuners do a great job of correcting SWR's at the feed line of up to about
4:1 to provide a non-reactive 50-ohm match to the transmitter.

These tuners were commonly equipped with a balun at the output at well.
Originally, the baluns were designed to feed a resonant 300-ohm antenna
system, such as folded dipole cut to the right length. Such an antenna needs
a 300 ohm balanced feed. A 4:1 balun at the output of the tuner converted
the 300 ohm impedance at the feeders to 50 ohms for the tuner output. The
tuner could then correct for fairly small mis-matches in the balanced
antenna system, just as it did for coax-fed antennas.

When the "WARC bands" were opened, Hams were faced with a dilemma. An
antenna system that would cover all the bands and provide a low SWR at the
transmitter became very complex for any Ham who couldn't put up several
antennas. Random end-fed wires and non-resonant "doublet" antennas fed with
open wire lines started to appear in numbers that had not been seen in Ham
installations since the 1930's and 1940's. The common commercial tuners were
pressed into service to match these antennas to the rigs. While the tuners
would often provide a low SWR to the transmitter, the combination was not
always an efficient antenna system.

The common "T" match tuner sold by MFJ and others, for example, are fine
when correcting small SWR's, but they can become very lossy when handing the
extreme sort of mismatches found when feeding "random wires" and "doublets".
Sometimes the coils inside the tuner would overheat and melt. Sometimes the
balun would arc over or overheat and the core would actually fracture. More
often, simply a lot of RF was lost in the losses and the operator never
realized he was transmitting a much weaker signal than the output power from
his rig would suggest.

In recent years there has been a lot of interest in a truly wide-range
antenna tuner that is efficient across the HF bands. The "L-network"
consisting of an inductor in series and a capacitor to ground at either the
rig or antenna end of the inductor is recognized as one of the most
efficient impedance matching networks. Elecraft uses this circuit in all of
their tuners. It is, unfortunately, a "single-ended" system for feeding coax
or an end-fed wire, so operators using balanced feeders resort to a balun at
the output of the tuner, again with mixed results.

Arguably the most efficient antenna tuner for balanced lines is a balanced,
link coupled tuner such a described by Cebik (See
http://www.cebik.com/link/link.html for examples). This is a design that
goes back to the earliest days of short-wave work. It is similar to some
commercial tuners made half a century ago by companies like E.F. Johnson.
The above link describes the Johnson "Matchbox" tuners in some detail. In
more recent years a German company made a beautiful, if expensive,
fully-balanced tuner using a similar circuit. I believe the German
manufacturer is no longer making them. This circuit offers the greatest
matching range with the lowest losses for either balanced or unbalanced
loads, but it has the disadvantage of needing a number of adjustments. (Yet
even the "Matchbox" tuners traded off convenient no-tap adjustments for a
more limited matching range). I use such a tuner and, while I can change
bands in a few seconds, certainly less than a minute, it still requires
setting 6 taps and adjusting two capacitors and sometimes even requires
changing coils. Such a tuner is very difficult to make "automatic".

Several years ago the ARRL published a new idea - or if not really new at
least they brought it to the attention of a lot of hams.  This is a
compromise that uses a fairly simple matching network that is "floating" so
it will drive a balanced line. The matching network is made to "float" above
ground by putting a balun at the input to the matching network. Commercial
manufacturers have picked up on the idea in their latest "balanced" tuners.
That's the sort of tuner you are using.

When the matching network is adjusted for resonance, the reactance of the
system is zero. No matter how big the coils or capacitors in the tuner are,
they become 'transparent' to the RF which sees only resistance in the load.
That's the definition of "resonance" - an absence of reactance. So, by
putting the balun at the input to the ATU, it will never see a reactive load
when the system is properly adjusted and so the balun can be expected to
behave predictably and with low loss.

Some make the claim that since the reactive elements in the tuner are not
exactly symmetrical - identical elements in each side of the feeder - that
they still introduce some unbalance. The currents flowing in each feeder may
be identical, but they may not be exactly 180 degrees out of phase. I've run
tests on such tuners and found the currents in the feeders very well
balanced in amplitude but I have not attempted vector measurements to see if
they are exactly 180 degrees out of phase. The effect of imbalance is that
the feeders will tend to radiate (or pick up signals). In many antenna
setups, this is actually a help rather than a problem.  

In any case, this sort of tuner, such as you are using, is catching on very
quickly. It is simpler to adjust than a fully balanced link-coupled design,
offers good efficiency and a much wider matching range than the unbalanced
tuner with a balun between it's output and the feeder combination.

Ron AC7AC


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