[Elecraft] More Balanced Current Measurements - Voltage Balun

[Tom W8JI]

> I had previously posted some RF Current data using an 
> MFJ-835 Balanced RF
> Current meter.  I measured the amount of current flowing 
> in each side of my
> ladder line

I'm pretty sure that unit doesn't actually indicate balance. 
I'm pretty sure it only indicates equal currents by sampling 
scalar currents in each conductor. I'll look into that, but 
everything in the manual seems to indicate it is a scalar 
measurement of current in each conductor.

Anyway the important point is that it is possible to have 
equal currents in each conductor and have perfect UNbalance.

To measure balance we have to measure phase, not just scalar 
currents. We also have to either measure current balance at 
two points some large fraction of a wave apart (like 1/8th 
or 1/4 wave), or measure BOTH voltage and current balance at 
one point.

>and compared the Johnson Matchbox, the internal KAT3 
>antenna
> tuner with the Elecraft BL2 balun in both the 4-to-1 and 
> 1-to-1 positions,
> and a direct connection from the KAT3 to the ladder line 
> with no balun.
> Tonight, I came across a 4-to-1 voltage balun left over 
> from the days of the
> Ultimate Transmatch (QST July, 1970 - wow! - that is a 
> long time ago but
> seems like yesterday).

There are only a few cases where we might be better off 
using a 4:1 balun rather than a 1:1 balun to transform 
random impedances to a tuner. Those cases would be where the 
antenna system (at the balun connection point) presents a 
reasonable impedance to the balun, the balun is designed for 
that frequency and impedance, and the antenna system balance 
presented to the balun is pretty good.

It would be even rarer to need or want a voltage balun. I 
can't think of many of any cases where I would want to use a 
balanced voltage source to feed a simple antenna. As a 
matter of fact I virtually never use a 4:1 unless I'm 
matching a folded dipole to coax.

> Anyway, I put the voltage balun in line and repeated the 
> measurements.  Here
> are my conclusions based on my results:
>
> 1) Without the MFJ 835 current meter, I would have been 
> happy with any of
> the configurations.  All of them had a perfect 1-to-1 SWR 
> when matched. All
> of them resulted in good QSOs.
> 2) The Johnson Matchbox has the best balance and least 
> loss on all bands.
> 3) A balun on the output of an unbalanced tuner is a 
> compromise and there is
> loss there.
> 4) The BL2 has good balance on all bands 160m to 6m but 
> gets very hot on
> some bands indicating loss. I have received reports that 
> the Johnson MB is
> several S units better than the balun on some bands.
> 5) The voltage balun works pretty well on 160m, 80m, and 
> 40m but is poor on
> 30m and higher. It is useless on 6m.
> 6) The direct connection had very poor balance on some 
> bands.
>
> So, IMHO, a balanced tuner is the best matching device for 
> a balanced
> antenna fed with ladder line. We must find a way to 
> convince Elecraft to
> offer a balanced version of the KAT3.

As a general rule it is far less expensive to build a good 
1:1 balun to use on the output of a unbalanced tuner than it 
is to build a balanced tuner. This is especially true if the 
load is not perfectly balanced, or on higher frequencies.

When I built high power tuners for a 25 kW AM SW BC station, 
I looked at all options and used 1:1 baluns on the matching 
system output. It was several thousands of dollars cheaper 
than using a balanced network and worked just as well.

I'd encourage them to build a good 1:1 balun, or perhaps 
two...one for lower bands and one for higher bands. The 
expense of a balanced tuner generally just isn't worth it, 
and a 4:1 is almost never needed.

I'll look at that balance indicator if I can get my hands on 
one. It might not be, but from the manual it sounds like it 
is a scalar device that does not consider phase.

73 Tom