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
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