Not to agree or disagree with any of the statements thus far on this
subject, I would like to add a real-world application that does seem to be
working fine with no RFI issues at legal limit. This is just food for
thought as it seems to work well for me and my specific application.
I have an 80M loop that surrounds my back yard, and for years it was fed via
coax to a homebrewed balun specifically wound to match impedances between
the coax and the loop at the loop's resonant point. This worked great for
roughly +/-150Khz from the resonance point of the antenna. Then I read an
article discussing a balanced to balanced tuner (two inductors & one
capacitor) fed with either ladder line or OWL that would open the
possibility of using the loop for other bands. I decided to give it a try,
certainly couldn't hurt, and I can always fall back to the previous method
if it becomes too much of an RFI hassle. So I built the tuner and first
deployed it in the study for testing. That testing involved 450 Ohm ladder
line from the study through a window all the way up to the loop. Even
though I cut the ladder line perfectly, there were still impedance
differences between the two wires such that RFI was experienced. Then I
placed a heavy duty 1:1 balun on the tuner, between it and the station and
this did seem to clean it up quite a bit, but I would still get RFI on a
couple of the higher bands. At this point, I knew the 1:1 Balun was doing
all it could do, but the ladder line itself has to be radiating as well. So
the next step was to deploy the tuner in the attic and use a short run of
OWL from the loop through the attic to the tuner. I then used coax from the
tuner to the station. This meant I had to add a few more features to the
tuner such that I could control the placement of the capacitor and operate
the stepper motors via an Ethernet connection. Once completed, I placed the
tuner in the attic ran a short piece of OWL from it to the loop, still using
the heavy duty 1:1 balun at the tuner, connected a coax run from it to the
study. With the tuner in close proximity of the antenna connected via a
short OWL run, and a 1:1 balun that is between it and the final coax feed
line, I am no longer seeing any RFI issues at all.
Conclusion: If using a tuner, the closer you can get the tuner to the
antenna while using a 1:1 balun to isolate the tuner from the coax to your
station, it should work well in most applications. I would also think the
same remote capabilities could be achieved using a link coupled tuner
design, you would need just one more stepper motor for the additional
capacitor.
73,
Mike
W5CUL
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Rob Atkinson
Sent: Sunday, September 25, 2011 7:27 AM
To: amps@contesting.com
Subject: Re: [Amps] RF in the Audio
The perceived problem with parallel balanced feedline has nothing to
do with the ability to achieve an acceptable balance in the system and
everything to do with the way most hams use the line and type of
matching network employed. Roughly 90% of so-called balanced tuners
are either non-symmetrical, inadequate in design or (this is the
closest to honesty) make no claim of being balanced but somehow claim
to handle balanced systems. The Johnson Matchboxes are genuine
balanced tuners that do the job right by putting RF currents in the
line that cancel and collapse the field. I've tested this with my
system using current meters and field strength measurements around and
in between my line in several random points. A better tuner is the
very hard to fine TMC TAC tuner, probably the best commercially
manufactured tuner ever made available to hams. But many hams express
dissatisfaction with the Matchboxes usually over their alleged limited
matching range. That gets us into a separate discussion about the
education of hams regarding tuners and their expectations, and is a
topic for TowerTalk.
The common mode problem exists where you have a balanced system, but a
noise point source is closer to one side of the system than the other,
so while you have equal and opposite transmit currents, you can have
c.m. on receive from a local point source near the antenna, such as
noise from a router or power supply in a neighboring home.
Jim,
I don't think I'd employ DX Engineering as some sort of imprimatur or
validator for your work, as they are in the business of making and
selling products for hams.
Having read the rest of your email, I understand your points and your
statements are convincing, on paper at least, but such a choke as you
describe seems to be a solution to a problem that doesn't have to
exist, if an operator were to employ a method of impedance matching
and transfer from balanced feed to unbalanced that would allow for the
isolation of the balanced feed to prevent a complete common mode
circuit.
I can see such a choke being worth a try for someone trying to force a
transfer with an unbalanced network, or with one of the symmetrical
tuners that contain a pair of synchronized roller inductors and a
single common capacitor. I operated with one of those for a few years
and did in fact experience c.m. issues such as conducted out of band
RF (a very strong electric service spark gap) detuning a vswr
analyzer, but in my case all these problems vanished once I started
isolating the balanced feed lines with inductive coupling (the
aforementioned Matchboxes). I believe that is a more robust and
reliable solution.
73
Rob
K5UJ
<<<MANY of the DX Engineering so-called baluns are common mode chokes --
indeed, what is commonly called a "current balun" IS a common mode
choke. Many DXE baluns that transform impedance are ARRAYS of common
mode chokes connected in series and parallel. If you open up some of
these you will clearly see chokes would not with coax, but with parallel
wires. And DXE DOES sell a common mode choke. I haven't bought one,
because I can rolll my own that are probably better for one-sixth of the
cost.
I HAVE inserted the bifilar chokes between the output of a Titan 425 and
the antenna tuner and tested at 1.5kW keydown for several minutes from
1.8 MHz to 28MHz. At that point, the choke sees ONLY the differential
field, and there is VERY little heating because the field from one
conductor cancels the field from the other. Dissipation due to common
mode current is a very different matter, and is discussed at length in
the tutorial. In essence, if the choke as sufficiently choking high
impedance and the antenna is not very poorly balanced, the common mode
current, and thus the common mode dissipation, is reasonably small. If
conditions of the application (for example, impedance transformation)
place very high common mode voltage across a choke, the common mode
impedance must be much higher. In a testing situation, I have set up
very high common mode voltages and placed two chokes in series to
withstand them. DXE builds some of their impedance transforming arrays
of chokes that way.
As to mismatch -- a study of the fundamentals of transmission lines
would lead one to the conclusion that the loss due to mismatch in the
short length of 100 ohm line that comprises the choke is quite small.
After all, one of the most common uses of parallel wire line (notice
that I do NOT repeat the fiction of calling it a balanced line) is to
minimize the loss due to mismatch when feeding antennas that are wildly
mismatched, like the "one-size'fits-all" dipole that is nowhere near
resonance on most frequencies where it is used. Think about this --
we're connecting an antenna that could be anything from 5 ohms to 5,000
ohms, plus reactance, to a feedline that is, perhaps, 400 ohms. The
insertion of a 24 inch piece of 100 ohm line simply modifies (and not
very much) the impedance of the antenna as seen by the line. And, if
wound using #12 copper, as the chokes I have described are, the loss is
VERY VERY small, as confirmed by my tests.
Now, I'm a guy who plays by the rules, and shares my work FOR those who
play by the rules, and my testing is done at that power level, at duty
cycles consistent with serious contesting. Someone who wants to run more
than 1.5kW can design and test his own solutions. :)
73, Jim Brown K9YC>>
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