In reading the description {http://www.palstar.com/at5k.php} and looking at
the schematic, I consider the tuner in the "balanced" output mode to
actually be a matching network that is unbalanced above ground. It is not a
balanced tuner in that the elements, the L and C values, are not equal in
each side of the balanced line. The switch that selects balanced simply
lifts the output low side of the inductor above ground. For a truly
balanced tuner, see the info from Rich Measures. He goes into quite a bit
of detail on the topic.
I have built a balanced tuner, being a L network, {I don't care for T
networks} using two roller inductors and two variable C's. I use a RF choke
operating at 50 ohms on the input as well so that it always sees 50 ohms in
and 50 ohms out.
Although the Palstar is a "rock" in their terms, clearly it is a well built
tuner that will handle the power as described. Likely the fact that it is
not a balanced tuner and it is being connected to a balanced load is the
reason for the "havoc" caused when running the amp.
73
Bob, K4TAX
----- Original Message -----
From: <d.e.warnick@comcast.net>
To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
Sent: Wednesday, March 18, 2009 4:21 PM
Subject: Re: [TenTec] Problem on 160 meters, Whats up?
The problem of heating an unmatched balun is one of the reasons that I
chose the Palstar AT5K. It has a 1:1 balun at the input side of the tuner,
that way it sees a 50-ohm load on each side when the tuner is matched.
There's a great writeup on that on page 25-15 of my 2006 ARRL Antenna
Book. Dean Straw, N6BV, reasoned that a 4:1 balun at the output would
seldom see a matched condition and could overheat or arc at 100 watts, let
alone full power. The design in the Antenna Book uses a 1:1 balun on the
input side of the tuner also
Dave
WA3MKB
----- Original Message -----
From: "Jim Brown K9YC" <k9yc@audiosystemsgroup.com>
To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
Sent: Wednesday, March 18, 2009 4:53:26 PM GMT -05:00 US/Canada Eastern
Subject: Re: [TenTec] Problem on 160 meters, Whats up?
On Wed, 18 Mar 2009 13:19:12 -0600, Dr. Gerald N. Johnson wrote:
Another VERY different thing that is CALLED a balun is really a common
mode
choke. Put a bunch of ferrite cores around coax, or wind a bunch of
turns of
coax through one or more ferrite cores. Those are common mode chokes,
commonly (wrongly) called "current baluns." But when done well (enough
turns
to have a high enough choking impedance), they work VERY well.
And if they don't have enough core cross section, they too can saturate.
But saturation is more difficult achieve with only one turn per core.
Saturation is VERY unlikely in a coaxial choke if the choking impedance is
high
enough, because the core sees ONLY the common mode component. It sees NONE
of the
differential field -- it's all confined to the dielectric of the coax.
That is
NOT true in a bifilar winding, which has a lot of leakage flux. So the
core in a
common mode choke that is bifilar wound (that is, a pair of wires) IS
likely to
saturate if you run enough power through it.
Their big claim to fame is that the allow the antenna to set the balance
where the transformer forces voltage balance.
That's the 30 year old version of why common mode chokes are much better
than
voltage (transformer) baluns. That's certainly true, but they have another
HUGE
advantage -- they prevent the feedline from becoming part of the antenna.
This
keeps RF out of the shack and your neighbor's living room, and it also
prevents
NOISE received on the feedline from coupling to the antenna and making it
harder
to hear weak signals. That is, the only noise you hear is what's picked up
on the
antenna, which is usually further from the noise source (in your home and
your
neighbor's) than the feedline.
73,
Jim Brown K9YC
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