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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