On Fri,4/8/2016 10:13 PM, Jim Thomson wrote:
## say what ? AFIAIK, he uses FOUR type 31 cores for the CMC-230-5K ,
2 x cores for each assy.
Could be -- can't tell from the photos. That helps with power handling,
and it also adds L, which lowers the resonance.
8000-11,200 ohms of RS is nothing to sneeze at. Its
rated at 5 kw pep out. It wont blow up with 1.5 kw cxr. Its been tested.
2-30 mhz
## The BL2006-3k is a 14-54 mhz balun. Single core. 5400- 8600 ohms of RS
is again, nothing to sneeze at. Rated for 3 kw pep out. It wont blow up on
20-6m,
with 1.5 kw cxr.
There's a serious fallacy here about power ratings for chokes that use
the resistance at resonance (which is the right way to do it).
Dissipation in this kind of choke depends STRONGLY on the common mode
voltage, which in turn depends strongly on the antenna system, INCLUDING
the feedline and its length.
## Your single core overheated on 80m. His BL2006-3k Is rated for 14-54
mhz.
Its not intended to be used on 80m!
Operating frequency is only one factor. The choke I fried was more turns
to move the resonance down so that it would cover 80M, and it provided
about 5K ohms resistive. But that antenna SYSTEM needed at least twice
that choking Z to run US legal limit.
Dissipation in a choke is the SUM of the differential mode power and the
common mode power. N6BV wrote an excellent piece that ran in QST a
couple years ago on the issue of differential mode dissipation in chokes
with severely mismatched antennas (he predicted disaster), and ran it
past me first. I told him that his analysis of differential mode was
right on, but that he had failed to consider common mode, which makes
matters even worse!
Another thing that bothers me about these chokes is that they are in
what appear to be sealed enclosures, which can limit the ability of the
assembly to conduct heat away from the choke, reducing its power handling.
The key to power handling is to understand that power is I squared R,
and that the choking Z must be high enough to reduce the current so that
dissipation is not excessive. Because power is increasing (or
decreasing) as the square of the current, you CAN outrun your tail by
making the choking Z high enough, because the power is dropping twice as
fast as the current. :)
The differential mode dissipation must be modeled using transmission
line techniques -- a Smith Chart or N6BV's TLW (free with the ARRL
Antenna Book) will get you right answers. We must model the common mode
part of the problem by adding the equivalent circuit of the choke(s) to
an NEC model, where the choke is added to a single conductor the length
of the feedline running between the feedpoint and ground (or wherever
the coax is connected). Remember -- the equivalent circuit of a ferrite
common mode choke is NOT best described as a series R + jX, but rather
as a parallel resonant circuit, where L is the mid-to-low frequency
inductance, R is the Z at resonance, and C is the stray C that resonates
with L to establish the resonant peak. That simple circuit works for #43
and for #31 above about 7 MHz. For #31, it's more complicated than that,
because it has both a circuit resonance and a dimensional resonance that
give it a double humped response (like a stagger tuned IF), which is
what gives #31 its extended bandwidth.
Much of this is developed in the tutorial stuff on my website.
73, Jim K9YC
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