On 12/28/2017 2:46 AM, Rick@dj0ip.de wrote:
The problem that we are fighting is common mode current
One fact that is being missed in this discussion is that the
transmission line as a common mode element of the antenna system, and,
like any antenna, current varies along the length of every element based
on the boundary conditions. The most common "boundary conditions" are
the open circuits at the ends of conductors that force the current to
near zero (limited by capacitance at the ends to free space and the
antenna's surroundings). When we add a common mode choke with a high
choking Z, we force current to near zero, creating another boundary
condition AT THE POINT WHERE THE CHOKE IS PLACED. A choke at the antenna
feedpoint forces near zero current there, but the feedline may still
connected (as common mode element) via the tuner to ground. A choke at
the tuner can another near-zero condition.
With a choke at the tuner and no choke at the feedpoint, the feedline is
still part of the antenna, and is vulnerable to picking up noise on
receive. This can be clearly seen (and studied) by a simple NEC model of
the antenna that includes the feedline and the choke(s) using the Load
function in NEC. My tutorial on chokes shows how to determine the
parallel RLC equivalent circuit of a choke based on measurements of its
impedance vs frequency. NEC computes currents vs length for every
element of the antenna, and can display them graphically and as table.
In all of the tests that I have run on dipoles and OCFD antennas, the lower
bands, being electrically closer to the ground, have significantly more problem
with CMC than the higher bands.
Therefore I chose to run with #43 when 80m is required.
#31 is several dB better on 80M and a lot better on 160M.
I agree the point of using bifilar.
If the device in question is operating as a transformer (that is, two
windings coupled magnetically but with no DC connection), from a common
mode point of view, that's a BAD idea, because it increases the
capacitance between turns, and reducing the common mode impedance.
Without that capacitance, they have the same effect as a good common
mode choke. Transformers are fundamentally different from common mode
chokes in that the core carries the entire signal. For receiving only,
loss in the core may not matter, but for transmitting, loss can burn a
lot of transmit power. At low power levels, this simply makes the signal
weaker. At high power levels, losses in the core are likely to cause it
to self-destruct.
It should be obvious that cores having low loss at the frequency of
interest should be used for transformers. Likewise, the core should
provide a magnetic path (dimensions) capable of handling the flux
density associated with the power level. Fair-Rite #61 is quite
efficient below 10 MHz, with losses beginning to increase above that
frequency. Fair-Rite #67 handles more power to higher frequencies.
N6RK, a pretty smart EE retired from HP's instrumentation group, uses
#67 for high power transformers in his antenna system.
The only advantage of a bifilar-wound transformer is slightly greater
mutual impedance between windings, which increases efficiency and power
handling.
Separating the windings (that is, placing them on opposite sides of the
core) reduces (greatly) the capacitance between windings, increasing its
common mode impedance.
73, Jim K9YC
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