On 11/12/2013 9:14 PM, Jim Brown wrote:
On 11/12/2013 5:28 PM, Peter Voelpel wrote:
Any choke is inductive.
If not it is no choke at all.
WRONG. A proper common mode choke inductively couples loss from the
ferrite core. A parallel resonance is formed by the inductance of the
winding (the wire passing through the core) and the stray capacitance
between the winding. The coupled resistance is what forms the common
mode choke, and because the core material is very lossy, the circuit Q
is quite low (typically on the order of 0.5), so the resonance is
quite broad.
I may be wrong, but I've always looked at the solenoid coil as if it
forms two resonances. The choke itself forms a parallel resonant
frequency with a rather high Q So it may work on one or two bands and it
also forms a second resonant frequency with the overall coax length
which can be worse than no choke at all. Even when one works and does
not create a problematic resonance with the overall line length, it is
no where near as effective as the choke wound on the ferrite core(s).
Using the small coax rated for high power lets one make a compact choke
for the tribander while a couple more cores and turn (based on your
tutorial) will take you down to 160. You can wind a choke large enough
to be effective on 160 by running 6 turns of the LMR-400 feedline
through 5 or 6 2.4" #31 mix cores. OTOH using the smaller coax you can
easily add several more turns with out crowding.
These are essential as antennas like a center fed, half wave, sloping
dipole for 75 can be far from being a balanced load and difficult to
keep the common mode voltage on the coax down to reasonable values. It
may even take two chokes to keep the RF "out there" instead of lighting
up every thing in the shack.
73
Roger (K8RI)
A single turn through a ferrite core resonates in the 150 MHz range.
Like any coil, winding multiple turns multiplies L by the square of
the turns, capacitance increases approximately linearly with the
number of turns, the resistance, because it is coupled inductively is
multiplied by the square of the turns. This moves the resonance to the
part of the HF spectrum where choking action is needed, and it is the
RESISTANCE that provides the most reliable choking action. A proper
choke wound on a suitable core material can provide resistive choking
Z in the range of 3K - 8K Ohms over a 2 octave frequency range
(because the circuit Q is so low).
Again, any choke that is not strongly resistive at frequencies of
interest will resonate with the line (which behaves as an antenna) at
frequencies which depend on the length of the feedline and the
reactance of the choke.
For a thorough development of these concepts, see my RFI tutorial,
cited in my prior email.
73, Jim K9YC
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