On 8/18/2013 9:41 PM, MU 4CX250B wrote:
I agree that strings of ferrite beads don't present a lot of inductive
reactance on the low HF bands, but you lost me with your comment about
the reactance canceling the capacitive reactance of the cable. I've
not thought about this deeply, but it seems to me the cable
capacitance is differential, between the center conductor and the coax
braid.
You're confusing the differential circuit with the common mode circuit.
A choke is in the common mode circuit, but not the differential
circuit. In the common mode circuit, the feedline is a longwire antenna
-- it's that current on the outside of the shield in the commonly
discussed diagram. An antenna shorter than a quarter wave looks
capacitive, longer than a quarter wave looks inductive, and that repeats
in increments of half waves.
There's a simple analysis of this in my AES paper, and in several
tutorials on my website. When I did literature search for the AES paper,
I found app notes by major EU ferrite mfrs from'50's/'60s that made it
clear that they understood this (because of the advice they gave),
although the concept was not directly stated.
If you doubt this, build a simple NEC model with a short feedline and an
inductive choke and compare currents with and without the choke. W7EL's
manual for EZNEC discusses how to model the common mode behavior of a
transmission line, and you insert the circuit model of the choke as a
load in that line.
Think about this -- a common method of matching a vertical antenna to 50
ohm line is to make it a little long so that it's 50 +j xxx, then tune
out jxxx with -jxxx ( a series cap). I'm doing this with one of my 160M
TX antennas, and I'm doing the opposite with a 160 antenna that's a bit
short, adding a bit of L at the base. In both cases, adding the
reactance lowers the impedance of that antenna, and that's what an
inductive choke does in a capacitive (short) line. And when we lower the
common mode Z, we increase the common mode current, which is the
opposite of what we want to achieve.
73, Jim K9YC
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Topband Reflector
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