Hi Jim,
I think we may talking at cross purposes! I certainly agree with your
explanation about how a choke is in the common mode circuit, but not the
differential circuit. In fact, that just what I said in my posting. I also
agree that in the common mode equivalent circuit, the coax feedline looks
like a single wire whose electrical length is shortened by the inductive
reactance of a common mode choke. So we're in complete agreement on those
points.
What I was wondering about, however, is why you believe the distributed
intrinsic cable capacitance, in pF/m, has anything to do with this
explanation? The intrinsic cable capacitance is what I was referring to as
"differential," and it, along with the distributed inductance per unit
length of the cable, is what determines the cable's characteristic
impedance. Any _series_ (differential) inductance or capacitance added to
the cable will electrically shorten or lengthen it, just as you said, but a
common mode inductance, such as a ferrite bead choke, will not do this
because it adds reactance to the inner and outer cable conductor equally. In
other words, I believe a common mode inductance always reduces the common
mode currents along the feedline, as one can easily visualize by drawing out
the equivalent circuit. I can't think of any mechanism that would cause a
common mode inductance to increase the common mode currents, which is what I
thought you were asserting.
73,
Jim W8ZR
> -----Original Message-----
> From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of Jim
Brown
> Sent: Monday, August 19, 2013 12:28 AM
> To: topband@contesting.com
> Subject: Re: Topband: Bead balun waterproofing
>
> 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
>
>
> _________________
> Topband Reflector
_________________
Topband Reflector
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