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:[email protected]] On Behalf Of Jim Brown > Sent: Monday, August 19, 2013 12:28 AM > To: [email protected] > 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
