Topband: Bead balun waterproofing

[Jim Brown]

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