Topband: 160m noise

[Jim Brown]

On Wed, 10 Nov 2004 12:07:08 -0500, Jeff Maass wrote:

>It looks like several more turns than three are required
>for use on 1.8 MHz to get >800 ohms with #43 material cores.
>What number of turns have you used and measured?

I've measured up to 10 turns, and used up to 14 turns. In research for an 
EMC paper  published by the Audio Engineering Society, I wound 14 
turns of a mic cable around the #43 toroid to reduce the shield current 
on a 125 ft long mic cable set up about 1/2 mile away from WGN (50 
kW on 720 kHz).  Without the choke, there was massive interference to 
most of the equipment that I tested. With the choke the interference was 
either gone or drastically reduced in level.  

An important comment on impedance measurements of these chokes. 
In general, we want to use resistance, NOT inductance in chokes 
designed for EMI suppression, because we want to dissipate the RF, 
and because we don't want the X of the choke (either capacitive or 
inductive) to resonate with the length of the cable and increase the 
current rather than reduce it.  

I've used both the MFJ-259B and the AEA CIA-HF to measure these 
chokes. Neither device could be considered a serious bridge, but the 
CIA-HF can measure Z up to 1K ohms, both can measure the phase 
angle, but neither can tell you the sign of the phase angle, and I would 
describe the accuracy of both instruments as a first approximation of the 
actual R and X.  Obviously, if I had better test gear I would use it. :) But I 
learned long ago that one can accomplish quite a lot with less than 
perfect tools if you are both careful and disciplined.