[Lowfer] Common Mode Chokes (was Active Whips)

[Clint Turner]

I've had very good luck using the core extracted from the flyback of a 
discarded TV/monitor:  It doesn't take too many turns to get many mH of 
common-mode inductance.  About a decade or so, I grounded the active 
antenna (An LF Engineering LF-400B that I got in the mid 80's) to my 
metal roof - but this didn't fix the problem entirely.

Inserting a couple dozen turns of RG-58 on a flyback core (I didn't have 
RG-174 handy at the time) and then applying the local shack ground to 
the shield of the coax on the radio (not the antenna) side of the choke 
effectively decoupled the coax down to below 10 kHz.

The trick to getting a flyback core - other than maybe finding a junked 
monitor - is extracting the core.  I've used a heat gun for this, gently 
heating the entire thing until the glue softens and it can be pushed 
apart.  Often, I've broken the core, but ferrite breaks cleanly and can 
be reassembled with Super Glue with negligible loss of permeability if 
it is properly aligned.  There's usually a shim on the ferrite to 
maintain a gap (to prevent saturation, etc.) but this may be either left 
in, or removed - depending on what provides the best alignment.  They 
almost always have a wire spring or clamp that holds them together.  
Best of all, they are often free!

The other place that I've used these cores is on the USB cable that goes 
to my SDR-14, and also in connecting the USB and audio cables to an SDR 
- all three cables (USB, audio in, audio out) are wound paralleled in 
the core and it nicely isolates the computer and power supply from the 
receiver.

73,

Clint
KA7OEI/CT