[RFI] Core for common-mode choke?

jeff_list at burnshome.org jeff_list at burnshome.org
Tue Jan 11 17:31:49 EST 2005

I have made some progress on solving the TV interference problem referenced
below, but I can use some more help.


A colleague at work gave me five split cores with an inside diameter of 0.5
in. Some are type 25 material the others are unknown material. I was able to
wrap 3 turns of the AC line on a core. Four of these cores stopped the TVI
with all other cables disconnected from the TV. 


I wrapped two turns of the TV cable on the one core I had left and
reattached it to the TV. The TVI is back. Obviously, I need more impedance
on the TV cable.


Replies to my previous post indicated that type 31 material is the optimal
choice to make the RF chokes I need. This is what I have been able to find
for sale: 


DX Engineering

Split beads, Type 31,

 for up to 0.275 inch Cable, Pack of 10 for $23.25  for up to 0.525 inch
Cable, Pack of 5 for $22.25  for up to 0.750 inch Cable, Pack of 5 for $25.8


>From Amidon

Type 31 non-split beads

OD 0.485in ID 0.20in Length 1 in. $0.80 each OD 0.562in ID 0.25in Length 1
in. $1.50 each



With the split beads I can put one to three turns of a pre-made cable on a
bead depending on its size. The non-slit beads will need to be put on before
the connector is attached to the cable. I will need to use the bigger size
beads unless I can find a 75 ohm cable that is smaller than 0.20in in


What is the most cost effective bead to purchase for this application?





-----Original Message-----

From: rfi-bounces at contesting.com [mailto:rfi-bounces at contesting.com] On
Behalf Of jeff_list at burnshome.org

Sent: Monday, December 13, 2004 22:47 PM

To: rfi at contesting.com

Subject: [RFI] Core for common-mode choke?


I am trying to eliminate RFI problems with my TV. Even with all the inputs
disconnected and terminated I still get moderate blanking of the screen when
I key my rig at 100w. Hopefully, adding a common-mode choke to the AC line
will solve the problem. Since I will probably need chokes on several other
lines once I start reconnecting the various cables I want to find the most
cost effective source for the chokes. 


Amidon lists FT-240-77 toroids for $9 each. Is there a cheaper source for
these? MFJ-701 RFI Choke Kits are only $15 for four. Are these effective
enough on the AC line?


If I find that I need a choke on the component video cable from my DVD
player what should I use? It may be too big to wrap on an FT-240-77 toroid. 


Jeff Burns





RFI mailing list

RFI at contesting.com



More information about the RFI mailing list