Re: Common Mode Chokes

Everett N4CY


If the FT240-31 is all you have, then that i what you will have to use. For the area of coverage you want it is the worst of the bunch. All you can wind of the RG316 on a FT240 core is 35 turns. Below is the chart with the 3 toroid types, each with 35 turns. So you can see what the 31 mix will do compared to the other mix types.

Everett N4CY

In a message dated 11/15/2019 10:30:11 AM Central Standard Time, aunumero73@... writes:

Hi Everett,

Unfortunately, I have no TDK N30 on hand.
Only a FT240-31 ring and a couple of material 75 ferrite clamp on.

Le ven. 15 nov. 2019 à 17:08, Everett N4CY via Groups.Io <> a écrit :

If you only want to use just one toroid, than use the TDK N30 with 35 turns on RG316 will cover the frequency area of interest.

Everett N4CY

On Nov 15, 2019, at 8:02 AM, Patrick <aunumero73@...> wrote:

Hi there,

<< You will wind each toroid with 35 turns and they are connected in series, not stacked >>
With 35 turns, what is the "frequency coverage" ?
How many turns would be required for this setup to be effective in the range 500-1200 kHz ?


Le ven. 15 nov. 2019 à 12:14, Everett N4CY via Groups.Io <> a écrit :

You will wind each toroid with 35 turns and they are connected in series, not stacked.


In a message dated 11/14/2019 10:54:48 AM Central Standard Time, writes:

Thanks Everett.  Fantastic information.  However, I'm having trouble envisioning the configuration using two toroids in each individual common mode choke.  Are the TDK N30 and FT240-31 toroids 'stacked' together, with the RG-316 looped through the 'stack' to tie them together?  Do you have a simplified drawing or photo of the configuration you use?

Thanks again.  Also, a belated Veterans Day thank you for your military service in the Air Force.

Ken, WBØOCV East Falmouth, MA USA
41.5997N, 70.5614W  FN41ro

On Thursday, November 14, 2019, 08:55:20 AM EST, Everett N4CY via Groups.Io <> wrote:

Hi Ken,

You need only one of the TDK N30 and one FT240-31 cores, each must be wound with 35 turns of RG316. Also as Paul said you will need one at the antenna and one at the receiver. Also another thing that will help is a isolation transformer between the coax and your receiver, which can be made using a BN73-202 binocular core with 6 bifilar turns. One winding will go to your coax and the other to your receiver.

You can get all of the ferrite from Mouser Electronics. You have the part number for the TDK core and the part number for the FT240-31 is 2631803802, the part number for the BN73-202 is 2873000202.. So you will need 2 of the FT240-31 and 2 of the TDK30 and you might want to get several of the BN73-200 cores as they are great for making up matching transformers.

Your best bet for the RG316 coax is eBay...




Join to automatically receive all group messages.