Common Mode Choke FT240-31 Ferrites in Series

[VK1JA]

I'm about to build a line isolator/common mode choke (it seems they have so many names) to help prevent the feed line on my 6BTV (160m-10m) radiating and to reduce noise pickup - that's the theory anyway...

Looking at the good work that G3TXQ has done here - http://www.karinya.net/g3txq/chokes/ and http://www.karinya.net/g3txq/chokes/type31.jpg it appears that using the FT240-31 ferrites I have on hand it may be better to wire two in series with 12 turns of RG58/RG316 on each rather than stacking two FT240-31 and then winding the turns around them both.

It appears that wiring the two in series will give me a broader choking ability, where the two stacked will lower the freq and reduce where Rs > Xs.

If that is the case, I'm happy to wire two in series, but (and here's the question) can I then fold the two wired chokes to sit on top of each other to save space/enclosure size?  Will having the two chokes physically sitting on top of each other upset the choking abilities at all?  I'll use some sort of plastic to separate them, but they'll be within 10mm of each other I'm guessing.

I've attached a quick picture, imagine the second ferrite folded back to sit on top of the first, the wiring will remain in series.

Thanks

Jayson

[VK4ADC]

" I'll use some sort of plastic to separate them, but they'll be within 10mm of each other I'm guessing."

From memory, the fields around the toroids are fairly minimal while intense though the core so the coupling core to core should be low even when in relatively close physical proximity.  

Here is a question for you that might help with your issue - could you use two pieces of (eg 6mm) plastic material with a thin gauge metal sheet in between them as the spacer, preferably one that has defined magnetic characteristics (eg tinplate) rather than non-magnetic (eg Alfoil) ??? The metal doesn't have to be connected to anything (ground..) but should be a somewhat larger than the toroid diameters to create a shield effect.

Make sense ??

Doug VK4ADC

[VK1JA]

Hi Doug,

Thanks for the reply.

I should be able to make space for two pieces of plastic with a thin sheet of tin plate between them.  I'll take one of the FT240-31's into Jaycar during the week and see if I can find a suitable enclosure.

Another project in the making!

Regards

Jayson