On 2/9/2010 9:32 AM, UKMonitor wrote:


Also how much impedance could I expect to gain every time I added another
FT140-43 ferrite onto the stack?


You need to double the number of beads each time to make a noticable
difference.

For worthwhile common mode suppression you need at least ten and
ideally eighty type 43 beads over a length of cable.

It's more cost effective to wind more turns. 2 x the turns 4 x the
inductance.

UKM

If I am understanding the data associated with the device (bead) you
suggested, correctly--"EMI/RFI Dämpare och ferriter Z=375 OHM @ 100
MHz", it would, indeed, take a number of them. Personally, I like to
see a Z of 10 times that of the Z of the coax. For example, for 50 ohm
coax, I would like to see a choke which would provide a "resistance" (Z)
of 5,000 ohms to any rf currents which would be tempted to ride the
braid(radiate.) While 10 of these devices may be sufficient (and would
be affected\effected by the frequencies they would be used with); I
would probably choose to use a few more.

If cost\design-needs is\are any sort of factor(s) in your design needs,
I would tend to go multiple turns on a proper core size(power handling
requirements of correct material(frequency(s) designed for.)

While ferrite may have advantages over iron cores at vhf\uhf\shf--and
become quite noticible, I have found any losses or heating tolerable at
hf and below--again, given proper core size\material--and I have used
them at much higher freqs.

The beads you mentioned are very useful though. If in a hurry, if
experimenting and time is valuable--just slap too many on the line and
go for it!

Regards,
JS