[TowerTalk] Toroid Core Material Selection

Michael Tope W4EF at dellroy.com
Wed Mar 23 00:27:06 EST 2005

----- Original Message -----
From: "Dave" <k9xd at k9xd.com>

> Specifically what kind of RFI are you cleaning up. RFI effecting a stereo,
TV, or a phone in your house or in your a neighbors? What bands where you
TX'ing on and how much power.
> Thanks,
> XD


I am not sure who you addressed this question to, but here are
some examples of what I use toroids for. The following is a
common mode choke that I use at the output of my Honda
EU2000i inverter type generator to attenuate inverter hash that
falls in the ~14 to 30 MHz range. This choke is made using 5 or 6
turns each on a pair of FT-240-43 cores with the inductors
placed in series with each other. Each turn consists of 3 wires
(AC hot, neutral, and safety ground):


Another is example application is the UPS in my shack. It puts
out horrible switching spurs (S9+20dB) on 160 meters spaced
about 25 KHz apart (the spurs drift around with temperature and
almost invariably fall on top of rare DX stations). Instead of
connecting my various loads to the multitude of loads on the back
of the UPS, I make only one connection to the UPS output with a
single outlet strip. The cord for the outlet strip is wound around a
pair of FT-240-77 toroids in series. This forms a common-mode
filter similar to the one I use on my generator output. All my users
loads (computer, router, cable modem, etc) plug into the filtered
outlet strip. I also have the input power cord for the UPS wound
around a single FT-240-77 toroid. With these filters in place, the
switching noise on 160 meters is now just a whisper.

BTW, after having seen the impedance data from the type 31
core K9YC sent me, I believe I might be better served to switch
the 43 mix and 77 mix toroids in the aformentioned filters over
to 31 mix material as 31 material seems to provide significant
series impedance over most of the HF band and even into the
lower VHF range, whereas the impedance of the 77 toroid falls
off pretty quickly above 2 MHz and the 43 toroid falls a little short
 below 2 MHz.

73 de Mike, W4EF

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