[RFI] Ferrite Materials

EDWARDS, EDDIE J EDWARDS, EDDIE J" <eedwards@oppd.com
Tue, 7 Dec 1999 10:15:32 -0600

> -----Original Message-----
> From:	Jon Ogden [SMTP:jono@enteract.com]
> Anyhow, to me, it's pretty clear from this that type 77 IS the material to
> use for HF common mode chokes.  Perhaps Palomar or Fair-rite materials
> numbered 43 or 77 are different from Amidon's.
> Given this, I would like to know why people have found type 77 material
> not
> to work at HF. Comments?
	[Ed-K0iL]  Perhaps you are not considering which signal is actually
causing the interference.  Is it the fundamental signal on 3.5Mhz, 7Mhz,
14Mhz, 21Mhz, or 28Mhz?  (I know, I left out the WARCs).  Or is it one of
the Harmonics?  

	Harmonics are there although not as strong.  If you're running an
amp, they can be plenty strong however!  It may not be the fundamental
signal causing the specific interference being experienced.  Perhaps the
speaker wire picking up the interference is a quarter wave-length of one of
the harmonic frequencies, but is not the fundamental.  Then the harmonic
might appear stronger to the susceptible device.  If a harmonic is causing
the interference, a filter for the fundamental might work, but it might not.

	I've found that 28Mhz causes the most audio rectification in my Sony
XBR TV set with each band below causing less and less interference until at
20 or 30 meters there's no practically interference.  Is it each fundamental
causing this interference?  Perhaps, or maybe it's just one contributing
signal among others.  When I open it up and pulled out the speaker
wires--almost 5-6 feet, it occurred to me that they might be about a half
wavelength for the 3rd harmonic and multiple-quarter wave for the 2nd
harmonic from 10 meters!  By adding the Radio Shacks cores (43 or whatever
they are--they haven't called me back) I not only added the RF choke
impedance, but I also shortened the antenna (spkr wire) length.  Maybe that
had more effect than the choke!?

	The thing to do when eliminating RFI is match the filter attenuation
to the signal causing the interference.  Just don't always assume it's only
the fundamental causing the interference in something as complex and
EMI/RFI.  This discussion had made me think that maybe I need both 77 & 43
on my computer speakers to rid it of RFI from both low and high band
signals!  Thanks!

	de ed -K0iL

Submissions:              rfi@contesting.com
Administrative requests:  rfi-REQUEST@contesting.com
WWW:                      http://www.contesting.com/rfi-faq.html
Questions:                owner-rfi@contesting.com