[TowerTalk] ferrite beads was...Re: [RFI] HIGH POWER and RFI...

[Marlon K. Schafer]

Speaking of ferrite beads.  I need some for ethernet cable and haven't been 
able to locate a source.

Anyone have a good suggestion?

thanks!
marlon

----- Original Message ----- 
From: "Jim Lux" <jimlux at earthlink.net>
To: "Jim Brown" <jim at audiosystemsgroup.com>; <towertalk at contesting.com>
Sent: Tuesday, March 06, 2007 9:33 AM
Subject: Re: [TowerTalk] [RFI] HIGH POWER and RFI...


> At 07:32 AM 3/6/2007, Jim Brown wrote:
>>On Tue, 6 Mar 2007 09:06:45 -0500, Dan Zimmerman N3OX wrote:
>>
>> >measures, inaccurately, on my MFJ-259
>>
>>Ferrite chokes are essentially very low Q parallel resonant
>>circuits. Antenna analyzers are not very useful for measuring
>>ferrite chokes, for at least two reasons. First, they have a
>>fairly low input Z (typically 10K resistance in parallel with 12
>>pF. The capacitance is the major problem -- it combines with the
>>R, L, and C of the choke to move the choke's resonance down in
>>frequency to a new false resonant point, and above that false
>>resonance gives an impedance that is falsely quite low.
>
> Could one estimate that parasitic C by sweeping the analyzer and
> looking for the resonance?
>
> And, if you can assure yourself that the resonance is "far" away from
> the frequency of interest, one can ignore the C.
>
> Since there's really a limited number of possible mixes in use, it
> might be useful to come up with some sort of "diagnostic method" to
> identify the material using simple ham tools (like the MFJ
> box).  Once you know the mix, and the mechanical dimensions, then you
> can go to the mfr charts and find the "real numbers"...
>
> At a first glance, I would think you could just try and identify the
> mu of the mix.  Pick a low frequency to test at (so the parasitic C
> doesn't bite you) and measure L and work backwards from that?
>
> I'm sort of lazy and don't want to page through the FairRite catalog,
> and K9YC probably knows these things off the top of his head, but are
> all the "usual" mixes for this application sufficiently different in
> mu that you could use that as a sole distinguishing
> characteristic?  Or, are there mixes with the same mu, but different
> loss properties?
>
> Jim, W6RMK
>
>
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