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Re: [TowerTalk] [RFI] HIGH POWER and RFI...

To: "" <>
Subject: Re: [TowerTalk] [RFI] HIGH POWER and RFI...
From: "Jim Brown" <>
Date: Tue, 06 Mar 2007 10:11:00 -0800
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On Tue, 06 Mar 2007 09:33:05 -0800, Jim Lux wrote:

>Could one estimate that parasitic C by sweeping the analyzer and 
>looking for the resonance?

Good question.  See below. 

>And, if you can assure yourself that the resonance is "far" away from 
>the frequency of interest, one can ignore the C. 

Yes. If you're buying surplus clamp-ons and cylinders used for RFI 
suppression, it's most likely that they're #43, and a single turn (that is, 
the wire going through the core once) is going to resonate somewhere above 
100 MHz. So single turn data is fairly good (+/- 25%) to several tens of 
MHz if the "SWR" is less than about 4:1. 

>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"...

It might help, actually, if I also presented some of the data from my own 
antenna analyzer measurements so that folks could see how the errors 
distort the data. I'll do that when I have time. 

>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?

Yes. If you're more than a couple of octaves below resonance, the materials 
are pretty high Q -- that is, the impedance is strongly inductive, so you 
can get good numbers for mu. You know your data is looking right when 
you're well below resonance and Z is all X and increasing proportional to 

>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?

If it's a 2.4" toroid, it could be #31, #43, #61, #77, or #78. All are 
commonly available. #31 and #43 would be hard to distinguish unless you had 
both of them. #61 is readily identifiable by its high Q resonance, even 
though it will be a LOT lower in frequency on the antenna analyzer. #77 and 
#78 have a strong resonance around 1 MHz, no matter how many turns you put 
on them, but the MFJ-259 doesn't go down low enough to let you see it (the 
AEA does). 

At some point, you need to study the Fair-Rite catalog. Like anything else 
in life, you really start learning when you really start studying. :)  Once 
you've put in that time, then you've got the possibility of using simpler 
instrumentation to figure out what you have. BUT: when I started learning 
about ferrites, I was using only what I bought at hamfests. The best advice 
I ever got on the subject was to quit playing around and start looking at 
known parts. 



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