John cross-posted his original question to both TowerTalk and the RFI
list. I saw it on the TowerTalk first, and responded there. Here's my post.
73, Jim K9YC
-------- Forwarded Message --------
Subject: Re: [TowerTalk] Snap-on Ferrites
Date: Sun, 18 Jun 2017 23:33:44 -0700
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-To: jim@audiosystemsgroup.com
To: towertalk@contesting.com
On Sun,6/18/2017 10:06 PM, VE1DT wrote:
I have again reviewed the tutorial (Rev 6 Dec 2016) and found the
paragraph
that does indeed repeat what you refer to. I thought that basically
only the
material mattered, not the shape. So, if we are comparing #31 toroids
to #31
Clamp-Ons, they are basically going to give about the same results.
I've noted that, in general, impedance is proportional to the length of
the core that a single turn of the conductor passes through. And yes,
this depends upon the material. It also STRONGLY relates to the number
of turns. Those relationships are clearly addressed in the tutorial
(but you've got to STUDY it, not just skim it looking for the answer to
a narrow question).
You also "recommend" many Big #31 Clamp-on options for HF usage in your
cookbook. In fact, there are also other places in the tutorial where the
clamp-on chokes are discussed and yet none of the sections say that the
Clamp-On is a poor choice for usage at HF.
I do not share Joe's concerns about the small air gap for clamp-ons that
are manufactured to very good "flatness" or whatever the correct word is
to define the perfection with which the mating surfaces can meet.
Fair-Rite data sheets show very little divergence between clamp-on and
solid cylinders having the same dimensions. I'd probably change my mind
if I saw what I believed to be credible data showing that they ARE
significantly different when they come off the production line. :)
What the article probably was saying (and I haven't read it) is that
string of beads chokes don't work unless that material is like Fair-Rite
#73, which is what W2DU correctly chose for his first chokes. Most
ferrite beads are resonant in the high VHF range. As the tutorial shows,
an effective choke is self resonant at the frequency where suppression
is required.
My tutorials develop these concepts. I encourage all to study it. I
spent a lot of time learning it and writing about it, and I refuse to
repeat it here for those who are too lazy to study what I've written for
them! The first tutorial, from 2010, is k9yc.com/RFI-Ham.pdf Another,
which ran in National Contest Journal last year, is
http://k9yc.com/KillingReceiveNoise.pdf It offers a conceptual
discussion of how ferrite chokes work in a sidebar.
73, Jim K9YC
In reviewing Appendix 1, I cannot see how the #31 Big Clamp-On is a bad
choice. The first 4 Manufacturer's Published Data charts in Appendix 1
seem
to show about the same performance between circular and clamp on cores.
Comparing the 2.4" OD toroid against the Big #31 Clamp-On seem to provide
similar data.
#31 Toroid: Impedance/ESR - 5 turns (Dark red curve) peak at about 1,500
ohms
#31 Clamp-On: Impedance/ESR - 5 turns (Dark red curve) peaks at about
3,000
ohms
Here's another section where you certainly don't have anything bad to say
about Clamp-Ons.
The Big Clamp-Ons appear to be expensive but they're not - one is roughly
equivalent to three or
toroids! They are wonderful for portable and Field Day operations,
because
you can easily apply
them to coax without taking the connector off. There is another
advantage -
chokes for 160-40
meters using the Clamp-On weigh about a half pound less than one using
toroidal cores.
Color me confused. Where have I gone astray?
Gerald, VE1DT
He's absolutely correct. Many times I've posted a link to my tutorial
on the
topic. I spent years learning how this stuff works, and months writing it
up. I won't repeat it here. Only the link. k9yc.com/RFI-Ham.pdf
73, Jim K9YC
On Sun,6/18/2017 7:44 PM, john@kk9a.com wrote:
I was reading the recent QST article RF Mayhem by WB8IMY. The author
said to
avoid using snap-on ferrites for problems caused by HF signals as they
rarely work well at HF. It is best to stick with circular cores.
Is this correct? I was not aware that snap-ons were less effective.
John KK9A
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