On 11/8/19 8:36 PM, Jeff Blaine wrote:
A single turn through a given core has much less Z than the multi-turn
variety. And as frequency goes down for a given ferrite (of any type),
the Z is also lower.
Two equations that help:
L (and Z) goes as number of turns squared - so 5 turns through a core
has 25 times the inductance (or impedance) as 1 turn.
Z = L * 2*pi*f
So something that is 1000 ohms at, say, 144 MHz will be 100 ohms at 14.4
MHz.
So it's not that they only work at VHF, it's that the contribution to Z
is pretty small, especially on HF. Which is why you see that sleeve
type chokes need a LOT of ferries to provide anywhere a reasonable
choking Z.
Big cores (high mu) with lots of turns (big N^2) is the way to go..
(Except that at high frequencies, the capacitance between the turns
short circuits the impedance of the windings - so there *is* a happy medium)
73/jeff/ac0c
alpha-charlie-zero-charlie
www.ac0c.com
On 11/9/19 1:01 PM, Bob Shohet, KQ2M wrote:
Hi Jim,
Thank you for your reply and the link to your extremely comprehensive
documentation! I read though it at length today and there was much
that I did not understand very well. I am going to read it again over
the weekend.
You mention that the bead only works at VHF but I did not recall
seeing anything addressing that. Is it that the choking resistance
is not great enough or is there some other reason(s)? Could you
please explain?
Tnx & 73
Bob KQ2M
From: Jim Brown
Sent: Wednesday, November 06, 2019 10:52 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Ferrites 31 vs. 77 material
On 11/6/2019 6:28 PM, Bob Shohet, KQ2M wrote:
I prefer to use large ferrite beads that fit on the coax – roughly
1/2” ID - 1” OD x 1 1/4” height and then tape the beads in place in
physical proximity to each other next to the feedpoint.
You may prefer that, but that sort of bead only works at VHF. My
detailed recommendations are here. http://k9yc.com/2018Cookbook.pdf
Yes, #31 material is the weapon of choice for 160M - 6M.
73, Jim K9YC
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