[TowerTalk] Ferrites 31 vs. 77 material

[jimlux]

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 at 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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