The performance of split beads is very sensitive to the length of the
air gap between the faces, which is why the faces are so highly polished
and why you shouldn't touch them with your fingers (or get jam on them).
I once measured the impedance of a Fair Rite split bead in one of those
plastic enclosures that hold the two halves together. I then squeezed
the enclosure with my fingers to press the two halves together more
firmly. The impedance went up. Only a few percent (10 or so?) but
quite noticeable.
I wouldn't expect a shattered bead with all those air gaps squeezed
together only by vinyl tape to work very well.
FWIW
73, Jim Smith VE7FO
On 8/23/2010 8:57 AM, Kimberly Elmore wrote:
> Could they have been immersed (more or less) in water then exposed to
> freezing?
> That could have imposed enough stress. Also, as long as the pieces of the
> beads
> themselves remain held together, they will still work just fine. That's why
> split beads work, even though they're in two pieces.
>
> Kim Elmore, N5OP
>
>
>
> ----- Original Message ----
> From: Pete Smith<n4zr@contesting.com>
> To: Ken Winterling<wa2lbi@frontiernet.net>
> Cc: RFI List<rfi@contesting.com>
> Sent: Mon, August 23, 2010 9:12:27 AM
> Subject: Re: [RFI] Ferrite bead disintegration
>
> They were lying in the grass, largely during the winter months, under no
> physical stress at all, unless sun heat might have been a factor due to
> the use of black vinyl electrical tape to wrap the beads after they were
> put on the RG-59. But this is another reason to be worried about the
> bead chokes at my array feedpoints, because they are up year-round,
> subjected to whatever stresses are imposed by the wind, weight of the
> coax, etc.
>
> 73, Pete N4ZR
>
> The World Contest Station Database, updated daily at www.conteststations.com
> The Reverse Beacon Network at http://reversebeacon.net, blog at
> reversebeacon.blogspot.com,
> spots at telnet.reversebeacon.net, port 7000
>
>
> On 8/23/2010 9:29 AM, Ken Winterling wrote:
>> Pete,
>>
>> I have seen mechanical stress do this. You didn't indicate the
>> physical construction technique used to build the choke but it is
>> possible that movement of the beads against one another caused them to
>> crack.
>>
>> Ken, WA2LBI
>>
>>
>> On Mon, Aug 23, 2010 at 09:10, Pete Smith<n4zr@contesting.com
>> <mailto:n4zr@contesting.com>> wrote:
>>
>> I took apart a home-made 100 bead receiving-only common mode choke
>> yesterday. As I peeled off the electrical tape I was surprised when
>> each bead that came out of the wrapping disintegrated into dozens of
>> fragments. I think I saved about 5 beads out of a 100-bead choke.
>>
>> What could have caused this? The choke was used over 300 feet
>> away from
>> my transmit antennas, and never used for transmitting. Morte to the
>> point, can I expect to see the same thing in the feedpoint chokes
>> of my
>> 80m 4-dipole lazy vee array, which have been up and in use for ~10
>> years?
>>
>> --
>> 73, Pete N4ZR
>>
>>
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