On 05/20/2015 07:51 PM, Jim Brown wrote:
Here's a series of CorCom filters that looks promising.
http://www.te.com/usa-en/product-1-6609037-1.html#pdp-docs-features
Click on the "see all documentation" link, then choose the top pdf,
which includes graphs. You might also try calling them or using Live
Chat to ask about VHF.
Thanks Jim. I will see if I can get any information about VHF from
them. I have several similar filters which I have tested/used in
various applications over the years, but I have yet to see one that
offered much help beyond 6M.
As to the effectiveness of the number of cores and their type -- for a
single turn (that is, passing the cable through it once), the type you
chose, #43, is ideal for for 2M. Winding turns would lower its
resonance, and thus its effectiveness to around 6M. Choking Z will be
proportional to the total length of the ferrite material. This is going
to be somewhat logarithmic -- that is, for 6 dB, the current must be
reduced by half, which corresponds to doubling that Z. So -- assuming
it's all common mode, it would take 3 more cores to reduce that common
mode current by half.
Understood. The first three cores has a significant affect. Adding
more did nothing measurable. I stopped at 12 cores.
When using these cores at lower frequency, or any core at a
frequency where I haven't found published data on the optimum number
of turns, I have a process which has worked well for me. I "measure"
the noise level on the "bothered" device or at several points
further down the cable I'm working on (away from the noise
generating device) while adding turns to the core one at a time.
When adding a turn causes the noise to increase rather than
decrease, I remove that turn and leave it alone, adding more cores
with same number of turns as necessary.
Ideally, the choke should be as close as practical to the noisy device.
Fair-Rite #61 would be the weapon of choice for 440 MHz.
In this case I placed the first one right against the IEC plug on
the offending device.
By the way, I keep a copy of RFI-Ham.pdf in my RFI "go kit".
--
Paul Kelley, N1BUG
RFI Committee chair,
Piscataquis Amateur Radio Club
http://www.k1pq.org
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