[RFI] Line filters for VHF?

N1BUG paul at n1bug.com
Thu May 21 07:59:51 EDT 2015

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

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