[RFI] An article that happened to pop up today

[Grant Saviers]

Some of the ferrite "small chokes" (inductors) work well since the DC 
resistance is low and the SRF is high.  Others not so good with high R 
and SRF well below 1MHz, so might work for some RFI but not most ham 
bands.  In testing a bunch of the Chinese ~$5 inductor kits for RF uses, 
there have been both cases.  Otherwise, consider Digikey/Mouser etc for 
the brand name specs - Bourns 5300 and others with DC operating and 
saturation current and SRF specs.

Grant KZ1W

On 12/16/2020 19:37, Rob Atkinson wrote:
>> What kind of chokes, with what limitations and filtering purposes? A "small choke" is not a panacea for all applications. Please explain yourself more fully.
> 
> Sure--here's an example:  You have some piece of equipment that uses
> relays, the kind that are small pc board mounted jobs.  Maybe they
> switch when you go from transmit to receive.  They have 12 v. DC
> coils. But you have RF coming in on a DC line from a wall wart and the
> relays chatter because your transmit RF is on them.  A pair of small 2
> or 3 pie chokes in series with the DC at the entrance to the box or
> just inside along with bypass caps will fix it.  Sometimes the bypass
> caps alone will work but not always, at least not here.  Maybe you
> have some circuit you want to protect from overload on transmit so you
> want to keep RF out on all power and control lines going into it.  If
> a cheap pie wound choke with axial leads will do the job why not use
> it.  These small chokes are often found at hamfests for 25 cents each.
> They're usually around 1 mH and rated for a few hundred ma.   By the
> way, if you are going to buy ferrite that's fine, but never buy
> "mystery material" ferrite at hamfests.
> 
> 73
> 
> Rob
> K5UJ
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