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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