On 10/13/17 7:50 PM, David Gallatin via TowerTalk wrote:
Hello group,I have to admit I have read any number of tome's on suppression, chokes, etc. I still don't comprehend it well. So a question. When we use ferrite cores and beads on a wire are we seeking to keep what ever the wire is going to from sending trash up the wire or are we keeping outside trash from using the wire to ingress the device ?
A bit of what you say, and as well, keeping the "outside" of the cable
from serving as an RF pathway from one place to another. You can't
cover the entire wire.
But imagine you've got coax going from inside your house (full of
switching power supplies emitting all manner of noise) heading out and
up your tower - next to the antenna - without some form of RF choking,
you've provided a path that is lower loss than just the free space
propagation.
There's another factor, and that's "detuning" and "interaction" with the
antenna. If I have a dipole up, and I hang another wire that's the
"wrong" length close by (even not connected to anything), then some of
the power from my dipole couples to that new wire and radiates
(potentially cancelling my dipole signal). The coupling itself might
change the tuning, and on a Yagi,that screw up the relative resonances
in the elements that are used to get the elements' currents and phases
in the right relationship with other. Proper choking makes sure that
your feedline (or power cable, or rotator control cable) aren't "that
wire with the wrong length")
If I put #31 cores on the wire WHY am I using that material vs
another? 73, David
Every material has different properties at different frequencies - it
has to do with what kinds of metals, how finely ground the powders were
before pressing, etc. It's impossible to make a material that is truly
broadband (the HF ham bands cover 4 octaves, and if you add in VHF and
UHF it's really tough). So you choose your material according to the
frequencies you're concerned about.
For consumer equipment, they're typically concerned about VHF emissions
from harmonics of the processor clocks inside the box (e.g. there's a 24
or 48 MHz oscillator inside that USB device) that will cause them to
fail FCC Part 15 testing. So they use a choke material optimized for
the frequencies they care about.
For HF hams, 31 material has the right properties at the frequencies we
care about - Jim's RFI-Ham book online goes through the various
materials - if you're mostly a top-bander or you're activity is on 10 or
6 meters, maybe 31 isn't the best choice.
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