[TowerTalk] Suppression

[jimlux]

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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