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Keithinatlanta
Advanced Member
Username: Keithinatlanta
Post Number: 865
Registered: 3-2003
| | Posted on Tuesday, March 30, 2010 - 8:53 pm: | 
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Tech 833, can you explain what ferrite beads are and the purpose of them? Thanks.
Keith in Atlanta
CEF 150 |
Tech833
Intermediate Member
Username: Tech833
Post Number: 160
Registered: 12-2001
| | Posted on Wednesday, March 31, 2010 - 12:33 am: |
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Hi Keith.
Ferrite beads are made of ferroresonant (usually suspended floating iron) material. The various density of the ferrous materials determines the frequency range they will be most effective. For the CB band, 43 material is about the most effective.
When you place a ferrite bead over a wire, that wire becomes resistive to RF currents traveling over the wire. Remember, RF travels over the surface of a conductor, not through it. So, placing a ferrite over a wire sorta slows down the RF by creating an 'uphill road' for it. Adding another ferrite, or passing the same wire through the same ferrite again doubles the resistance. Add more ferrite, or more turns through the same ferrite and you add more and more resistance.
If you want a wire to stop acting like an antenna, place ferrite chokes over it and the RF will stop flowing over the surface of the wire.
Your radio 'Mythbuster' since 1998
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Starface
Senior Member
Username: Starface
Post Number: 2771
Registered: 1-2005
| | Posted on Thursday, April 01, 2010 - 1:04 am: |
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Paul, this grab me:
Would this work much the same on ground wires?
Like say those that might live on 2nd floor and like to have a ground wire run to out side and to ground rod.
Now most of us know doing this the ground wire acts as part of the antenna it's self.
So would Ferrite Beads or turns to make a Ferrite chock stop the ground wire from acting as part of the antenna?
Thanks George
Everyone is trying to accomplish something big,
not realizing life is made up of little things.
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Keithinatlanta
Advanced Member
Username: Keithinatlanta
Post Number: 867
Registered: 3-2003
| | Posted on Thursday, April 01, 2010 - 12:06 pm: |
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Tech 833, thank you sir! That was a great simple explanation done in laymens terms and now I know the answer.
Keith in Atlanta
CEF 150 |
Tech833
Intermediate Member
Username: Tech833
Post Number: 161
Registered: 12-2001
| | Posted on Friday, April 02, 2010 - 2:33 am: |
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George,
Putting ferrites over a ground wire would be a lot like putting resistors in series- not a good idea. Might as well disconnect it!
If your ground wire is acting like an antenna, then it is too long, or the ground itself is inadequate.
Your radio 'Mythbuster' since 1998
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Starface
Senior Member
Username: Starface
Post Number: 2776
Registered: 1-2005
| | Posted on Friday, April 02, 2010 - 7:46 pm: |
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Thanks Paul
Everyone is trying to accomplish something big,
not realizing life is made up of little things.
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Nitethang
New member
Username: Nitethang
Post Number: 2
Registered: 3-2011
| | Posted on Tuesday, April 05, 2011 - 5:24 pm: |
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Ok I'm doing my homework... It's been since 1990 since I had a mobile or a base; a lot has changed since then. I have a Cobra 2000 (ch's & peaked) D&A pdx-400, Kris BIG Boomer, Dosy 4001 meter, bought a used antenna off eBay a Shakespeare Army Big Stick 5/8 wave NBS 2010, and a new Sirio SY 27-3 3 element Beam. I remember when I ran power into a big stick before I got tons of tvi. Now I see that if I use ferrite beads that should help. If my coax run is between 50-100' how many of these beads is needed and what is overkill? Plus I would like to mount the Big Stick over the beams - how much space is needed? Will the beam act a ground plane? Thanks for all your help!
Tim KD8KZC |
Sitm
Intermediate Member
Username: Sitm
Post Number: 419
Registered: 1-2004
| | Posted on Wednesday, April 06, 2011 - 10:18 am: |
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RFI or radio-requency interference creates TVI
Ferrite beads or sometimes ferrite chokes goal in life is to reduce EMI (electromagnetic interference) and RFI (radio-frequency interference).
Simply, TVI is the interference of TV reception by transmissions from another source. In ham radio we are specificaly talking about transmissions from an amateur staion, although it is not uncommon for an amateur to be blamed when the interference is coming from somewhere else. Generally the signal from source is received on the shielding of the coax of the affected product received on its length and rides into product.
TVI can take a number of forms:
interferene with picture only
interference with sound only - this is more common in situations where the audio system is separate from the TV
interference with both sound and picture
interference on certain TV channels
interference when operating on certain bands
On analogue TV stations, TVI will typically result in "patterning" in which either
a pattern is superimposed on the TV picture with little actual distrotion
the superimposed pattern causes significant distortion to the picture
On digital TV stations, TVI typically causes picture freezing or breaking up.
There are several things that can be done to help cure TVI at the source or at the receptor. Proper grounding, running a clean station, not pushing transistors past the point of saturation or overmodulation. Sometimes in the cb world these signals are called "dirty", "overdriven" or from the standpoint of the offender "boy I'm loud".
Installing a low pass filter on your rig or a high pass filter on your tv can help as well as installing ferrite beads/chokes on all affected equipment as well.
A ferrite bead is simply a hollow bead or cylinder made of ferrite, which is a semi-magnetic substance made from iron oxide (rust) alloyed with other metals. It snaps over the cable , or it can be snapped around the cable in two pieces after the cable is installed. The bead is encased in plastic -- if you cut the plastic, all that you would find inside is a black metal cylinder.
A ferrite bead has the property of eliminating the broadcast signals. Essentially, it "chokes" the RFI transmission at that point on the cable -- this is why you find the beads at the ends of the cables. Instead of traveling down the cable and transmitting, the RFI signals turn into heat in the bead.
If you look at the cable going into most computer monitors, you will find a ferrite choke up near the back of the unit. These are generally installed on all monitors to eliminate the interference caused by the computer tower itself.
Ferrite beads can be ordered at ebay through radio shack (way overpriced) or other electrical outlets. |
Sitm
Intermediate Member
Username: Sitm
Post Number: 420
Registered: 1-2004
| | Posted on Wednesday, April 06, 2011 - 11:25 am: |
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Personally I have had good luck with TDK ferrite chokes, they were around $11.00 for ten of them, plus $3.00 shipping from china.
I think that if copper carried these, they would be flying off the shelves.
Personally, I have had two of the five televisions in the house with tvi. The two televisions affected had amplified speaker systems built in to the tv. All the televisions are sony and of the same quality, with the exception of the two with amplified speakers. After installing two ferrite chokes up near the back of the televisions the problem was completely cured. I also had a weather radio that was receiving interference so I installed a low pass filter on my ten meter rig which cured that problem as well.
Remember sometimes if you are having tvi either at your home or at a neighbors it can be at no fault of yours. Sometimes cheaply made computer speakers or other products can be very prone to radio frequency interference.
Although it may not be your fault a properly placed ferrite choke/bead on up near the back of the coax of their television or computer speaker wire wound accouple times that cures the problem can make for good neighbors and a happy hobby |
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