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Author Topic: Whats the difference between a Current Balun and a Ferrite Bead Choke  (Read 5049 times)
W4VR
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« Reply #15 on: November 21, 2012, 08:46:09 AM »

Decades ago I used to make my own current baluns with a T-200 ferrite core and a tri-filer winding using enameled #14 solid wire.  If I needed additional power handling capability I would stack two cores.  I would immerse the assembly in epoxy for weatherproofing.  These days I buy my current baluns, usually the ones with a piece of Teflon coax with beads around it inside a PVC pipe.
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W6GX
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« Reply #16 on: November 23, 2012, 08:22:35 PM »

Two of antennas came with #3 type baluns at the feedpoint.  To enhance the choking impedance could I purchase a #2 type balun such as the model 1115du by Balun Designs and mount it inside the shack?  Thanks.

73,
Jonathan W6GX
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W5DQ
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« Reply #17 on: November 26, 2012, 10:26:10 AM »

Quote
It takes a ton of beads to get significant choking action on the lowest bands and that gets expensive and heavy.

This depends on the beads used and the amount of 'choking' (Z) needed from the choke? I have used large heavy beads from Amidon (number escapes me right now) and using 6 or more gives the required impedance needed for 80-10M. But you are right .... they are heavy (each bead weighs a few ounces but they're not that expensive).

Gene W5DQ
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Gene W5DQ
Ridgecrest, CA - DM15dp
www.radioroom.org
AA4HA
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« Reply #18 on: November 26, 2012, 11:03:53 AM »

Quote
It takes a ton of beads to get significant choking action on the lowest bands and that gets expensive and heavy.

This depends on the beads used and the amount of 'choking' (Z) needed from the choke? I have used large heavy beads from Amidon (number escapes me right now) and using 6 or more gives the required impedance needed for 80-10M. But you are right .... they are heavy (each bead weighs a few ounces but they're not that expensive).

Gene W5DQ

As a general statement about ferrites, any energy that the ferrite is going to "eliminate" is going to end up as heat in the ferrite material. Think of 1.5 KW through a coax with even 20% of that being eliminated by the ferrite, that is 300 W of heat that would need to be dissipated in a tiny area. Sooner or later you are talking about heat that can actually melt the feedline insulation, somewhere around that area you are going to cross the curie temperature of the ferrite material and change the permeability of the core material (oftentimes this is a permanent change). Once that happens then all bets are off, you do not know what you will end up with. You can have something that initially looks like a success and then after a half-hour of rag chewing the SWR is much worse than anything you had seen before.

That rule about heat applies to any ferrite, even on conventional wound designs for impedance matching.

While some folks luck out and have bags of different cores and materials to put a dozen or more toroids on a feedline most folks may only have 2-3-4 toroids hiding away in their junk box. The ferrites strung along a length of feedline can get expensive and sooner or later you cross the point where it is cheaper to actually wind a balun and stick it in a plastic box with SO connectors on either end.
« Last Edit: November 26, 2012, 11:06:10 AM by AA4HA » Logged

Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
G3TXQ
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« Reply #19 on: November 26, 2012, 02:48:41 PM »

If you form a choke by winding coax on a ferrite core, the only current that can cause heating of the ferrite is the Common Mode current. If a choke had sufficient impedance to "eliminate" the CM current totally there would be zero heating of the ferrite. To suggest that the CM energy the choke eliminates is somehow "absorbed" by the choke is quite wrong!

If a choke gets hot, it's because it does not have enough CM impedance to reduce the CM current to a safe level.

73,
Steve G3TXQ
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W5DQ
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« Reply #20 on: November 26, 2012, 03:14:17 PM »

Quote
It takes a ton of beads to get significant choking action on the lowest bands and that gets expensive and heavy.

This depends on the beads used and the amount of 'choking' (Z) needed from the choke? I have used large heavy beads from Amidon (number escapes me right now) and using 6 or more gives the required impedance needed for 80-10M. But you are right .... they are heavy (each bead weighs a few ounces but they're not that expensive).

Gene W5DQ

As a general statement about ferrites, any energy that the ferrite is going to "eliminate" is going to end up as heat in the ferrite material. Think of 1.5 KW through a coax with even 20% of that being eliminated by the ferrite, that is 300 W of heat that would need to be dissipated in a tiny area. Sooner or later you are talking about heat that can actually melt the feedline insulation, somewhere around that area you are going to cross the curie temperature of the ferrite material and change the permeability of the core material (oftentimes this is a permanent change). Once that happens then all bets are off, you do not know what you will end up with. You can have something that initially looks like a success and then after a half-hour of rag chewing the SWR is much worse than anything you had seen before.

That rule about heat applies to any ferrite, even on conventional wound designs for impedance matching.

While some folks luck out and have bags of different cores and materials to put a dozen or more toroids on a feedline most folks may only have 2-3-4 toroids hiding away in their junk box. The ferrites strung along a length of feedline can get expensive and sooner or later you cross the point where it is cheaper to actually wind a balun and stick it in a plastic box with SO connectors on either end.


True if you have SWR that's thru the roof but with SWR close to normal and using the cores as a choke to prevent return currents off the outside of coax, there isn't much heat buildup. While I don't run 1.5Kw, I do run 500W and at that level, 8 of the Amidon FB-77-1024 provided enough current choke to remove all RF from the shack coming back on the outside of the coax driving my 6BTV on the lowbands. Without it I had some, not alot, but some RFI in the shack. I took a piece of 1.5" PVC pipe and ends, drilled holes in the end of the caps and ran the coax thru one end, thru a stack of 8 cores taped together and out the other end and added the PL259 to the feedline. Made a nice compact choke and with it laying on the base of the vertical, it is out of the way and I don't worry about the weight.

Referring to the graph at https://www.amidoncorp.com/specs/2-15.PDF, using the #77 material curve and a filter conversion factor of 3.7x, I calculated that for 80M, 8 of the FB-77-1024 cores provide a Z of appx 500 ohms. On 40M, Z is around 660 ohms, which is where I had the worse RFI and coincidently, the most interest of operating so it all worked out for the best. The curve for Z in this choke rises to a peak around 890 ohms at 10M. The cost for this choke was around $20 for the 8 cores (when I bought them) and the rest from home repair junk box. I am sure this setup would work at full power providing the SWR at the choke poinit wasn't above a 2:1 or so. It works for my needs and that is what is the bottom line here.

Gene W5DQ
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Gene W5DQ
Ridgecrest, CA - DM15dp
www.radioroom.org
W5DXP
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« Reply #21 on: November 27, 2012, 05:32:11 AM »

As a general statement about ferrites, any energy that the ferrite is going to "eliminate" is going to end up as heat in the ferrite material.

The common-mode energy is usually not "eliminated" by the ferrite choke. Common-mode RF signals obey the rules for the distributed-network/wave-reflection model. Typically, the common-mode RF wave encounters the high impedance of the choke and is reflected, i.e. the common-mode signals form a standing wave, e.g. on the outside of the coax braid. One can prove this to oneself by sliding an RF current meter up and down the coax. Just remember that there is a standing wave voltage maximum point at every standing wave current minimum point. A small florescent light bulb will light up reflecting the magnitude of the E-field.

The characteristic impedance for common-mode signals on the outside of the coax braid is in the ballpark of a few hundred ohms. That's why we need to have a high impedance choke - so the reflection coefficient will be high.

If a choke had sufficient impedance to "eliminate" the CM current totally there would be zero heating of the ferrite. To suggest that the CM energy the choke eliminates is somehow "absorbed" by the choke is quite wrong!

The question then becomes: If the CM energy doesn't make it through the choke and it is not absorbed, what happens to it? The answer is that most of it is reflected by the high impedance of the choke, i.e. the CM signal energy reverses direction at the choke and forms standing waves on the transmission line.
« Last Edit: November 27, 2012, 05:45:00 AM by W5DXP » Logged

73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
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