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Author Topic: Whats the difference between a Current Balun and a Ferrite Bead Choke  (Read 4704 times)
KD2CJJ
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Posts: 369




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« on: November 19, 2012, 06:06:31 PM »

I have both and havent found any info on what the major difference are between a 1:1 Current Balun (that uses a donut Torrid) and a simple multi ferrite bead choke.  I know both reduce/remove common mode current but when is one better over the other?

Thanks!
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73

Mike
KD2CJJ
WB6BYU
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Posts: 13039




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« Reply #1 on: November 19, 2012, 07:23:04 PM »

A ferrite bead choke IS a current balun, although there are other designs of
current baluns as well.

The main issue is getting enough choking impedance for the balun to be
effective.  Since the inductance increases with the square of the number
of turns, winding the coax multiple times through a single core becomes
more effective than a single turn through a large number of cores (which
is the equivalent circuit of the ferrite bead balun.)  But both can work
effectively as long a they are designed properly for the band of operation.
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W5DXP
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Posts: 3546


WWW

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« Reply #2 on: November 20, 2012, 04:20:19 AM »

A ferrite bead choke IS a current balun, ...

The ARRL Antenna Book describes such as a "W2DU balun".
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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.
W5LZ
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Posts: 477




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« Reply #3 on: November 20, 2012, 06:33:05 AM »

The biggest difference is in the word "balun".  If the idea is to get rid of CMCs then both can do that.  But a choke of any kind can't change from an unbalanced state to a balanced one.  That's the difference.  They may both get to the same destination but the routes are different.
 - Paul
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NR4C
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Posts: 306




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« Reply #4 on: November 20, 2012, 07:05:58 AM »

Coax with beads on it, or coax looped thru a toroid is an UNUN or UNbalanced to UNbalanced device.  A BALUN or BALanced to UNbalanced device will transform a BALanced antenna fed with some form of twin-wire, balanced feedline to an UNbalanced feedline such as coax.

....bill  nr4c
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N3JBH
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Posts: 2358




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« Reply #5 on: November 20, 2012, 08:54:55 AM »

Bill  nr4c

Am i to understand by your definition that you can not make a balun by using co-ax to wind through a ferrite core? I be interested in why this can not be done. Jeff
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N4RSS
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Posts: 258




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« Reply #6 on: November 20, 2012, 10:02:44 AM »

The biggest difference is in the word "balun".  If the idea is to get rid of CMCs then both can do that.  But a choke of any kind can't change from an unbalanced state to a balanced one.  That's the difference.  They may both get to the same destination but the routes are different.
 - Paul


Really ?  If there is no common mode current then the only current paths are center conductor and inside of shield, each of which is connected to a separate dipole leg.  Doesn't this yield a balanced current condition ?  If so,  why would it matter how it's achieved ? Idealized conditions of course, for example, choke right at feed point, etc.
« Last Edit: November 20, 2012, 10:05:33 AM by N4RSS » Logged
KD2CJJ
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Posts: 369




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« Reply #7 on: November 20, 2012, 10:36:11 AM »

I would rather stay away from conjecture and just build a conclusion from real principles - which I am lacking at this point - and yes I have read the antenna book many times and it doesnt answer my SPECIFIC question... and if we soley reliend on Google then why bother allowing me to post questions on here.

I appreciate the response that are constructive.

We have 3 ways to construct a choke (ie a 1:1 balun).

1.  "Ugly Balun" coax wrapped around PVC, etc.
2.  A Guanella Balun which is Bifolar wire wrapped around a torrid
3.  Simple Choke that is just a bunch of ferrite beads slipped over coax.

They all remove common mode current (assumption - but am I wrong?)

My questions are:

1.  Are they are all Baluns - like some of you said transform a balanced connection to an unbalanced connection?

2.  Is there a design that is more effective for a dipole than the other?  If so which one and why?

3.  Whats the best way to determine which one is effective for a specific application - More of a generalized question beyond a dipole?

Thanks... 
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73

Mike
KD2CJJ
AC5UP
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Posts: 3825




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« Reply #8 on: November 20, 2012, 11:11:06 AM »

We have 3 ways to construct a choke (ie a 1:1 balun).
1.  "Ugly Balun" coax wrapped around PVC, etc.

There's a reason why that's called an Ugly Balun... Rarely the best choice. They tend to be frequency selective and lack choking ability on the lower bands. Maybe for VHF where it will offer low losses, but for HF think of this as a Q & D expedient until you get a permanent balun.

2.  A Guanella Balun which is Bifolar wire wrapped around a torrid

That's what I use for HF. Typically with a 1.5" #43 mix core using 18ga stranded silver Teflon. Small, light, effective, I mention the silver Teflon as it has an extremely high voltage breakdown rating and I scored a pantload of it on the cheap many years ago. Best wire you can buy. If you want to roll your own, locate your nearest Avionics / Airframe repair shop and inquire about wire scraps. Sometimes they'll pull a perfectly good harness out of a wing for no better reason than it's 10-20 years old. Better to replace it before it breaks because 30,000 feet at night can be a rough neighborhood without instruments.

BTW:  " Bifilar " and " Toroid "

3.  Simple Choke that is just a bunch of ferrite beads slipped over coax.

If it wasn't for the "Ugly Balun" this would be the worst choice for HF. It takes a ton of beads to get significant choking action on the lowest bands and that gets expensive and heavy. For VHF it's a reasonable choice and the advantage of a bead balun is that you don't coil the coax which makes for the lowest possible loss.

The answers to your questions depend on the application. And if it's an HF dipole you're thinking about.......... Pick Door # 2
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WB6BYU
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Posts: 13039




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« Reply #9 on: November 20, 2012, 11:14:24 AM »

Quote from: KD2CJJ

We have 3 ways to construct a choke (ie a 1:1 balun).

1.  "Ugly Balun" coax wrapped around PVC, etc.
2.  A Guanella Balun which is Bifolar wire wrapped around a torrid
3.  Simple Choke that is just a bunch of ferrite beads slipped over coax.



What is common in all of these approaches is that the coax is made into a coil.  With
enough impedance in series with the outer shield, the common mode currents will
be greatly reduced.

The only difference among them is how this is done:  #1 and #2 use multiple turns to
increase the inductance.  (#2 can use coax rather than a bifiliar winding of wire.) 
#2 and #3 use ferrite to increase the impedance per turn.  In all cases the choke
must have sufficient impedance across the operating frequency range to be effective.
Usually the lowest effective frequency is determined by the inductance, and the high
end by self-resonances.

Chokes using ferrite tend to have a wider operating frequency range than those on
an air core, but the ferrite material must be chosen to give enough impedance without
saturating at full output power.  Using ferrite and/or multiple turns gives more inductance.

G3TXQ ha taken actual measurements on many different designs, and the results and
recommendations are available on his web site:  http://www.karinya.net/g3txq/chokes/

Further reading on how baluns work and why a choke balun really is a balun rather than
an unun in the classic article by W7EL:  http://www.eznec.com/Amateur/Articles/Baluns.pdf


There are other types of 1 : 1 baluns as well, though many are more common at VHF/UHF.
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KD2CJJ
Member

Posts: 369




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« Reply #10 on: November 20, 2012, 12:04:41 PM »

OK... thanks guys... between the recent comments and the article by W7EL I am fully baked on the topic...  My understand is crystal clear now...  I think this was a great discussion!  Now on to the UNUN research...  Grin
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73

Mike
KD2CJJ
W5DXP
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Posts: 3546


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« Reply #11 on: November 20, 2012, 12:21:18 PM »

But a choke of any kind can't change from an unbalanced state to a balanced one. 

The nature of coax is that the current on the inside conductor is perfectly balanced with the current on the inside of the coax braid. If a choke eliminates common mode current on the outside of the coax braid, the resulting currents must necessarily be balanced..
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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.
N4NYY
Member

Posts: 4742




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« Reply #12 on: November 20, 2012, 02:25:37 PM »

Quote
2.  A Guanella Balun which is Bifolar wire wrapped around a torrid

That's what I use for HF. Typically with a 1.5" #43 mix core using 18ga stranded silver Teflon. Small, light, effective, I mention the silver Teflon as it has an extremely high voltage breakdown rating and I scored a pantload of it on the cheap many years ago. Best wire you can buy. If you want to roll your own, locate your nearest Avionics / Airframe repair shop and inquire about wire scraps. Sometimes they'll pull a perfectly good harness out of a wing for no better reason than it's 10-20 years old. Better to replace it before it breaks because 30,000 feet at night can be a rough neighborhood without instruments.

BTW:  " Bifilar " and " Toroid "

This is what I have and got it from Balun Designs. He makes great baluns guaranteed forever, outside of a lightning hit.
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AC5UP
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« Reply #13 on: November 20, 2012, 02:37:39 PM »



The perfect balun for you would have a bipolar winding.    Grin


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N4NYY
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Posts: 4742




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« Reply #14 on: November 20, 2012, 03:14:59 PM »

Doh! Shocked
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