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Author Topic: Yagi Balun  (Read 2748 times)
EI5GUB
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Posts: 60




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« on: July 15, 2013, 03:22:18 PM »

Hi im rebuilding my beam and want to use a better balun this time instead of the air wound version I had,can I use a current balun using 2 60 mm stacked ferrite rings and winding 2 strands of copper wire around them,we built a 1:1 with 3 coils but the swr and impeadance is crazy
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K8JD
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« Reply #1 on: July 15, 2013, 03:56:36 PM »

What you have at the feedpoint of a driven element depends on the parasitic element spacing and  a few other factors.
If you changed the diamater or spacing of the elements, that is a BIG factor.
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73...John
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N3JBH
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« Reply #2 on: July 15, 2013, 05:40:14 PM »

Might be wrong mix as well on the ferrites
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W9FI
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« Reply #3 on: July 15, 2013, 07:10:57 PM »

It sounds like you have both coax and copper wire going through that toroid, turning it into a transformer with primary and secondary windings.

So in addition to verifying that the mix is correct for the frequencies on which you're using the antenna, I'd pull off the copper wires and replace them with something nonmetallic to hold the cores together: plastic ties, nylon ties, or fiberglass tape come to mind as suitable substitutes.  That will simplify the interpretation of your next set of test results.

Good Luck and 73, Jim W.
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WB6BYU
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« Reply #4 on: July 15, 2013, 09:33:52 PM »

Quote from: EI5GUB

...we built a 1:1 with 3 coils but the swr and impeadance is crazy



I'm assuming that this means you had 3 turns of feedline (made with 2 parallel or twisted wires)
around the stack of cores.  Most likely this is too few turns to provide enough inductance to
work well as a balun, but unless you wound it wrong somehow I wouldn't expect it to throw off
the SWR and impedance (unless you had a the worst case feedline length that causes a lot of
common mode current.)

If you had wound it for a 1 : 1 voltage balun then it certainly could cause such problems.

You have to know what type of cores you have to know how many turns it needs (and whether
they are at all suitable for this application) but you can get some idea of the number of turns
needed for various core types and frequency ranges from G3TXQ's web page here:

http://www.karinya.net/g3txq/chokes/
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EI5GUB
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Posts: 60




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« Reply #5 on: July 16, 2013, 02:15:53 PM »

ok thanks guys for replies,this beam is a 4 element 6 band beam,it has 2 driven elements 2 feet apart,it is factory built and does work quiet well  even though it has more traps that Grizzly Adams,I originally used 6 coils of coax as a choke balun but need something neater this time,the balun i spoke about was copied from the net and had 2 60 mm cores stacked,there was 3 wires of 2.5mm insulated wire wound 12 times around this core,but the swr was highand resistance was over 100,i scrsaped that lastnight and built a 1:1 current balun with same core and 2 copper wires wound six turns then sent across core diameter and then 6 turns up the other side but havnt had time to check if its ok,any ideas
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WB6BYU
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« Reply #6 on: July 16, 2013, 05:28:30 PM »

Quote from: EI5GUB

...there was 3 wires of 2.5mm insulated wire wound 12 times around this core...



That is a traditional way of making a 1 : 1 voltage balun.

Performance is highly dependent on the choice of core material.
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EI5GUB
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Posts: 60




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« Reply #7 on: July 18, 2013, 09:59:22 AM »

ok ive checked this new balun and its the same as before,high swr and high impeadance,on 14mhz,when i go down to 1.8 mhz its is perfect,I dont have any information on the ferrite rings as I got them on ebay,looks like I will just wind some RG213 around a 6 inch plastic pipe,i cant understand why a 1:1 current balun which isnt a transfromer is so far away from 50 ohms
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WB6BYU
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« Reply #8 on: July 18, 2013, 01:45:05 PM »

Quote from: EI5GUB

...i cant understand why a 1:1 current balun which isnt a transfromer is so far away from 50 ohms...



Did you try winding the cores with thin coax instead of two parallel wires?

Assuming that you used a two-wire transmission line, with one end connected to the coax and
the other end connected to the load, if the load is a perfect 50 ohms then the impedance seen
by the coax will depend on the characteristic impedance of the two wire line used to wind the
balun and the electrical length of it.

If the two wires have an impedance of 50 ohms, then there shouldn't be any impedance
transformation effect.  But it is difficult to make a 50 ohm parallel-conductor line because it
requires a spacing that is less than the wire diameter (at least for round conductors.)
Sometimes you'll see recommendations that the two wires be twisted together before being
wound on the core - this lowers the characteristic impedance even further.

Why does this matter?  If the characteristic impedance of the line (made from the two wires
wound on the core) is 100 ohms, then with 1m the SWR would be 2 : 1 on 20m just due to the
impedance transformation.  The situation gets a bit more complicated when the turns are wound
adjacent to each other on the core because a single wire can couple to the turns on either side
of it.

Twisting the wires together and/or covering with heat shrink tubing to hold them together may
help by lowering the impedance and/or providing more coupling between the desired wires and
less to the adjacent ones.  Winding the core with the same number of turns of coax cable is
even better at providing the desired impedance and minimizing unwanted coupling between turns.
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