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Author Topic: transition from coax to 450 ladder line  (Read 780 times)
KD8Z
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Posts: 169




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« on: December 02, 2008, 01:32:35 PM »

I need to transition from coax coming from the shack, through the wall and then to ladder line 450 ohm. What device does this well?  Some form of adapter.

ty
73
kd8z
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W4VR
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« Reply #1 on: December 02, 2008, 01:34:57 PM »

Most people use a 4:1 current balun.  I believe you can purchase these devices from DX Engineering or Radio Works.
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WV4L
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« Reply #2 on: December 02, 2008, 02:38:49 PM »

Yes, a 4:1 Current balun is what you need. I have one from DX Engineering it is well made and sealed off from moisture egress as well.
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N5LRZ
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« Reply #3 on: December 02, 2008, 03:22:00 PM »

HOW about a 9:1 balun to convert 50 0hms to 450 termination...

Ask and ye shall receive

http://www.dxengineering.com/Parts.asp?ID=60&PLID=50&SecID=10&DeptID=9&PartNo=DXE-BAL450-H10-A
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K3AN
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Posts: 787




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« Reply #4 on: December 02, 2008, 03:22:47 PM »

Why not a W2DU-style bead balun? The impedance at the bottom end of the 450 Ohm line can vary all over the place, depending on the antenna's impedance and the length of the line. Baluns get lossy when they don't work into something close to their design impedance.

Varney (G5RV) recommended no balun for his antenna.

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K1BXI
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« Reply #5 on: December 02, 2008, 03:27:34 PM »

Check this:

http://www.w5dxp.com/notuner.htm

John
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N3JBH
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« Reply #6 on: December 02, 2008, 03:51:28 PM »

Check out the Model #4115 From Balun Designs it be the perfect gizmo for the job....

www.balundesigns.com


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W8JI
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« Reply #7 on: December 02, 2008, 03:56:23 PM »

Since the ladder line is not matched, you really don't want a 4:1 balun. You especially don't want a 9:1 balun.

What you want is the shortest possible run of very good coax with good connectors into a good 1:1 balun. If you run a bead balun it will probably overheat with less than a kilowatt. It just won't take the voltage from the ladder line terminals to earth.

You need a strong current balun with a lot of core material and a high choking impedance. I'd use a DXE 1:1 high power balun if you are going to run an amp into an untuned line.

73 Tom



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W5GNB
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Posts: 419




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« Reply #8 on: December 02, 2008, 04:39:16 PM »

What's wrong with using an antenna tuner with a balun installed in it for ballanced line??
That is what I do here and I have absolutely NO problems and a great match on any frequency.

Gary - W5GNB
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G3LBS
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Posts: 11




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« Reply #9 on: December 02, 2008, 11:36:15 PM »

Better still a true differential tuner like a Johnson Matchbox, with no balun
W2/G3LBS
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WA3SKN
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Posts: 5494




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« Reply #10 on: December 03, 2008, 03:49:59 AM »

The first choice would be to run the ladder line directly to the tuner in the shack.
Second choice would be to use two pieces of coax inside and connect the centers to the ladder line outside.
Third choice would be a current style 1:1 balun at the ladder line with coax to the tuner. (assuming typical amateur antenna).
Fourth is the 4:1 current balun design.
Try to make the coax part short and the ladder line long.
73s.

-Mike.
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W5GNB
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Posts: 419




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« Reply #11 on: December 03, 2008, 05:34:02 AM »

To clarify my installation, I have the outside ladder line attaching to a pair of feedthrough insulators at the window sill.  On the inside I have a Pair of RG-8 coax cables running parallel to the tuner with the two center conductors as a parallel line.  The shield is commonly grounded at the tuner chassis but is NOT grounded at the window sill end.

This provides a shielded parallel cable INSIDE the shack and also provides a ballanced connection to the outside open wire feeders.

I Do use a ballanced matchbox that requires No balun (Johnson Matchbox) but I have noticed absouletly no difference in performance with tuners that do use a balun.

When running the Full legal limit, I have no rf problems at all with this setup......

Regards!
Gary - W5GNB
 
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N5YPJ
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Posts: 642




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« Reply #12 on: December 03, 2008, 07:12:36 AM »

I have been using a W2DU 1:1 balun for a little over a year now and this setup has proven to be the best for me. I have less problems loading up than any other system. Somebody may claim that I'm losing power somewhere and I may well be but good contacts are made with 25 watts.

Previous setups were parallel coaxes to the built in 4:1 tuner and coil baluns just outside the shack window. Also I had the MFJ-993 remotely mounted with ladder line running directly to the balun. Besides more difficulty in matching I dislike having to disconnect parallel lines from the tuner balun when thunderstorm approach, I find the conductors more apt to break when moved a lot.

Best of luck to you.
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KD8Z
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Posts: 169




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« Reply #13 on: December 03, 2008, 10:23:21 AM »

Was thinking about using my Dentron MT-3000A tuner for the job, but was kind of preferring to use coax in the shack, rather than 450 ladder line inside, I always seem to get plenty of rf in the shack with ladder line inside.
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VK1OD
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Posts: 1697




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« Reply #14 on: December 03, 2008, 12:07:31 PM »


Conducted common mode current is possible on both open wire transmission lines and coaxial transmission lines.

If you have common mode current on the existing open wire line, transitioning to coax does not of itself significantly reduce the prospect of common mode current.

If you placed a voltage balun at the transition point, or utilised a voltage balun built in to an ATU, it will do almost nothing to reduce common mode current.

The shielded coax pair with the shields grounded at the ATU only does not significantly reduce common mode current or shield its effect.

One or more common mode chokes or 1:1 current baluns as recommended by some may be effective in mitigating 'RF in the shack' due to common mode current.

Owen

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