Current Balun vs Voltage Balun
I am building a 135' dipole which I would like to feed with 450 ohm ladder line. However, I don't want to bring the ladder line into the shack so I would like to use a 4:1 balun to transition to RG-8X for the last 15-20 feet. I am unclear on when to use a Voltage vs Current balun. Also, is there a length of Coax that I should avoid?
A 4-1 voltage balun would seem to be appropriate, with the high impedance facing the line and the low impedance side facing the co-ax.
Licensed Since 1970
A 4:1 current balun is most appropriate. See http://www.radioworks.com for their "Remote Balun" product, which is intended for specifically the application you have. It's 4:1, current balun, with a hook to hang it outside under the eaves or something, and terminals for the balanced line, plus an SO-239 for the coax connection.
Originally Posted by KC9QQ
I too would have to vote for the current balun. The current balun will force equal currents in the coax feed. This will reduce noise and rf in the shack. Check out these sites for plain language discussions on the baluns.
Originally Posted by KC9QQ
Hello Martinsville from Carmel, IN!
For matched antenna the 4:1 voltage balun is good. In unmatched I think you need the current balun. So depends on how you will operate the antenna.
This probably explains it where anyone can understand.
I made a fundemantal error in my first reply.
The voltage transformation should, of course, be 3:1, giving an impedance transformation of 9:1, thereby matching the 50 Ohms with the 450 Ohms. Therefore, use a 9:1 voltage balun to match the impedance of the ribbon to the impedance of the coax.
There is nothing wrong at all with also including a 1:1 "current" balun to achieve common mode rejection and promote balance. However, the impedance transformation is a necessity. A "current" balun on its own will not work.
Sorry for the confusion
Licensed Since 1970
Trouble is, the "450 Ohm" line is not going to present 450 ohms at the shack end when used to feed a 135 foot antenna operated over several bands.
The impedance at the feed end will be some other reactive value over a wide range depending on the frequency you operate the feedline/antenna system at.
"450 Ohm" line will only present 450 ohms at the feed end when it is terminated in it's characteristic impedance at the load end...450 ohms....Resistive.
Don't worry about the impedance transformation, just use a 1:1 current balun to couple the coax to the balanced feeders. It will work fine.
Current Balun is the Answer
Heftier 4:1 voltage baluns might handle the freight, but I can tell you that my LDG 4:1 voltage balun got very crispy inside when I was "multi-banding" an 80 meter doublet fed with window line. Not major power either, just attempting to find a match on 20 meters with less than 35 watts output. Fractured the toroid & burned through the enameled wire.
I was able to get rid of most RF in the shack from my 200 ft center fed antenna with a 6:1 balun on the window-line, and a 1:1 current balun inboard of that facing the tuner. Of course neither is actually seeing the impedance it is designed for, and it's possible I'd do as well or better putting the 1:1 between rig and tuner.
I've had a "KW" 1:1 balun get hot enough to boil spit at 100 watts. Misuse of the device, sure. But instructive.
Use a current balun when you want to force equal currents, and a voltage balun when you want to force equal voltages. In this application we usually want to prevent feedline radiation - that requires equal currents.
The original poster didn't tell us what length of ladderline he is using, so the impedance, just on 80m, could be anything between 50 ohms and 3200 ohms; given that tuners have an easier time with a high impedance than a low one, why would we want to step the impedance down?
Given the information that we have, the most appropriate choice is a 1:1 Current Balun.
Check out the excellent advice on the DX Engineering web site:http://www.dxengineering.com/TechArt...-3B654F804C4F} which includes this quote:
Length of the coax? As short as possible - it'll be handling a high VSWR if you use the antenna on several bands.
Often, a 4:1 balun is suggested for Multi-band Dipoles; however, the best balun to use for this application is a 1:1 ratio. The impedance at the end of the feedline is going to vary greatly from very high to very low. Tuners have an easier time with high impedance than a low one. A balun with a ratio of 4:1 or more will transform already low impedance to an even lower one that will make the antenna hard to tune. The 1:1 ratio balun will just pass the low impedance through.