Toroidal Balun vs. Choke Balun

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W2XR:
A quick question for the group:

I presently feed the 50 ohm input to my homebrew balanced L-network antenna tuner with a choke balun wound with about 27 feet of RG-213 on a 4.0" O.D. piece of Schedule 40 PVC pipe. It seems to work well, with the bulk of my operating on 75 meters, but I was wondering if there would be any advantage or disadvantage to replacing this existing coaxially-wound balun with a conventional current balun wound on a toroidal core?

Array Solutions of Sunnyvale, TX offers a Model W1JR-50-1 Toroidal 5 KW/10 KW PEP 50 ohm/50 ohm balun that covers the frequency range of 1.8 to 30 Mhz., so this is one possible option if the toroidal current balun is considerered a better solution in this application.

Thanks & 73,

Bruce, W2XR

K1JJ:
Quote from: W2XR on December 04, 2006, 08:44:16 PM

A quick question for the group:

I presently feed the 50 ohm input to my homebrew balanced L-network antenna tuner with a choke balun wound with about 27 feet of RG-213 on a 4.0" O.D. piece of Schedule 40 PVC pipe. It seems to work well, with the bulk of my operating on 75 meters, but I was wondering if there would be any advantage or disadvantage to replacing this existing coaxially-wound balun with a conventional current balun wound on a toroidal core?

Array Solutions of Sunnyvale, TX offers a Model W1JR-50-1 Toroidal 5 KW/10 KW PEP 50 ohm/50 ohm balun that covers the frequency range of 1.8 to 30 Mhz., so this is one possible option if the toroidal current balun is considerered a better solution in this application.

Thanks & 73,

Bruce, W2XR


Bruce,

I personally think that the toroidal balun is a better isolator than a simple RF choke comprised of a coil of coax.  The optimum amount of coils is about four turns for 10M, while for 75M it's about 12 turns - all on a 4" pvc pipe. So, on 20M-10M the reactance degrades if using 12T or more.  It's even worse if you wanna cover 160-10M.

In contrast, the toroidal unun or balun is pretty consistent across this spectrum if swr is any indication. Hook a 50 ohm resistor to the sec and sweep the pri with the MFJ-259B and you'll see.

I also used some of WX0T's unun's here to go from 50 to 75 ohms. (50 ohm Coax to 75 ohm hardline)   

After paying $100 a pop for a few I decided to wind my own using W2FMY's book and parts. Do a search and wind your own for a few bux cost of the toroid and wire. I use Teflon wire on mine.

73,
T

W2XR:
Hi Tom,

Thanks for the reply!

You indicated that the isolation with the toroidal current balun would be better than that of the coaxial choke balun. Why would the isolation be an important or more optimum consideration here? By isolation, I am assuming that you are referring to the isolation between the input to the balanced-L tuning network, and the output of the transmitter.

Is this correct?

BTW, I have done some research on winding my own toroidal current balun, and you are right; I would be better served to wind my own.

73,

Bruce

K1JJ:
Hi Bruce,

What I mean by isolation is the balun's ability to keep the unbalanced components from the balanced components... ie, like an effective transformer's primary referenced to ground and the sec floating with a balanced dipole, etc. The output balanced currents are not skewed and phase distorted by the input's unbalanced currents.

An RF choke as created by a coil of coax will vary in impedance depending on it's number of turns. Too many turns on the higher bands and it's performance degrades markedly due to intercapacitance of the turns, etc.. But you need the many turns to cover 160 and 75M, etc.  There was a thread here where I posted the freq vs: turns impedance. It shows the problem clearly when using it on a broad spectrum like 160-10M. (The idea with the RF choke is to stop unbalanced RF currents flowing on the outside shield, as you know.)  Many guys use the proper permability ferrite beads over the coax too, to do the same job.

But I figure, why not start with no unbalance in the first place by using an unbal pri to bal sec RF transformer? Plus, the toroidal balun is pretty flat (broadband) across the whole 160-10M spectrum. Just get the correct core permability and you're all set. The downside is they are prone to permanent saturation damage from lightning pulses. Up on the antenna it's a problem. But in the shack you will probably be OK cuz the ant usually gets disconnected during stroms. You will know your ferrite has a problem if the swr is high... (the core cannot pass much power)

Don't get me wrong...a coax  RF choke works FB for semi-mono band use. I use them on many of my antennas here at the feedpoints except for some 20 ohm Yagis where I need a 4:1 coaxial balun to interface a delta match to give 50 ohms.. 

As a precaution...  Make sure you wind it like a toroid is supposed to be. If you do it like a conventional pri to sec power transformer, the coupling is poor and you will see the core heat up, even though the match may be good. Just follow the instructions and it will work FB..

73,
T

W2XR:
Hi Tom,

Thanks for clarifying this isolation issue for me. Yes, I completely understand the point you are making here, and I understand the concept and need for the isolation you have described.

But out of curiousity, why would the toroidal current balun offer better isolation vs. the coaxial RF choke current balun? And if the choke balun and the toroidal balun were both to be used on one band only, say 75M, and the choke balun was optimized for that band, would the isolation be essentially equivalent between the two devices?

Thanks & 73,

Bruce

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