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I have been researching how to construct a 1:1 current balun. I have a few questions and am looking for feedback on what I plan to do.

First, I want to use the balun from 10m to 160m at 100w.

I plan to use an Amidon FT-140-61 toroid bifilar (2 wires) wrapped with #18 magnetic wire 10 to 12 turns. I do not plan to use tape.

Do I need to use tape? How do you use tape? Why? I've seen toroids covered in tape and I've seen tape that appears randomly wrapped on the magnet wire. Do you need to prevent the magnet wire from touching a bare ferrite toroid?

When would you need a different material type for the toroid? I've read Amidon's documentation on frequencies for their toroids, but I've also seen different frequencies shown elsewhere. Is there a chart that shows which material is best with which range of frequencies?

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  • $\begingroup$ I have a balun I just put together a few days ago. Good for up to 250 watts and it supports both 1:1 and 4:1 transformer. Bandwidth is 500 KHz to 55 MHz. It is a kit, available from Elecraft for $39.95. How can you beat that. Of course, you may want to design your own but I just needed a nice balun for my portable ops. It is called the Elecraft BL2. $\endgroup$ – K7PEH Mar 5 '17 at 5:50
  • $\begingroup$ Thanks for mentioning this. That's an interesting design. I really want to understand balun construction. It's very simple, I want to understand ferrite selection and the details. Cost for materials is only about $5 unless you get the larger toroid, then it's around 11. Installing in my attic, so I don't need an enclosure for weather proofing. $\endgroup$ – billabel Mar 5 '17 at 16:58
  • $\begingroup$ I certainly understand your desire to learn more about baluns and designing your own. And, I agree, picking the right toroid size and ferrite mix to achieve the desired results is an area where guessing and experimentation are sometimes necessary. As far as basic balun operation and understanding, I have found Tom's (W8JI) descriptions to be most clear. Here is his description of balun lore that appears on the DX Engineering site: dxengineering.com/techarticles/balunsandfeedlinechokes/… $\endgroup$ – K7PEH Mar 5 '17 at 17:15
  • $\begingroup$ I'd guess the tape is just there to protect the enamel insulation from getting nicked by sharp edges on the core. $\endgroup$ – Phil Frost - W8II Mar 7 '17 at 15:36
  • $\begingroup$ Hello and welcome to ham.stackexchange.com! Please consider taking the tour to get the most from the site. 73! $\endgroup$ – rclocher3 Mar 10 '17 at 21:48
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Do I need to use tape?

With insulated magnet wire, no. Ferrites are at least somewhat conductive, so you wouldn't want bare wire touching them. I suppose tape around the core could protect the thin insulation from sharp edges on the core.

When would you need a different material type for the toroid?

See How does one read a ferrite datasheet? Fair-rite has very good datasheets on each of their materials.

In practice it can be difficult to predict the choking impedance since the core and winding geometry is very significant. More turns increases the flux in the core, but it also introduces more capacitance, moving the effective choking frequency lower and narrowing the bandwidth.

It's easier to determine the impedance empirically. G3TXQ has a nice page of measured data for several designs. That should give you an idea of where to start.

You can also build the choke and install it, then transmit a carrier and measure the common-mode current.

I've also measured chokes with a RigExpert antenna analyzer. I imagine other analyzers work fine. If you're using bifiliar windings, connect the pair together at each end (common-mode currents are equal on each wire), attach the two ends to a connector with leads as short as possible, and then measure the impedance with the analyzer. For a coax winding connect the shield.

I wouldn't trust an antenna analyzer to be very accurate, since most are designed to be accurate only around 50 ohms. But it's accurate enough, and gives an easy way to check the choking impedance at several frequencies, and to determine if a change is making it better or worse.

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    $\begingroup$ Phil. Thanks for your answer. Looking at G3TXQ's chart, it appears that resistive measurements are best. So, green with black bars are the ideal frequencies for the ferrite/turns he measured. Is that correct? As you move to yellow and then red, you get more reactive, which means the choke is less effective. Do I understand that correctly? $\endgroup$ – billabel Mar 13 '17 at 2:26
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    $\begingroup$ Yes. Although, a reactive choke isn't ineffective per se, it just means it may be effective, ineffective, or actually make things worse depending on what the feedline reactance happens to be. Resistive works always. $\endgroup$ – Phil Frost - W8II Mar 13 '17 at 2:41
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Type 61 is probably not really suitable for a choke type current balun for 160m, and definitely not in a smaller core size like a T140. The AL of the FT140-61 is about 120, about 1/10th that of the higher permeability materials like Type 43, 31, or 73/77. You'll find it difficult to achieve sufficient choking impedance to get good common mode suppression on 80 and 160m, as you're really looking for something on the order of 2,000-5,000 ohms. You just can't cram enough turns on those lower loss materials, and you don't want low loss to begin with. A 1:1 choke type current balun only works because the material is lossy.

A truly effective and low loss 160-10m balun is a bit of a tricky creature to construct, because in order to get good common mode impedance at one end of the range, you'll likely have to sacrifice to some degree on the other, either by injecting additional loss on 10 meters, or accepting somewhat reduced impedance on 160 meters.

You'd do best for the entire range with something like an FT240-43 (which run about $4.50 from Mouser), with 14 or so bifilar turns of good quality magnet wire on the core.

Given the current status of the solar cycle, personally I'd probably opt for something like a T240-31 with 12-14 bifilar turns, knowing I was injecting some loss on 10m and to a lesser extent 15m, but getting better performance on 160 and 80m. Most of the action for the next few years will be on 80-15 meters, and a in a couple years that will probably drop to 160-17m before starting to rebound.

This document is often referred to as the "RFI bible" or "Balun bible", and with good reason. All the knowledge you seek is contained within, and in fairly easy to understand language. It's well worth a read and, in my opinion, should live on every ham's bookshelf.

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I am no expert, but this site has loads of information, http://www.dj0ip.de/balun-stuff/

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