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DIY: Winding the FCP's Isolation Transformer

If one does not wish to construct their own isolation transformer, it can be procured from Balun Designs. For dual banding use a 160 isolation transformer, DIY or procured.

Balun Designs is the only firm known to us supplying ready-to-use isolation transformers per the specifications on this web site. We will list other suppliers if they emerge. We do not receive royalties from the sale of Balun Designs products.

We frequently get questions, like instead of a balun, why not just any old balun? Why that toroid material, why that size toroid, why that wire, why the teflon sleeving? Suspiciously, these questions often occur close to a major contest involving 160 meters, with procurement now unlikely before the contest. So we will take a little aside to outline the decidedly non-theoretical reasons from the original Burn 'em Brothers, the crack them, arc them, melt them, set them on fire wrecking crew, W0UCE + K2AV.

You can skip those explanations and farther down the page.

How Come that Particular Stuff?

The choice of wire and insulations came at the end of a period of failed transformers subjected to 1500 watts, tough RF situations and some number of close lightning strikes at W0UCE which also took out appliances and other ham equipment. The fix for blown transformers was heavy polyimide insulated wire plus standard wall #12 teflon sleeving.

#2 powdered iron cores wound with this combination no longer failed unless they were directly struck by lightning or dropped on their windings from height onto concrete garage floors or otherwise damaged from harsh mistreatment. Undamaged, they ran stone-cold in FCP related applications, and are still in use.

The double (heavy) polyimide insulation would not melt, but unfortunately was easily nicked in miscellaneous handling of a core wound with #14 double polyimide only. A nick in the polyimide led to carbon tracks, and burned and shorted devices. However, if the same wire was immediately inserted into teflon sleeving before exposure to any working or contact, it was protected from nicks and abrasion.

Teflon sleeving also increased the breakdown to 55 kVDC between adjacent wires in the winding and 27.5 kVDC between wires and the core and mounting materials, or 19.5 and 9.75 kV RMS at RF respectively**.

The wire/insulation diameter of teflon-sleeved #14 double polyimide dictated that the T300 series toroids had the smallest inside diameter that could fit a single 20 turn laid-flat bifilar winding on the inside surface. Two or more layers require a different and more difficult and expensive winding design, usually done by machines.

Balun guru W2FMI(SK), in his detailed book on baluns and ununs***, described an antenna tuner balun he designed to handle "worst cases". Jerry settled on a T400A-2 powdered iron core, and used 21 bifilar turns of the same teflon sleeved #14 wire for his antenna tuner balun. He had done much testing to arrive at his choice of toroid core material. After our collection of failed attempts with different cores, we settled on W2FMI's choice and sizing of core material. And having already arrived at the same wire and sleeving, we were now synchronized with W2FMI.

There were negative reactions to our using a T400A-2 core, on price and on physical size. The next smaller available diameters were the T300xx toroids. As the T300xx were the smallest usable inside diameter for the windings, and we settled on and tested with the inch-thick T300A-2 core.

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Isolation Transformer Options

On 160 meters, an isolation transformer wound on a T300xx or T400xx powdered iron core exhibits some non-one-to-one impedance transformation. The "Regular" transformer versions exhibit more than the "Heavy" versions. But the variation in the transformation is usually absorbed without effect in the typical 160 meter pruning and tuning exercise. In many cases the residual inductive reactance is actually helpful for overall tuning of the L/IsoT/FCP system.

A 160 L/IsoT/FCP sometimes gets trimmed/adjusted/tuned to 50 ohms easier with a "heavy" version T400A-2 based transformer. But the T400A-2 will not fit in the 4x4 inch sealing cover plastic electrical box often used for transformer/balun housing. An alternate "heavy" version is a two core, inch and a half stack of a T300A-2 and T300-2, minimally taped together and wound as one core. This is very close to the overall characteristics of a T400A-2 and will still barely fit in a 4x4 inch box. This can also be done with a three-stack of the half inch T300-2.

We cannot specify the benefits of a "heavy" version vs. the "regular" with any certainty due to the huge variation in inverted L characteristics as actually installed due to height, placement, and environment. This subject is treated in depth in We highly recommend that you study the "Taming" section if you are interested in an "advanced" isolation transformer deployment.

Using a larger core is an extra expense that may or may not render an actual benefit. Other than sometimes mildly more monkey business to get a 1:1 SWR to the feed coax, use of the standard single T300A-2 or the equivalent double T300-2 does not in any way effect the distant end signal strength performance of a 160 L/IsoT/FCP.

Micrometals' identical cores for Amidon's inch-high T400A-2 and T300A-2 are the T400-2D and T300-2D respectively. The half inch high versions are labeled the same. The Micrometals cores are frequently listed on eBay.

**See double polyimide coated wire and teflon tubing specifications. Typical "heavy" or "double" Polyimide (14.6 kV), plus #12 standard wall teflon tubing 800V/mil times .016 in (12.8 kV) gives a 27.5 kVDC rating for the combination. This results in 27.5 kVDC insulation between the wire and the core and environment, and 55 kVDC between wires in the winding.

***Jerry Sevick, W2FMI(SK) "Understanding, Building, and Using Baluns and Ununs -- Theory and Practical Designs for the Experimenter", pp 58-62, (C) 2003 CQ Communications, Inc, Hicksville, NY

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Isolation Transformer Construction

Most of this section on winding an isolation transformer is based on experience and things that have gone wrong. Purchasing a commercial version allows you to skip this section, but if you are going to wind your own, this is must reading.

Transformer** Core Configurations Winding Turns and Lengths
3 Inch Regular (a) qty 1 T300A-2 (T300-2D*) 2 x 7'6" (2 x 2.3 m)
(b) qty 2 T300-2 40 total, 20 bifilar turns
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3 Inch Heavy (c) qty 1 T300A-2 + qty 1 T300-2 2 x 9'2" (2 x 2.8 m)
(d) qty 3 T300-2 40 total, 20 bifilar turns
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4 Inch Heavy (e) qty 1 T400A-2 (T400-2D*) 2 x 12'6" (2 x 3.8 m)
(f) qty 2 T400-2 50 total, 25 bifilar turns

* Micrometals designations for inch thick cores. Amidon and Micrometals designations are identical for the half-inch thick cores
**Discussion of these transformers is above in

Do not bother to cover the toroids with fiberglass tape. As we experienced early on, in several years of outdoor and damp indoors locations, the irregular surface of the fiberglass tape became a point for moisture, mold, and airborne material such as smoke and airborne tree resin particles to accumulate and become a dielectric and carbon track issue. With temperature changes, even a "sealed" box allowed air egress at hardware penetration points. "Project boxes" with doors required weep holes in the bottom of the box to blow out droplets when sunlight struck the box and heated it.

The teflon sleeved wire should be wound directly on the bare toroid core. In addition to our experience, W2FMI flatly states in his book that fiberglass tape is not needed if teflon sleeving is used.

If your core consists of multiple toroids, then tape them together at 120 degree intervals with one perpendicular turn of narrow electrical tape plus an inch overwrap. This is just to hold them together to start the winding. After winding, the windings and hold-down tiewraps will hold the toroids together.

Do not tape two toroids together by going "around the equator". This will create a void, inside the tape and between the toroids, which will at some point outdoors will accumulate water, causing sneaky, gradually increasing loss.

It is important to avoid nicking the transformer wire's polyimide insulation or subjecting it to any kind of abrasion except at the soldering ends of the wire. Teflon sleeving protects the polyimide insulation. Although the polyimide insulation retains its heat resistance and insulating properties beyond common soldering temperatures, we found it easy to nick the polyimide, not so after sleeving. You may wish to do the cutting and sleeving of the wire on a carpet for this reason.

Transformers situated outdoors usually accumulate traces of moisture between the wire and sleeve. RF at a nick in the wire insulation can use the moisture to begin a carbon track. A nick presents a point where only the teflon has to be penetrated to get to the wire. The strategy is to require penetrating both teflon and the polyimide to get to the wire.

Do not use any kind of lubricant on the wire. Teflon is already very slippery. It is possible to push 20 feet of #14 heavy polyimide wire inside #12 standard wall teflon tubing. Some lubricants will make it harder to insert the wire. The lubricant can also be of a composition that deteriorates or introduces loss. To avoid digging into the teflon during insertion, burnish the wire's insertion end-cut with a fine fingernail file or sandpaper until it is rounded without sharp edges. When done filing be sure to wipe off any grit or other residue from the wire's insertion end.

Before you do anything else with the wire and tubing, measure twice and cut the teflon tubing to size. To reduce friction, gently straighten the wire just as it is being pushed into the teflon. Push the #14 polyimide wire all the way into the #12 standard wall teflon sleeve until the far ends of wire and tubing are even. Then cut the near end of the wire even with the near end of the tubing. Teflon sleeving is now protecting the polyimide insulation from nicks. Do this for two wires, each wire at the length for your choice of core(s) in the table above.

Extremely important: On one of the winding wires mark both ends and the center and the 1/4 and 3/4 points with "flags" of tape, five flags in all. Later, if you don't flag, it will be difficult to correctly modify the turns ratio for Method B impedance matching in Don't use a marker for this as marker ink does not stick to teflon. Even permanent marker ink simply wipes off as the wire is handled during winding, resulting in one or both ends of the winding unmarked for polarity. Getting the wires flipped changes the isolation transformer into a weak choke. It will add loss to the system by driving power down the feedline shield, but otherwise seem normal. There is a check for this in the construction, but if you have already trimmed leads, you may have to splice wires to repair.

Pair the wires. Think of it as exotic zip cord. Although it is not required, it is easier winding (and looks better) if you tape the wire together roughly every four inches (100mm) with narrow tape. Ordinary width electric tape can be cut in half lengthwise with scissors for this purpose. Wrapping with a turn and a half around the wire pair is sufficient. Once the winding is on the toroid(s) there is no stress on the tape.

The wire lengths in the table include generous lead lengths to reach various points in a project box. Better too long than too short.

One flagged wire end goes to the FCP connection, the other flagged wire end goes to the aerial wire connection. If you shorten a flagged wire to clean up inside the project box, immediately reflag the wire. The flags will be essential if you later want to change the turns ratio.

Extremely important: After winding and preparing to make connections, perform this final test on the transformer: Place ohmmeter probes on the FCP connection and the aerial wire connection. You should see a dead short. Place the ohmmeter probes on the coax shield connection and the coax center conductor connection. You should see a dead short. Place ohmmeter probes on the FCP connection and the coax center conductor connection. You should see an open. If all conditions are not met, you have a wiring error to correct before installation. This is the most common issue isolated to a DIY isolation transformer.

We are often asked about a low power, or smaller core version for QRP. The transformers described above are probably good to 3 kW with an L over FCP. We have steadfastly resisted downsizing. In you will find downsized and miniature tuning devices listed as one of the miscreants. QRP particularly needs minimized losses everywhere in the antenna system, so you go for the low losses of the QRO capable transformer. QRP ops are frequently in the situation where their signal is in the noise at the other end, where all those extra dB shards can make a difference. Save the miniature stuff for back-packing, where you need it for minimal size and weight.

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