There are 2 basic types of broadband transformers used in most QRP work, conventional and transmission line style. Both types can be wound on ferrite toroids, potcores or rods, however for this discussion we will confine ourselves to the toroidal types used to give a 4:1 impedance transformation. These transformers are used throughout the various projects on this website. For MF and HF uses, a ferrite core permeability of 850-900 is generally required and the FT37-43 ferrite core is suitable. This will allow AC circuits to transform from unbalanced 50 ohms impedance up to 200 ohms unbalanced impedance or visa-versa. Shown below are three equivalent schematics of the 4:1 transmission line transformer. The center drawing is the easiest to conventionalize, however close examination will show that all 3 schematics represent the same thing. The high impedance is 200 ohms and the low impedance is 50 ohms in all cases. It is important to know that these transformers are symmetrical and the points labeled Ground or VCC can be switched with the point labeled High Impedance. There are a great many published references on RF transformers for further study.
Winding the 4:1 Transformers
These transformers are wound as bifilar (2 wires) which are generally twisted together. Winding these transformers is very easy. All that is required is two 7 to 8 inch pieces of #28 AWG enamel coated wire and an FT37-43 ferrite toroidal core. A vise, ruler and a brace and bit drill are also very useful. I got my brace and bit drill at a garage sale for two dollars. You need to twist the 2 pieces of wire together so that there is around 8-10 twists per inch in the wire. To do this, loosely twist the wires at one end so they are of equal length. Place the twisted ends in a vise about 1/4 inch. Next, place the free wire ends together in your brace and bit drill chuck (no drill bit) and tighten up the chuck so that the wires are held secure. The wires should be of equal length and tension. Start hand winding the drill to twist the wires together and every once and a while use the ruler to check how many twists are in a one inch space. When you get to 8-10 twists per inch you are done and can trim the very tips with a wire cutter in preparation for winding.
You generally need somewhere ~ 7 inches of wire for winding a complete transformer. Leaving a 1 inch lead, wind ten complete loops through the toroidal core leaving a small gap between the start and finish leads.
Untwist the leads a little so that you have 4 separate wires. One set of these wires wires will be called winding #1 and the other winding #2. You need to identify them and further break them into 1a, 1b and 2a and 2b. Generally I regard the the top two windings as A and the the bottom two wires B. Use whatever system works for you. Strip off all the enamel on all four leads and then get your ohm meter or better yet a beeping continuity tester. Start on one of the top (A) wires by connecting the ohmmeter or continuity beeper to it and then touch one of the bottom wires and then the other bottom wire. Whatever bottom wire shows continuity with the top wire should be marked along with the source top wire with paint, liquid paper or whatever you like. Designate the marked wire pair winding number 1. You can test for shorts as well, there should be no connection between wire set 1 and wire set 2 at all! So now you have two wires sets, winding set 1 is marked and winding set 2 is unmarked. The top two wires are arbitrarily labeled A and the bottom two wires are labeled B . Refer to the diagram above for clarification. Connect 1b to 2a and twist them together and then solder. Your transformer is done. It is really easy to make these things. More elaborate methods such as using 2 color wire and painting one wire maybe used. These transformers will also work if the wires are untwisted, when you twist them, 8-10 twists per inch is a guide only and is not critical. Never use bare wire for these transformers.
Homebuilding Diode Ring Mixers
Homebuilt diode ring mixers are also easy to make and can be quite a cost savings. A doubly-balanced diode ring mixer has two unbalanced to balanced transformers and a diode ring. The impedances at the three ports is 50 ohms. The transformers are wound with #28 AWG enamel coated wire on a FT37-43 ferrite toroidal core using a trifilar (three wire) technique. The wire twisting and winding technique is done as described above for the bifilar transformers. The connections 2b and 3a are twisted together and soldered. Again you will have to develop a technique to help you distinguish the wires from one another.
For optimal results Schottky or Hot-Carrier diodes should be used. However, common diodes such as the 1N914, 1N4148 or 1N4454 are all quite suitable and are much cheaper. The four ring diodes should be matched to help mixer balance and thus carrier suppression. At MF and HF the most critical matching required is the forward voltage drop across the diode and this is easily performed with a sensitive voltmeter. Set your voltmeter on the 2 volt scale to give you three decimal places for matching the voltage drops. Try and find 4 diodes close to one another. In addition, best results maybe obtained if all the diodes are the same type (ie. all 1N4148) and if they are all from the same manufacturer. Below is a easy schematic for matching your diodes with a voltmeter. Give the diode under test at least 20 seconds to warm up and stabilize before taking your voltage measurement.