Depending on the total number of turns, which is somewhat dependent on the size of the core (the schematic shows 9 turns, but 10 to 13 turns are fine), and tapping approximately 1/4, 1/2, and 3/4 of the way along the top winding, 1.5:1, 2:1, and 3:1 ratios can be obtained. Or nominally, 33 ohms, 25 ohms, and 16 ohms. Adding a tap at the far left side will supply a 1:1 ratio (50 ohms).

An FT240-61 ferrite core is an ideal size and mix. When wound with #14 awg Thermalese^®, it is more than adequate for 1,000 watts. The core, wire, and Scotch #27 glass tape maybe purchased from Amidon or Palomar Engineers.

The photo at left shows the unwound core covered with Scotch #27 glass tape which assures a neat and clean appearance. Note the small strips of glass tape holding the windings together. This is an important issue. For maximum efficiency (>97%) the characteristic impedance of the windings needs to be 25 ohms. Two closely spaced, Thermalese^® insulated, #14 awg wires is very close to 25 ohms. Thermal plastic and Teflon^® insulated wires should be avoided.

The photo on the right shows a partially wound core. Please note the evenly spaced, and tightly wound windings. Compare these to the sloppy windings in the photo below left. This is good example of how NOT to do it.

The length of the windings will vary depending on core size and the number of turns. The wire length should be approximately 28" long for an F240 core This will result in 11 complete turns which is the optimum number. This length will give you a little extra for handling, and connecting to the SO239s.

The photo below left shows the completed UNUN mounted inside an aluminum box. You could use plastic as well, but if you do, don't forget to connect the SO239s together. The six position ceramic switch is from Digi-Key and has been shown to handle 500 watts easily. Soldering spots for the taps are made by cleaning the insulation off the top winding using a Dremel^® tool with a wire brush attached. Just be careful not to damage the lower winding.

While a rotary switch is ideal, it is not strictly required. You can use banana plugs and jacks, but changing taps will not be as easy. In the real world, two taps will suffice for most five band mobile antennas. In this case, you can use a good quality bat handle switch to change taps with only a minor bump in impedance. A remotely operated relay is another possibility.

The right photo is the top view. Please note these are approximate impedances. The actual impedances will depend on how many windings you use and where you make the taps. Remember, the impedance is based on the square of the turns ratio (primary to secondary).

The aforementioned dissertation is not intended to be a construction primer, but rather a guide line, as there are just to many different configurations which will work. Just take your time, and remember to keep the windings tight and evenly spaced. Total cost will depend on the parts you use. Shop carefully, and you can do this project for as little as $10. The UNUN in the photos was closer to $75, as all of the parts were purchased new.

Odds & Ends

For a more complex description of how transmission line transformers work, I refer you to the various publications offered by the ARRL.

UNUNs have their place, as does inductive matching, capacitive matching, and the use of auto-transformers. However, if you're using a remotely tuned antenna, inductive matching has an edge as it makes the process fully automatic. More information on the subject is in my Antenna Matching article.

Bon Mobilite'