A nice tutorial on building a 1:1 ferrite-core balun for amateur radio HF use by a Spanish ham, with all the information you need to build one yourself presented visually, like Roy Doty's "Wordless Workshop" series:
(Bonus: includes the metal version of a cold 807!) In case pictures don't quite work for you, EA7IQZ kindly provides an English bill of materials in comments at YouTube. His drawing -- which still-frames quite readably -- is especially good. Oh, and the 2 mm enameled copper wire? Close enough to #12 AWG (2.053 mm) that you can use one for the other.
Variations and cautions: different ferrite formulations will have different saturation points and frequency coverage; I'd expect salvaged AM radio ferrite to be okay for 160 through 40 meters and maybe getting lossy around 20 meters. Power-handling should be okay up to a couple hundred Watts. You can (and should) test this before you glue up the housing by running some RF power through it into a dummy load for a few minutes, shutting it off and checking for heat, repeating the process at different frequencies (see below also) and power levels. It is possible that with a really crummy ferrite rod, it might break or smolder, which would be exciting, but considering the most expensive part of the project is the PVC pipe and eyebolts, not a big loss.
You can build air-core versions (I have one, somewhere, in a box with a nice toroidal balun, but I still haven't found them!), by scaling up the diameter and number of turns, but it's a matter of making a good estimate followed by cut and try. If you own an antenna bridge, it's easy to build them, terminate the balanced side with a 50 Ohm resistor, and use the antenna bridge to check frequency coverage. Power handling will be limited by wire size, insulation and coil form material -- PVC pipe is slightly lossy, polystyrene much less so.
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The real trick is making the balun as a transmission line al la Sevick, then the ferrite is there for the ride mostly so it's less critical but it still has e to have enough series
ReplyDeletereactance to break the ground return path.
For a 1:1 the w4du form (choke on a rope) style of ferrite balun is usually easiest for 1:1 apps and pretty cheap. I make them using FT37-43 on 10 inches of .195 teflon coax (rg series).
Eck!