Hi Eddy, > I am thinking of incorporating a homebrewed 4:1 balun as part of a > transmatch that I have here, which was designed to feed coaxial cable-fed > antennas only...
Why did you decide on a 4:1? Generally a tuner matches a much higher impedance than a 4:1 balun will safely permit, and has the most difficulty with low impedance loads. If you use a 4:1, it makes every load lower impedance. > Is there really & truly such a thing as an air core 4:1 balun that will > cover the entire spectrum from 1.8- to 29.7-MHz...? I've looked & I've > repeatedly searched on the internet, but can not quite come up with such > an animal. Maybe the frequency range of such a beast is far too great for > any practical design... > > Anyone have any leads in this regard...? Generally a 1:1 ferrite core balun has the least problem with grossly mismatched impedances, the widest useable frequency range, and the best balance for a given size and cost. Air core would be the most restricted in impedance and bandwidth, and have the poorest balance over a wide range of load conditions. It also would have very high Q, being mostly reactive in any "choking" impedance, which means it could actually make balance worse under many load conditions. Let me give an example. If I had an aircore balun with a common mode impedance of 5 +j500 ohms (Q=100) in series with a feedline having a common mode impedance of any reasonable R with a reactance of around -j250 or higher, the balun would make unbalance increase. Those big air core coax baluns you see will actually make many systems have worse balance, because they are largely a pure reactance. They only help balance under certain conditions, contrary to what is often assumed. As a general rule we do not want a high-Q balun, but something with very high resistance. We have to compromise that at times because of heat and impedance limitations, but almost always one of the very last things we want is an air core. I use air core baluns on some of my antennas, but I control the common mode impedance to ensure the baluns actually do what I want. I never use them in random situations. 73 Tom _______________________________________________ UR RST IS ... ... ..9 QSB QSB - hw? BK
