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Several-octave balun?
Hi all. I'm pondering how to make a 1:1 current-mode balun that will
cover a little over 8 octaves. Thankfully, I can guarantee pretty close to a 50 ohm load over that range...20MHz to 6GHz. I have some nice tiny coax to use in making it. To get to the low end, something like 250nH or perhaps even a little less should do OK, but I need to avoid resonances that will mess up the high end. An air-core solenoid coil of about 250nH will have a self-resonance below 1GHz, so that's probably out. Ferrite beads or binocular cores, perhaps? Anyone have practical experience with how they behave in the GHz region? Comments? Cheers, Tom |
#3
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Several-octave balun?
On 11 mar, 20:44, K7ITM wrote:
Hi all. I'm pondering how to make a 1:1 current-mode balun that will cover a little over 8 octaves. Thankfully, I can guarantee pretty close to a 50 ohm load over that range...20MHz to 6GHz. I have some nice tiny coax to use in making it. To get to the low end, something like 250nH or perhaps even a little less should do OK, but I need to avoid resonances that will mess up the high end. An air-core solenoid coil of about 250nH will have a self-resonance below 1GHz, so that's probably out. Ferrite beads or binocular cores, perhaps? Anyone have practical experience with how they behave in the GHz region? Comments? Cheers, Tom Hi Tom, Up to about 1-2 GHz you can use a series of common mode chokes with different turns and different ferrite material and shape of ferrite material. Saying this is easy, but doing requires lots of measurements. The reason is that above the useful frequency range of the ferrite your coil becomes capacitive (with not very low Q factor). That capacitive part may resonate with the inductive part of the higher frequency section (or transmission line effects) and reduce common mode impedance significantly. Some manufacturers claim to have materials that work up to 2 GHz (for example Steward ferrites, now Laird). When you have some space available, you might cover the upper part (above 1 GHz), with a gradually changing coax to parallel strip construction of about 12 inch or so. The lower part can be done as described above. I do have some experience with reducing spurious resonance with filling the open spaces with carbon filled foam. It did have effect above about 1.5 GHz (I hoped effect at lower frequency, but that was insignificant). Good luck with the design, Wim PA3DJS www.tetech.nl |
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