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