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Helical element matching

Just my contribution on matching helical elements.

A typical unmatched helix impedance is about 140 ohms resistive.  Normally
it is fed with a 50 ohm co-axial line.  A standard quarter wave transformer
of sqrt(50*140)~ 84 ohms should do the trick at your centre frequency.

Now if one helix winding is a wavelength long the first quarter turn can be
made into a quarter wave transformer by choosing its height and width to
give you an 84 ohm transmission line.  That is if you keep it parallel to
the groundplane for the first quarter turn and consider it as a 'microstrip'
line with air as the dielectric.

I have had good results with this approach in simulation and with one rough
and ready prototype.  If this makes sense, is common knowledge or complete
nonsense please let me know.


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