# Impedance Matching for Helical Antenna

I'm building a helical antenna at 1.7 GHz from scratch. Theory says that its characteristic impedance is 140 Ohm. The RF transmission line is 50 Ohm so there has to be an impedance transformer in between. What is the proposed custom implementation of such a transformer? I have come across the LC circuit as a possible transformer, but is it enough for 5 MHz of bandwidth that i require from the antenna? What's the simplest transformer that will do the job for me?

• at 1.7 GHz, 5 MHz is really not that much bandwidth – so a simple LC circuit really can't be too selective there. – Marcus Müller Jun 12 '18 at 12:21

The classic impedance transformer is a $\frac{\lambda}{4}$ length of transmission line, of the geometric mean impedance. In this case, sqrt(50*140) = 83 ohms.
75 ohms is quite close, and 140 ohms through a $\frac{\lambda}{4}$ of 75 ohms comes out to be 40 ohms. Not perfect, but it improves the return loss from -6.5dB directly to -19dB through the transformer. May be good enough for you.