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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?

Thank you for your time

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

If not, it does mean you have a much less extreme impedance ratio to tidy up with an L and a C.

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Hope that I am not oversimplifying your problem.

I am not fully versed on this antenna, but to me, an impedance of 140Ohms seemed a little off for anything that I have laid out for 1.7GHz. (Pardon my ignorance for your analysis, but just in case.) There are some impedance transformers for 1.7 GHz in Minicircuits, but I have the sense that those frequencies are prohibitively high for them. Depending on your output power, an LC impedance matching network or a microstrip matching network would work better. You might consult to Pozar for textbook examples. For broadband applications, I have used Richard Li's book for intuitive broadband matching tricks, there might be some use in checking that book also.

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  • \$\begingroup\$ There are impedance transformers that are actual transformers (two coils coupled to each other) I refer to them in impedance transformers for 1.7 GHz in Minicircuits \$\endgroup\$ – mehmet.ali.anil Jun 12 '18 at 12:21

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