I'm designing some low loss antenna switches for 160-10 meters, and my current plan is to construct the entire system as continuous 50 ohm microstrip feedlines on 2 sided FR4 PCB.

All examples that I've been able to find focus on applications above 100MHz, and I'm unable to find any information at all on whether there is a practical or theoretical lower limit to the frequency that such feedlines can operate at.

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    $\begingroup$ Unless you are making some very big PCBs, the characteristic impedance of your traces isn't very important since they will be a very small fraction of wavelength in length. $\endgroup$ – Phil Frost - W8II Feb 25 '16 at 0:07

Microstrips should in theory work just fine at low frequencies, as Z0 is independent of f. The advantage of microstrip technology becomes clear at high frequency when it is practical, and often easier, to create distributed element matching networks and filters than using lumped elements.

https://en.wikipedia.org/wiki/Microstrip http://www.microwaves101.com/encyclopedias/microstrip-calculator

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