![]() The criterion for cutoff is that the circumference at the midpoint inside the dielectric must be less than a wavelength. ![]() In order to minimize losses due to skin depth, you want to use the BIGGEST coax cable you can that won't support TE11 mode. For more info on connector species, check out our section on microwave connectors! To obtain good performance at higher frequencies, smaller diameter cables are required to stay below the cutoff frequency (thanks for the correction, Gary!) This is the reason that precision air-dielectric connector families have progressed from 3.5mm, to 2.9mm, to 2.4mm, to 1.85mm and now to 1mm as microwave applications have moved from X-band to W-band frequencies. To be sure that only one mode propagates, thus keeping the signal clean, you will need to stay below f c. Higher modes will be excited at small imperfections, bends, etc., but below cutoff they rapidly disappear along straight sections of coax. Higher-order mode that will screw up your loss and VSWR and as they have different propagation velocity than the TEM mode and will interfere with it. What is meant by the cutoff frequency f c? The desirable TEM mode is allowed to propagate at all frequencies, but at frequencies above f c the first higher-order mode called TE11 is also allowed to propagate. This page was split off from our coax page. Click here to go to our main page on coax
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