How do I measure the output power of a transmitter accurately using a power meter and sensor?
SIW Technology
SIW bridges the gap between microstrip (easy to fabricate, high radiation loss at mmWave) and rectangular waveguide (low loss, excellent shielding, but bulky and cannot be integrated on a PCB). The SIW structure supports the TE10 mode (same as rectangular waveguide), providing a completely shielded transmission medium on a standard PCB.
SIW transitions connect the SIW to microstrip or coplanar waveguide for interfacing with active components. A tapered microstrip-to-SIW transition provides wideband (30%+ bandwidth) impedance matching between the quasi-TEM microstrip mode and the TE10 SIW mode. The taper length is typically λ/4 to λ/2 at the center frequency.
SIW components have been demonstrated across the full mmWave range: bandpass filters with 1-5% bandwidth and 1-3 dB insertion loss, directional couplers with 20+ dB directivity, slot antenna arrays with 15-25 dBi gain, and power divider networks. The all-printed fabrication makes SIW highly attractive for mass-produced mmWave products such as automotive radar modules and 5G antenna arrays.
Frequently Asked Questions
When should I use SIW instead of microstrip?
SIW is preferred above 20-30 GHz where microstrip radiation loss becomes significant, for filters requiring high Q (SIW Q is 200-500 vs. 50-150 for microstrip at 30 GHz), for antenna feed networks requiring low cross-talk between channels, and for any application requiring waveguide-like shielding on a PCB. Below 20 GHz: microstrip is usually simpler and more compact.
What substrates work for SIW?
Any RF PCB substrate with controlled dielectric constant and low loss tangent. Rogers RO4003C, RT/Duroid 5880, RO3003, and similar materials are standard choices. The substrate thickness determines the SIW height and therefore the bandwidth: thicker substrates provide wider bandwidth but larger via length. Typical substrate thickness for SIW: 10-30 mils.
Can I make an antenna from SIW?
Yes. SIW slot arrays are a popular mmWave antenna type: slots cut into the top copper of the SIW radiate, and the SIW serves as both the feed network and the radiating structure. SIW slot arrays achieve 15-25 dBi gain with efficiencies of 50-80% at 28-77 GHz. The all-PCB construction makes them ideal for mass production.