The design of conventional stepped-impedance microstrip line low pass filter (LPF) is based on high (ZH) to low impedance (ZL) ratio. The width of ZH line, for ZH > 100 Ω, becomes critical and challenging, especially on high dielectric constant substrates. A concept of air-filled recessed ground plane below a microstrip line is introduced in this paper. The effect of dimensions of recessed ground on characteristic impedance, attenuation and propagation constant of a microstrip line are first studied. This simple approach is utilized to design the ZH line of stepped-impedance microstrip line LPFs. Prototypes of recessed ground stepped-impedance microstrip line LPFs with ZH/ZL (keeping ZL constant as 20 Ω)ratio in the range 6 to 10 are designed and developed on Rogers 4350B of height 0.508 mm with εr = 3.66 at 3 GHz. For LPF with ZH/ZL = 10, the measured 3-dB cutoff frequency (fc) is achieved at 3.12 GHz with return loss (RL) > 12 dB and insertion loss (IL) < 0.28 dB in its passband whereas the stopband attenuation (SBA) is better than 38 dB. In comparison to recessed ground LPF, the simulated results of conventional LPF with ZH/ZL = 10 (critical width of ZH line =) are as follows RL > 10 dB and IL < 1.07 dB in passband at fc = 3 GHz. The size of recessed ground LPF is reduced by 25%, when ZH/ZL is increased to 10 from 6. The approach of recessed ground microstrip line avoids the fabrication issues, reduces size, and improves the performance of LPF, which in turns confirms the advantages of recessed ground over conventional microstrip line.
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