CHAPTER SIX
Highpass and Bandstop Filters
In this chapter, we will discuss some typical microstrip highpass and bandstop filters. These include quasilumped element and optimum distributed highpass filters, narrow- and wide-band bandstop filters, as well as filters for RF chokes. Design equations, tables, and examples are presented for easy reference.
6.1 HIGHPASS FILTERS
6.1.1 Quasilumped Highpass Filters
Highpass filters constructed from quasilumped elements may be desirable for many applications, provided that these elements can achieve good approximation of desired lumped elements over an entire operating frequency band. Care should be taken when designing this type of filter, because as the size of any quasilumped element becomes comparable with the wavelength of an operating frequency, it no longer behaves as a lumped element.
The simplest form of a highpass filter may just consist of a series capacitor, which is often applied for direct current or dc block. For more selective highpass filters, more elements are required. This type of highpass filter can be easily designed based on a lumped-element lowpass prototype, such as one shown in Figure 6.1a, where gi denotes the element values normalized by a terminating impedance Z0 and obtained at a lowpass cutoff frequency Ωc. Following the discussions in the Chapter 3, if we apply the frequency mapping
(6.1) 
Ω and ω are the angular ...
