Issue link: https://resources.system-analysis.cadence.com/i/1325428
RF Electronics Chapter 7: RF Filters Page 240 2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0. Helical Filters For high Q value resonators at UHF frequencies, a cylindrical rod of one-quarter wavelength long is placed inside a cavity. This cavity can be cylindrical but in many cases, the cavity is rectangular for ease of construction. Such cylindrical structures are called coaxial filters, as the cross-section of the filter is a coaxial transmission line. At VHF frequencies, the size of the coaxial filter becomes too large for many applications. To reduce the length of the resonator, the centre quarter-wave resonator is wound as a helix. Often the helix is wound on a former or bobbin, as shown in figure 9.29 of Zverev [1]. One has to ensure that the material for the former is not lossy at the operating frequency. Polyethylene is suitable for low loss formers. The resonators have shields between them to confine the fields for the resonators. The resonators are coupled by having the appropriate shield height, so that some of the field of a resonator couples to the next resonator. The coupling is controlled by adjusting the difference between the height of the shield and the top of the helix as shown by dimension "h" in figure 7.53. Alternately, the coupling can be achieved by having a coupling hole in the centre of a full-height shield or by having a slot at the bottom of the shield. The choice of coupling aperture depends on the construction of the cavity. d H S h B Output Input Figure 7.53. Drawing of a helical filter showing the sizes used in the formulae. Formula for Helical Filters (from Zverev [1]) f is the centre frequency of the helical resonator in MHz S is the width of the rectangular resonator cavity. (units m) H is the height of the resonator cavity (units m) B is the height of the helix (units m) d is the diameter of the helix (units m) N is the number of turns of the helix Z 0 is the characteristic impedance of the helical transmission line in the cavity. is the skin depth. The wire diameter should be > 5 times the skin depth. Q 0 is the unloaded Q of the resonator cavity. If instead of a rectangular cavity, a cylindrical cavity is used, the diameter D of the cavity is D = 1.2 S. RF Electronics: Design and Simulation 240 www.cadence.com/go/awr