Described various effects of RF housing and methods to minimize it.

1. PRESENTATION ON
HOUSING EFFECT
ON
RF/MW SUB-SYSTEMS
BY,
RAMLAL BHARATH. J
R&D (RF&MW)- AEROSPACE SYSTEMS

2.  The package design for RF/MW Sub-systems require adequate
knowledge of EM field distribution inside the housing.
 RF/MW Sub-systems consists of MMICs, Active/Passive Devices
having different package styles.
 Housing design is very important for all Sub-systems as it
provides Environmental protection, Structural support and also
suitable path for heat transfer.
 Cavity electrical size increases at higher frequencies as a result no
of undesirable parasitic effects degrades the performance of the
circuit.

3.  The conductive surface of the enclosure supports several resonant
modes and produces oscillating charges, which in turn produces
surface currents on conducting walls of the package.
 In an assembled package, the effect of various parts like Feed
through, Packaged devices, Mounting screws are taken in account.
 The cavity resonance of the Sub-system degrades the electrical
performance and also causes unwanted oscillations which results in
failure of the Sub-system.
 Enclosure for the RF/MW Sub-systems are designed using Aluminium.

4.  EM design and analysis for the circuit is carried out using 2D EDA
software ADS , but EM analysis for enclosure is not carried out in it.
 3D EDA softwares like CST, HFSS, EMPro based on FEM (Finite Element
Method) is used for Eigen/Driven Mode Study.
 Eigen Mode solver calculates Resonance Frequencies and Fields at the
resonant frequencies.
 Driven Mode solver calculates S-Parameters.
 The following differential equation , known as Helmholtz wave
equation is solved by FEM.
ψ = Electric or Magnetic field.

5.  FEM breaks 3D structure into large number of tetrahedral mesh cells.
 3D EDA softwares computes wave patterns (TE, TM modes) on each
ports of the structure supported by transmission lines.
 TM wave in a cavity resonator is any wave which has no magnetic field
in Z-Direction of cavity. i.e. Ez ≠ 0 ,Hz = 0.
In TM mnp Modes , m,n = 1,2,3…..
p = 0,1,2,3……
 TE wave in a cavity resonator is any wave which has no magnetic field
in Z-Direction of cavity. i.e. Ez = 0, Hz ≠ 0.
In TE mnp Modes , m,n = 0,1,2,3…..
p = 1,2,3……

6.  The field pattern of the travelling wave is calculated by using
Maxwell’s equation.
 The theoretical resonance frequency for rectangular cavity is
calculated by
c= Velocity of light
μr = Relative Permeability
εr = Relative Permittivity
a = Width
b = Height
d = Length

7.  To minimize the effects of cavity resonances , different mitigation
techniques are adopted : 1) Pre-design Techniques
2) Post-design Techniques
 Pre-design techniques are related to Package geometry, Package
material selection and Surface Treatment.
 Post-design techniques are implemented when packages are already
fabricated. They are : 1) Optimization of Package geometry
2) Analysis of Package Material Selection
3) Selection of Absorber