Cavity Resonance

Microwave Circuit Enclosure 

Note how energy is distributed throughout the cavity when empty, but when absorber is inserted, the energy resides in the absorber allowing the circuit to operte properly. 

• Cavity absorbers
• Circuits operating at microwave frequencies tend to radiate, causing standing waves in an enclosure.
• Radiation patterns inside the cavity are difficult to characterize but resonant conditions with an unpredictable effect are common with microwave circuits and can impede circuit performance.
• The phenomenon occurs in cavities with a high quality factor or "Q".
• Absorbers reduce the amount of stored energy and hence the "Q" and improve circuit performance.
 

Circuit Board Housing

Many circuit boards in microwave components use some sort of housing to protect them. Often this housing is made of or coated with metal to provide some electromagnetic isolation or physical protection. However this can cause problems of its own.  An active microwave circuit element housed in a conductive enclosure will often emit spurious signals that will create a resonant VSWR inside the enclosure. This VSWR might adversely affect the circuit operation and is difficult to predict beforehand.

Inserting a type of microwave absorber will often eliminate the VSWR through a variety of mechanisms.  Usually, a magnetically loaded absorber such as Eccosorb MCS or Eccosorb BSR is recommended due to the fact that the E field goes to zero at the cavity walls while the H field is at a maximum giving a material with high magnetic loss the best absorption characteristics. In addition, the low conductivity of magnetically loaded elastomers will prevent shorting out of components if they come in contact with the absorber. Their relatively thin thickness also makes them optimal for eliminating cavity resonances. If thickness and conductivity are not an issue (and price is) a dielectrically lossy foam such as Eccosorb LS can be used. Often, inserting a high permeability or permittivity material in the cavity will modify the characteristics of the VSWR pattern such that the EM energy being emitted by the device no longer excites the cavity.

Choosing the best absorber for your application can be tricky. Virtually all absorbers will absorb some of the energy and many different types will absorb enough to make your problem go away.  The trick is finding the most cost effective solution. Even a simply shaped cavity with a known mode of excitation can become an exceedingly complex electromagnetic problem. Emerson & Cuming MWP provides a free Problem Solver Kit which includes several types of absorber material complete with a peel and stick Pressure Sensitive Adhesive. This kit enables engineers to try several candidate materials in their circuits to choose the best.

Adhering the absorber to the walls of the cavity is the next issue. Once the circuit board covers are on, you do not want to have to take them off to replace or re-adhere absorber. ECMP offers two solutions. All our absorbers are available with a peel and stick Pressure Sensitive Adhesive (PSA)  with outstanding adhesion properties. For a more permanent solution, ECMP offers its Mold-in-Place (MIP) process which uses a liquid dispensing system and an epoxy based absorber for a true permanent solution.