Microwave terminations are the precise loads required in microwave measurement and calibration systems. Most common among them are matched terminations and variable precession short-circuits. A matched load or a matched termination absorbs all the power incident on it without any reflection. In this respect it is equivalent to an absolutely black body in microwave range. On the other hand, a microwave line terminated in a matched load does not have any reflection and hence is equivalent to terminating the line with its characteristic impedance. The short circuit termination produces and adjustable reactive load at the desired point on a microwave line.
Matched waveguide terminations are constructed by mounting a power-absorbing card or pad in the space near the closed end of a waveguide section as shown in figure .A typical attenuator card consists of powdered iron or carbon mixed with a binder and deposited on a dielectric strip or porcelain containing silicon carbide.
The reflections arising from the end are minimized by tapering the card. The card is placed parallel to the dominant TE10 mode at a place where the electric field is maximum to have maximum attenuation. As the card has finite thickness, the reflections arising from it cannot be ruled out. Moreover, it is located at Emax which will give maximum reflected power. To avoid this the pad is kept closer to the side wall and its length is increased. Tunable matched tennination is shown in Figure .Matched loads may also be designed by loading the whole of the waveguide with lossy materials such as powdered metal, lossy dielectric,
wood, flowing water, sand, etc. as shown in Figure c, d and e.
Variable waveguide short-circuit termination:-
Waveguide short-circuit terminations provide standard reflection at desired, precisely measurable position. The basic idea behind it is to provide short-circuit by changing reactance of the termination.
The simplest form of an adjustable waveguide short-circuit is shown in above figure (a). It consists of a sliding block of a good conductor (such as copper) which makes as snug fit in the waveguide. The position of the block is varied by means of a micrometer drive. However, the electrical short circuit position deviates from the physical short circuit position in a random manner owing to the erratic contact between the sliding block and the waveguide wall. Furthermore, some power leakage past the block may occur thereby making the reflection coefficient less than unity. This too is highly undesirable. These problems are overcomed in choke-type shorting plungers shown in the above Figure b and c.