the present invention is based on the recognition that microwaves can be used to determine the status of loading of traps or filters . the loading may be soot , particulates , ash or any solid / liquid . in addition to determining the total amount of loading , the microwave system to be described herein is useful in determining the distribution of the loading throughout the trap . the microwave sensing used in this invention can be inexpensive as inexpensive oscillators and detectors in the frequency range of interest are commercially available . in the case of a diesel particulate filter , the particulates are made from soot and other organic compounds ( solid and / or liquid ), and ash . for the purposes of this disclosure , the combination of carbon , organic compounds and ash will be referred to , for simplicity , as soot . those of skill in the art will recognize that soot and organic compounds are removed through regeneration but ash loading will remain . usually , diesel particulate filter units are made of cordierite material which has a dielectric constant at frequencies around s - band , slightly higher than 4 , with a weak temperature dependence . the effective dielectric constant of the cordierite filter , which is mainly void with a fraction of cordierite , is around 1 . 5 - 1 . 7 , and it is slightly anisotropic because of the orientation dependence of the trap . the presence of soot ( which can be as much as 10 g / liter of trap , with the size of the trap being about two liters for 5 . 66 inch traps ) changes the microwave characteristics of the microwave cavity , as the soot has a dielectric constant that is different from the gas ( air or exhaust ) that it displaces . thus , the maximum soot loading for this trap could be as high as 20 g with a volume of about 20 cm 3 . this amount of soot corresponds to a substantial volume and a correspondingly large change in the dielectric characteristics of the trap . it is noted that the dielectric constant of some types of soot is approximately 2 . silicon carbide is also suitable for the manufacture of a diesel particulate filter . the microwave properties of silicon carbide also make it suitable for the use of microwaves for loading sensing . those skilled in the art will recognize that the microwave load sensing technology disclosed herein can be used , for example , to determine the loading of fiber filters ( organic and inorganic fibers ), such as those used in bag - houses , and in other applications where substantial masses / volumes of materials that have non - unity dielectric constants are collected . ash content , which is not removed through regeneration , can be monitored if substantial ash amounts build with time on the trap . low order cavity modes as well as high order modes can be used to monitor the trap loading . different cavity modes have different electric field patterns with peaks and nulls that vary across the volume . since for a given cavity mode only the presence of soot in those regions with high electric field affects the microwave response in the cavity . by choosing different modes in the cavity it is possible to sample different regions and thus obtain information on the soot distribution . the theory on which the present invention is based will now be discussed briefly . the presence of soot affects the cavity response in several ways . the resonant frequency shifts to lower frequencies with soot buildup . in addition , the cavity quality q is affected by the presence of absorbing soot . further , the amplitude of the signal at resonance decreases with soot buildup . all three of these parameters can be used to determine the soot level . several modes can be used to monitor the loading in various regions of the diesel particulate filter . the invention will now be described in conjunction with the figures . with reference first to fig1 , a diesel particulate filter unit 10 includes a metallic cylinder ( referred as the can ) portion 12 and transition cones 14 and 16 . the cone 14 connects to an exhaust pipe 18 . in this embodiment , a pair of rod antennas 20 and 22 are located on opposite sides of a filter 24 . because of the conical transition sections 14 and 16 , the frequency of operation can be chosen so that the modes are operating below cutoff at the small inlet and outlet pipes of the trap , with the frequency such that the modes are operating below cutoff on main exhaust pipe 18 . it is not necessary to provide screens to confine the microwave radiation . in the embodiment of fig1 , one of the conventional rod antennas , 20 and 22 , serves as a transmitter and the other serves as a receiver . it should be understood that both of the rod antennas 20 and 22 could be located on the same side of the filter 24 rather than flanking it . in this case , the preferred location for the rod antennas 20 and 22 will be downstream from the filter element 24 to minimize soot on the surface of the transmitter , receiver or associated components . with reference now to fig2 , it is possible to implement the transmitter through the use of a loop antenna 26 , or through the use of a waveguide 28 . the waveguide 28 will likely be filled with a high dielectric material . it is also contemplated to use the loop antenna 26 and / or the waveguide 28 to monitor the radiation by acting as receiving antennas . it is possible to use a single antenna ( rod or loop ), as well as a single waveguide , or to use two antennas or waveguides . in the case of a single antenna / waveguide , the information is in the reflected signal . in the case of separate antennas / waveguides for transmitter / receiver it is possible to choose between reflection or transmission modes . in the case of two antennas / waveguides , there are four elements in the coupling matrix that could be used to determine soot loading : transmission from one antenna / waveguide to the other , the reverse , and reflection in each antenna / waveguide . as shown in fig2 , one suitable location of the transmitter and / or receiver is in the central region of the filter . this location illustrates a clear advantage of the microwave system disclosed herein as the waves penetrate through the external surface of the filter and a sensor can thus be protected from soot deposition by the external walls of the filter . this arrangement can be done for either single or double waveguides , loop antennas or rod antennas . in operation , microwave energy is established within the cavity of the device 10 . there are a large number of modes that can be used to determine the trap loading . fig3 shows the transmission element s 21 as a function of frequency and fig4 shows the reflections from a single launcher / receiver system . in fig3 rod antennas were on opposite sides of the trap as shown in fig1 . the graph in fig4 was created with a single antenna and the information is in the reflected signal . fig5 is an expanded view of the transmission mode from fig3 and shows detail around the mode near 1 . 7 ghz . one can readily see how the graph changes from no soot to soot . it is this difference that allows a determination of trap loading to be determined . the microwave sensing system disclosed herein can use inexpensive components with the microwave source being a modified microwave chip such as those used in cell phones , and the receiver can be a simple diode with or without amplification . the detection system can use advanced detection systems such as lock - in detection , heterodyne detection and others . although the loading has been assumed to be of soot ( as from a diesel engine ), any matter that builds in a substantial amount on the surface of a filter can be measured as long as it has a dielectric constant different from the background filter material ( one in the case of air / engine exhaust ). the system can be used to monitor the health of the trap . when substantial cracks are present in the system , soot distribution changes , and becomes inhomogeneous . in addition , it may be possible to use the temperature dependence of the cordierite to monitor temperature across the trap . although the description refers to the use of a single dpf in the can , the approach is also applicable to the case of multiple filters in a single can . the filtering monitoring system can be either original equipment , as well as be used as refrofits . it is recognized that modifications and variations of the invention will occur to those of ordinary skill in the art and it is intended that all such modifications and variations be included within the scope of the appended claims .