The term millimeter waves generally refers to the portion of electromagnetic spectrum between 30 and 300 GHz, corresponding to wavelength of 10 to 1 mm. Thus, the millimeter wave spectrum lies between the microwaves and infrared portions ( see Figure 1).
One should notice that there are different terms for the millimeter waves spectrum , but the above is the most common one.
Millimeter wave characteristics differ from those of microwaves and infrared as well, and because of these differences the millimeter waves are ideal candidates for certain applications which the infrared and microwaves could not materialize. Millimeter waves have been recognized as a part of the electromagnetic spectrum since Maxwell conceptualized the spectrum. However, lack of suitable sources, devices, components, instrumentation, and so on has made progress in the area very slow. The interest in the area has been in spurts, with the development of high accuracy radars in the 1940s, and long distance communications in the 1950’s and again in the late 1960s. The current interest in millimeter waves arises from the realization that there are limitations to what can be accomplished with optical systems, in particular their disadvantage in fog, dust, nighttime viewing and so on and the overcrowding of the microwave portion of the spectrum. Thus there are today specific important applications that can best be satisfied using millimeter waves.
Besides the specific reasons mentioned above, the growth in the field is also due to the fact that numbers of technical problems in the area have been solved or are near a solution. Generation, transmission, and detection have all seen considerable attention over the past few years and many of the significant problems, such as generation of high-power coherent radiations, suitable solid state sources, transmission media, and millimeter wave components have been developed and are commercially available. Notice that the FEL, which is supposed to be the source to our project, is a high-power coherent radiation source in the millimeter waves portion. There was also an improvement concerning millimeter waves sensor, and today due to the fact that inherent resolution of sensors operating at millimeter wavelengths is not significantly lower than those in the infrared/optical bands and other techniques such as injection locking, phase locking, and power combining of multiple diodes, the ultimate in sensor system performance can be achieved.
Advantages and disadvantages of millimeter waves:
The three principal characteristics of millimeter waves are short wavelengths, large bandwidth and interaction with atmospheric constituents,
translates into both advantages and disadvantages depending on the applications in mind.
Some of the main advantages are:
Smaller wavelengths which allow one to reduce component size, to achieve narrow beamwidths, to work in greater resolution and so on.
Wide bandwidths around main carrier frequencies ( 35, 94, 140 and 220 GHz), that can provide a high information rate capability, wide-band spread spectrum capability, high immunity to jamming and interference, and so on.
Environmental interaction characteristics such as: atmospheric attenuation and losses that are relatively low in the transmission windows compared to infrared and optical frequencies, lower attenuation in aerosols and condition of dust, smoke, and battlefield contaminants compared to infrared and optical frequencies.
Some of millimeter waves limitations and shortcoming are:
Smaller component size, increasing the need for greater precision in manufacturing, thus, leads to greater costs.
Narrow beamwidths, which can be a problem in target search and acquisition. Thus millimeter waves are not suitable for large volume search.
smaller antennas, which collect less energy in a receiving system, thereby reducing the sensitivity.
Limited communication range due to atmospheric attenuation (10-20 km)
Reduced range capability in adverse weather
Poor foliage penetration, particularly in dense green foliage.
Keep in mind that the above are only a general comparison of the advantages and disadvantages of millimeter waves systems over microwaves and optical system, and it is necessary to explore these fully for the application in mind.
“MILLIMETER WAVE ENGINEERING AND APPLICATIONS” Bhartia Bahl, 1984 by J.Wiley & Sons.