What are Microwaves & mmWaves

Definitions, applications, properties for microwaves & millimetre or mmWaves being used for high speed low latency data communications, 5G, radar, & many other radio based applications.

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Microwaves and millimetre waves, mmWaves are both forms of radio frequency signal - the only difference is the frequency ranges they cover.

While the terms tend to be used rather loosely in some circumstances, there are definitions for both microwaves and millimetre waves, mmwaves which explain where these bands are located within the overall radio spectrum.

With the increased pressure on spectrum lower in frequency, and the need for faster wider bandwidth data communications, both the microwave and mmwave bands will see considerably increased usage in the coming years.

Often defining microwaves vs millimetre wave of mmWave it is necessary to distinguish between them, although many of the properties of both bands are very similar.

What are microwaves

Although the term microwave has been over-used in terms of microwave ovens, when used with RF design, radio communications, radar and the like, the term describes signals in a certain portion of the radio spectrum.

Video: What are Microwaves & mmWaves

Although not a defined aspect of microwaves, many people consider microwaves to be a region of the radio spectrum where the capacitance, inductance and resistance of all components need to be incorporated into the radio frequency design. Lumped components consisting of just resistance, capacitance, or inductance are not appropriate.

In view of this RF components and microwave components tend to be designed to reduce the levels of stray inductance and capacitance to ensure that the microwave circuit design is made as easy as possible.

Generally microwaves are considered to be the frequencies or bands between 300 MHz and 30 GHz, although often many people tend to consider microwave frequencies to start at 1 GHz, rather than 300 MHz.

With a frequency of 300 MHz at the bottom end of the UHF band, and microwave radio frequency design technology taking on different approach in view of the fact that lumped components are not appropriate, then a frequency of 1 GHz seems more applicable in many instances to be the border for where microwaves start.

Accordingly there are many electronic components designed specifically to address the microwave and millimetre wave design requirements.

Not only are specialised capacitors and resistors available, but so too are semiconductor devices. For these frequencies semiconductor devices tend to use compound semiconductor technology to enable the required performance to be achieved at these frequences. Technologes include Gallium Arsenide, gallium Nitride and the like are all used.

In addition to these there are components like waveguides, circulators, isolators and more are available as well.

Summary of Microwave Spectrum Properties
Band Type	Frequency range	Wavelength	Typical applications
Microwave	300 MHz - 30 GHz	1 - 0.1 metre	Terrestrial broadcasting, satellite direct broadcasting, cellular / mobile communications, Wireless LANs, satellite & terrestrial radio communications, radar, navigation.

One of the disadvantages of microwave usage is the cost of the microwave components. These can be much higher than those required for RF design at lower frequencies - tolerances are tighter, microwave components often need to be manufactured with advanced technologies, especially where semiconductors are concerned. This all adds cost and as a result, microwave components tend to be much more expensive.

Typical small horn antenna used for radiating and receiving microwave signals

The advantages of microwaves over the lower frequencies are that the available spectrum levels are much higher and this means that individual transmissions can utilise wider bandwidths. In turn this means that they can transmit data at much higher rates.

It should be remembered that the propagation of signals is rather different to those at lower frequencies.

The signals tend to be line of sight only, and objects in the path can cause significant levels of attenuation. This can be a disadvantages because signals do not travel as far and greater care needs to be taken in planing the signal paths available, although in some circumstances like with cellular or mobile communications, the short distances can be put to good advantage giving high levels of frequency re-use.

When combined with the higher data rates available, this can be put to good use in many instances. The downside is that more nodes or base stations are required to give coverage, although this will only be needed in areas of high usage.

This portion of the radio spectrum is being increasingly used for Wi-Fi and wireless LANs. With signals for the standard Wi-Fi variants using 2.4 GHz, 5 GHz and now the 6 GHz bands, wireless LAN and general wireless communications are increasingly using this portion of the spectrum.

Microwave signals can also be used for point to point links. Parabolic reflector antennas give high gain which provides higher signals to combat the increased path losses. Radio communications point to point links used to be a feature of general telecommunications networks, but now they are less widely used for telecommunications infrastructure because fibre optic links have a much greater capacity.

However microwave links are still very widely used for mobile base stations because they can easily link a number of base stations together and in places where no telecommunications fibre lines may be available.

What are mmWaves

Millimetre waves are the next band of frequencies up from microwaves. As the name suggests, their wavelength is measured in millimetres. In particular they are between 10 and 1 mm, and this corresponds to frequencies in the band from 30 GHz to 300 GHz.

In some cases millimetre waveband is considered to start at 26 GHz instead of 30 GHz because the 5G mmwave frequency bands are at 26 and 28 GHz. These 5G mmwave bands provide the mobile communications with short range very high bandwidth communications.

Summary of mmWave Spectrum Properties
Band Type	Frequency range	Wavelength	Typical applications
Millimetre wave(mmwave)	30 - 300 GHz	10 - 1 mm	Radar, mmWave imaging, medical scanning, investigations for short range links for wireless communications

When using mmwave signals for radio communications, radar or for any other purpose, the attenuation of the signal with the distance travelled is an important aspect. Not only does the attenuation generally rise with frequency, but there are also peaks where constituents of the atmosphere react with the signals and the level of absorption increases. Although this is also present in the microwave portion of the spectrum, it is more pronounced for mmwave signals.

Absorption of mm wave signals with frequency - notice the peaks in attenuation — Absorption of mm wave signals with frequency

The graph above is a very approximate plot of the signal attenuation of mm wave signals with increasing frequency. The plot will vary according to a variety of atmospheric parameters including temperature, humidity and the like, but it does give an overall impression of the levels of attenuation encountered above and beyond the normal 1 / d² signal attenuation experienced from any signal.

Again for these frequencies, electronic components intended for use on these frequencies are available. The electronic components tend to be ones intended for use at these frequencies so that the spurious levels of inductance, capacitance and the like to not mask the required performance. There is also a growing reliance ont he use of specialised monolithic microwave ICs and similar devices which can accommodate these very high frequencies.

At these frequencies, even short paths can introduce delays, capacitance, inductance and the like. Also semiconductor technologies use compound semiconductor approaches to achieve the performance at these frequencies.

Comparison of Microwave vs mmWave

In terms of their frequencies, both microwave and millimetre wave bands occupy very high frequency ranges and they can provide high levels of bandwidth to support high data rate wireless communications, wireless LANs and the like as well as providing narrow beamwidth radar systems.

However there are differences when comparing microwaves and millimetre wave signals.

Comparison of Microwaves vs mmWaves - differences and similarities
Parameter	Microwave Bands	Millimetre Wave Bands
Frequency range	30 MHz to 30 GHz	30 GHz to 300 GHz
Wavelengths	1m to 0.01m	10mm to 1 mm
Signal bandwidths available	High	Very high
Antenna sizes	Larger than those for mmWave bands, especially at the lower end of the microwave region. They are still smaller than those for lower frequencies.	Small, especially when compared to those for the microwave region - for example dipole elements are typically about half a wavelength long, so this means they could be between 5mm and 0.5mm.
Transmission distances	Dependent upon the frequency, but line of sight distances are easily achievable for high gain antennas high up without obstructions.	Distances achievable are limited because of atmospheric absorption, etc. mmWave is not normally used for long distance radio communications links.
Attenuation	Relatively resistant to rain, especially at the lower frequencies	High absorption is experienced if rain is in the signal path.

The comparison of microwaves vs mm waves is not always clear cut. The two bands adjoin each other and in any case the definitions of what constitute microwaves and mmwaves can vary. Accordingly the comparison should be taken as a broad indication of the properties and it should be remembered that the properties for the top of the microwave region will be very close to those at the bottom of the mmwave section.

In terms of their usage, both the microwave and mmwave bands are being sued increasingly for mobile communications. Although commercialisation of the 5G mmwave bands has not yet started, plans are in place for the development and future use of the mmwave bands for 5G and 6G.

Both microwaves and mmWaves are being used increasingly. As the technology for the microwave design and mmWave design advances, and costs fall, the benefits for using these regions of the spectrum increases. Also there is increasing pressure on the lower frequencies where spectrum is becoming more congested and where the bandwidths required for high data rate wireless communications is not available. With all these pressures, the usage of the microwave and millimetre wave bands will only increase.

One exciting area for the use of the mmwave is for mobile communications. Although 5G mmwave bands are not in use yet, this is one of the goals that will enable very high data transfers with low latency.