11.8.6 Why Do Different Radio Frequencies Behave So Differently?
- Why Does Frequency Matter?
- What Is the Relationship Between Frequency and Wavelength?
- Why Do Low Frequencies Travel Further?
- Why Do Higher Frequencies Require Line of Sight?
- Why Do Higher Frequencies Provide More Bandwidth?
- Why Do Antennas Become Smaller?
- Why Are Microwave Signals More Affected by Rain?
- Why Doesn't Everyone Use Low Frequencies?
- Why Doesn't Everyone Use High Frequencies?
- How Do Engineers Choose the Best Frequency?
- How Does Frequency Influence Modern Communication Systems?
- Why Is Understanding Frequency So Important?
- What Should You Remember?
Short Answer
Different radio frequencies behave differently because their wavelengths determine how they interact with the environment. Long wavelengths readily bend around obstacles, follow the Earth's surface, and penetrate many materials, but they require large antennas and provide relatively limited bandwidth. Short wavelengths support much higher data rates and smaller antennas, but they travel primarily in straight lines and are more readily absorbed by rain, atmospheric gases, and other obstacles. Selecting the appropriate frequency is therefore one of the most important decisions in the design of any communication system.
Why Does Frequency Matter?
At first glance, all radio waves appear to be the same.
They are all electromagnetic waves. They all travel at approximately the speed of light. They all obey Maxwell's equations. Yet a submarine communication system may operate at only a few kilohertz, an AM broadcast station at around 1 MHz, a television station at hundreds of megahertz, a mobile phone at several gigahertz, and an optical communication system at frequencies hundreds of thousands of times higher still.
The reason is that changing the frequency changes the wavelength, and wavelength has a profound influence on how electromagnetic waves interact with the environment.
What Is the Relationship Between Frequency and Wavelength?
Frequency and wavelength are inversely related.
As frequency increases, wavelength decreases. Low-frequency signals therefore possess long wavelengths, while high-frequency signals have much shorter wavelengths. For example:
- an LF signal may have a wavelength of several kilometres;
- an AM broadcast signal may have a wavelength of several hundred metres;
- an FM broadcast signal has a wavelength of about three metres;
- a mobile-phone signal has a wavelength of only a few tens of centimetres;
- a 30 GHz microwave signal has a wavelength of approximately one centimetre.
This enormous variation in wavelength explains why different parts of the radio spectrum behave so differently.
Why Do Low Frequencies Travel Further?
Long wavelengths interact relatively weakly with obstacles.
Instead of being blocked completely, they tend to bend around hills, buildings, and other obstructions through diffraction. At very low frequencies, radio waves may also follow the Earth's surface as surface waves, allowing communication well beyond the visual horizon. These characteristics make lower frequencies particularly valuable for:
- maritime communication;
- navigation systems;
- long-distance broadcasting;
- military communication; and
- submarine communication.
The price paid for these excellent propagation characteristics is relatively limited bandwidth and the requirement for physically large antennas.
Why Do Higher Frequencies Require Line of Sight?
As wavelength becomes shorter, diffraction becomes progressively weaker.
High-frequency signals therefore behave increasingly like beams of light. They propagate mainly along direct paths and are much more easily blocked by hills, buildings, and vegetation. Most VHF, UHF, and microwave systems consequently require an essentially unobstructed propagation path between the transmitting and receiving antennas.
This characteristic explains why communication towers are often located on hilltops or tall buildings. Greater antenna height increases the radio horizon and improves the probability of maintaining a clear propagation path.
Why Do Higher Frequencies Provide More Bandwidth?
One of the greatest advantages of high-frequency communication is the availability of much wider frequency allocations.
Information is transmitted by occupying part of the available spectrum. Higher-frequency bands generally provide much larger contiguous bandwidths, allowing far greater data rates. This is why technologies such as:
- Wi-Fi;
- 5G;
- satellite broadband;
- microwave links; and
- millimetre-wave communication
operate at frequencies far above those used by traditional voice radio systems.
Modern broadband communication depends upon these wider allocations to support video streaming, cloud computing, and high-speed Internet access.
Why Do Antennas Become Smaller?
Antenna dimensions are closely related to wavelength.
Efficient antennas are commonly a quarter wavelength or half wavelength long. Consequently:
- low-frequency systems require very large antennas;
- high-frequency systems require much smaller antennas.
For example, an efficient antenna at 100 kHz would be hundreds of metres long.
At 1 GHz, however, a quarter-wave antenna is only about 7.5 centimetres long.
The development of portable radios, mobile phones, Wi-Fi equipment, and satellite receivers has therefore been made possible largely because of the much shorter wavelengths available at higher frequencies.
Why Are Microwave Signals More Affected by Rain?
Another consequence of increasing frequency is greater interaction with atmospheric particles.
Raindrops, cloud droplets, and atmospheric gases become increasingly effective at scattering and absorbing electromagnetic energy as wavelength decreases. This effect is relatively insignificant below a few gigahertz. Above approximately 10 GHz, however, rain attenuation becomes increasingly important. Satellite systems operating in the Ku- and Ka-bands therefore experience noticeable reductions in signal strength during heavy rainfall.
Engineers compensate by incorporating additional fade margin, adaptive coding and modulation, uplink power control, and other techniques.
Why Doesn't Everyone Use Low Frequencies?
Given their excellent propagation characteristics, it might seem logical to use low frequencies for every communication system.
Unfortunately, several important limitations exist. Low-frequency bands offer relatively little spectrum. The antennas become extremely large. Available bandwidth is limited. The amount of information that can be transmitted is therefore comparatively small.
These limitations make low frequencies unsuitable for applications requiring very high data rates.
Consequently, modern communication systems increasingly employ higher frequencies whenever large bandwidths are needed.
Why Doesn't Everyone Use High Frequencies?
Higher frequencies also involve significant trade-offs.
Although they provide enormous bandwidth, they suffer from:
- greater free-space path loss when antenna gains are fixed;
- weaker diffraction;
- increased atmospheric attenuation;
- rain fade;
- more stringent antenna pointing requirements;
- shorter communication range for a given transmitter power; and
- greater susceptibility to obstruction.
Engineers therefore select the lowest frequency capable of meeting the communication objectives while providing adequate bandwidth.
This principle has guided radio-system design for more than a century.
How Do Engineers Choose the Best Frequency?
Selecting the operating frequency involves balancing many competing factors.
These include:
- required communication range;
- terrain;
- required bandwidth;
- antenna size;
- transmitter power;
- atmospheric conditions;
- spectrum availability;
- regulatory allocations;
- equipment cost; and
- application requirements.
For example, an international broadcaster may choose HF because worldwide coverage is required. A cellular network may select frequencies between several hundred megahertz and a few gigahertz to balance coverage with capacity. A satellite broadband provider may use the Ka-band because very high data rates justify the increased rain attenuation.
No single frequency band is ideal for every application.
How Does Frequency Influence Modern Communication Systems?
Virtually every modern communication technology reflects the trade-offs associated with frequency.
Examples include:
- LF and VLF for submarine communication;
- MF for AM broadcasting;
- HF for worldwide sky-wave communication;
- VHF for FM broadcasting and aviation;
- UHF for television, mobile communication, and land-mobile radio;
- SHF for microwave links, radar, Wi-Fi, and satellite communication; and
- millimetre-wave bands for emerging 5G, 6G, automotive radar, and ultra-high-capacity wireless systems.
Each frequency range occupies its own niche because its propagation characteristics best satisfy a particular set of engineering requirements.
Why Is Understanding Frequency So Important?
Frequency is one of the most fundamental design choices in communications engineering. It influences propagation, antenna design, transmitter power, receiver sensitivity, atmospheric attenuation, bandwidth, equipment cost, and achievable communication range.
Understanding how wavelength affects propagation allows engineers to select the most appropriate part of the radio spectrum for each application.
Rather than asking which frequency is "best," communication engineers ask which frequency provides the most appropriate balance of performance, practicality, and cost.
What Should You Remember?
- Radio frequencies behave differently because changing frequency changes wavelength.
- Long wavelengths diffract more readily, follow the Earth's surface more effectively, and generally provide greater communication range.
- Short wavelengths require more nearly line-of-sight propagation but support much greater bandwidth and smaller antennas.
- Antenna size is closely related to wavelength, making high-frequency equipment much more compact.
- Atmospheric absorption and rain attenuation become increasingly important at microwave and millimetre-wave frequencies.
- Engineers select operating frequencies by balancing propagation, bandwidth, antenna size, atmospheric effects, regulatory constraints, and system cost.
- Different regions of the radio spectrum have evolved to support different communication applications because each offers its own unique combination of advantages and limitations.