TWO WAY RADIO COURSE #3- Frequency and Spectrum


The electromagnetic spectrum is the total range of frequencies of electromagnetic radiation. It extends from the lowest audio waves (15 Hz) to the highest light waves (900,000 GHz). To make it more manageable, the electromagnetic spectrum is divided into bands.


The part of the spectrum we are concerned with are the frequency bands used for radio communications. These are:

  • VHF Lowband (25 MHz - 50 MHz)
  • VHF Highband (136 MHz - 174 MHz)
  • UHF Band (403 MHz - 512 MHz)
  • 800/900 MHz Bands

The characteristic behavior of frequencies in different bands in the spectrum is important when choosing a frequency for a radio system.


The frequency band characteristics most important to radio communications include:

  • Propagation
  • Noise Interference
  • Range


The atmosphere that surrounds the earth acts to attenuate and refract radio signals just as it does light. Just how much it is affected depends on the frequency. As a general rule, the lower the frequency, the less the attenuation, or loss of signal.

Lower frequency radio waves travel better through fog or dust than higher frequency waves. Low frequency AM broadcast radio signals will travel far beyond the horizon and can be reflected back to earth for reception at great distances.

Higher frequency television or FM commercial broadcast stations are absorbed by the earth's atmosphere and are therefore limited to line of sight transmission.

Below 300 KiloHertz, the characteristics are just the opposite. Here, radio waves follow the curvature of the earth for great distances. This type of propagation is called a ground wave. Radio communications over distances up to several thousand miles are possible by making use of low frequency ground waves.

Above 300 KHz to about 30 MHz, the ionosphere will sometimes reflect and/or refract the radio signals. when returned to earth, they are received hundreds or even thousands of miles away. This is called skip and these radio signals are called sky waves.

The frequencies from 30 MHz to about 900 MHtz are the most suitable for two-way radio transmission. Generally, this range of radio frequencies is characterized by line-of-sight propagation.


Electromagnetic noise interference comes from machines and engines. VHF lowband signals are very susceptible to noise interference because noise occurs in the lower frequency ranges. VHF high band has minimal susceptibility to noise interference. UHF and 800/900 MHz signals have virtually no susceptibility to noise interference. From this we can see that low band is not a good choice in high noise areas.


In rural areas, VHF lowband signals have the best range because they tend to follow the curvature of the earth. VHF highband signals have good range characteristics. The UHF and 800/900 MHz bands have only fair range characteristics because signals in this range can be attenuated by foliage and other terrain found in rural areas. From this we can see that the VHF bands are the best choices for use in rural areas.

In suburban areas, VHF lowband signals have the good range characteristics. VHF highband signals have excellent range characteristics. The UHF band has good range characteristics, and the 800/900 MHz bands have only fair range characteristics.

In urban areas, VHF lowband signals have poor range because they cannot penetrate buildings. Also, urban areas tend to have high levels of noise interference which affects lowband signals. VHF highband signals have good range characteristics and the UHF band and 800/900 MHz bands have excellent range characteristics because their signals can bounce off of, and penetrate buildings. From this you can see that lowband is not a good choice in urban areas.


There are many compromises in selecting a frequency band. To aid you there are two general rules that apply:

As the frequency increases, range decreases but so does the ambient electrical noise.

Reflections from buildings increase with frequency.

The result is that lowband VHF is generally more suited for large rural areas. Highband VHF is generally more suited for suburban and mixed urban and rural areas. The UHF and 800/900 MHz bands are general most suited for urban usage.


Because frequency spectrum is an infinite resource, and the number of users in many areas is high, many radio channels are becoming crowded. Channel loading is a term used to describe the number of users assigned to the same frequency. Channel loading is so heavy in some areas that additional users are no longer allowed on particular channels, or frequencies. The use of channels is authorized and licensed by government agencies in most countries.

In the United States, the FCC, or Federal Communications Commission is in charge of regulating communications by radio (both commercial broadcast and two-way), television, wire, satellite, and cable.

International regulations fall under the jurisdiction of the ITU, or International Telecommunications Union.

In all cases, a license to operate radio equipment is required and must be applied for with the appropriate governing body. The license is granted to operate on a particular frequency, or set of frequencies, with specific eligibility rules that must be met.

Spectrum management and conservation is a global issue. Not only are government agencies using regulations to help conserve and manage our spectrum, but they are also encouraging the adoption of new technologies—technologies that have the potential to make our use of the spectrum more efficient.

Stay Informed

Radio Wizard Free Online Radio Courses Motorola Battery Code Checker
Browse Our Catalog
Motorola Catalog Look at our catalog online or download the PDF file (5,768 kb)
Top 20 Site Motorola Dealer
ADVANCED RADIO SYSTEMS is an Authorized Motorola Dealer and only sells genuine Motorola 2-Way Radios for use in the United States. All our 2-Way Radios are FCC approved and licensed for sale within the United States. MOTOROLA and the Stylized M Logo are registered in the US Patent & Trademark Office. All other product or service names are the property of their respective owners. Motorola, Inc. 2015.