Survival Arts

Tactical Radio Communications, installment 2

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Frequency

Radio waves behave differently at different frequencies. The radio spectrum is broken into bands, usually by segments that have similar behavior, and sometimes by type of service (such as Short Wave, AM, FM and TV). The lowest range commonly used is the AM Broadcast band (0.5-1.3MHz). Next is HF (1 to 30 MHz), this band includes short wave radio. VHF-Low (30-50MHz), FM (88-108MHz), VHF (109-172), UHF (400-512MHz), Cellular and Paging (800-950MHz). Frequencies above 1000MHz (1GHz) are called microwave as the behavior of radio waves above this region changes significantly.

You will notice there are several large gaps between the bands listed. The military or TV stations use frequencies in these gaps. Some general characteristics that change with frequency are as follows. Distance traveled: the lower the frequency, the farther a radio wave will travel. Penetration of structures (due to bouncing and scattering) gets better as the frequency goes up. Antenna size becomes smaller as frequency increases. Another characteristic affected by frequency is node size. Nodes are areas where the signal dramatically changes strength over a very short distance. This is caused by two different reflections of the same signal meeting at the same place and canceling each other. You can see this by moving a portable FM radio tuned to a weak signal. You will notice that the signal comes and goes every few feet. What you are seeing are nodes.

Below is a list that describes some basic characteristics of the bands most used for two-way communication.

AM Broadcast and HF Bands

Range can be from hundreds of miles to worldwide. Radios are large and antennas can be over 100’ in length. Portable antennas are usually not practical although there are some exceptions. From 25 MHz and up portable equipment is available, but bulky. Reception in different parts of the band change dramatically with the time of the day. Signals can be reflected from the atmosphere, thus allowing the signal to bounce over very long distances. Penetration of structures is very poor, external antennas are usually required.

VHF-Low

Range can be from 10 to over 100 miles. Antennas are large, but can be small enough for mobiles (3’-5’ long) and large portables (1’ – 1.5’ long). Radios are large, but some portable equipment is available, most commonly CB, amateur and military radios. Nighttime conditions can cause the wave to randomly skip over large areas covering hundreds of miles. Penetration of structures is often poor. Nodes are many feet apart.

VHF

Perhaps one of the most useful bands. Range is between ½ to 20 miles depending upon terrain and power used. Reasonable penetration of structures. Nodes are only a few feet apart and weak. A wide variety of antennas and radios are available, including mobile and small portable equipment. Antenna size can be as small as 4” for a portable.

200MHz

This band is used mostly by the military and specialized commercial services. A small segment is allocated to ham radio, thus making some equipment available. Has the range of VHF with almost the penetration of UHF. Not covered by many scanners. Antenna and radio sizes can be small.

UHF

Another very common band for two-way communication. Range is 1/8 to 5 miles depending up terrain and power used. Antenna size can be as short as 1.5 inches for portable radios. Good selection of equipment is available in the Ham and commercial market. Excellent penetration of structures and built-up areas. Nodes are only a few inches apart and tend to be strong, rapidly changing from barely audible to full strength when moving and listening to weak signals.

800-950MHz

Used mainly for telephone-like services, public safety trunked systems and paging. There is also the 900 MHz ISM band for non-licensed radios such as cordless phones. Range is very short, less than 1 mile unless special antennas mounted in high locations and high power is used. Limited equipment is available, mostly used cellular or data transceivers, some commercial gear as well as spread spectrum devices. Antennas can be very small, sometimes built into the device. Nodes are 1” or less apart.

1GHz and up

Newer cellular phones, wireless data, satellite and numerous other services. Not suitable for most uses due to the very short ranges. There are portable radios available for amateur radio use on 1.2 GHz.

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Posted by Eric Cartman at November 21, 2002 12:31 AM | TrackBack