During the time when AM radio enjoyed high popularity people often listened to stations that were located hundreds of miles away. Frequencies under 2 Megahertz (MHz) gain the ability to traverse the Earth's curvature because they reflect off the atmospheric layers. AM radio signals reach radios hundreds of miles beyond the horizon when they operate in quiet environments.
The frequency range for most purchasable two-way radios and intercoms spans from 150MHz to 900MHz. These radio frequencies propagate in direct paths which restricts them to line-of-sight travel and prevents them from crossing the horizon or penetrating solid barriers.
However, there are exceptions to this rule. Even though radio signals travel along "line-of-sight" paths these frequencies can penetrate non-metallic materials enabling reception through walls and other barriers. The antennas of a transmitter and receiver may not always be visible yet the radio connection remains line-of-sight. Radio waves may bounce off surfaces which creates deviations from a straight line between radios.
The calculation of a two-way radio's maximum range requires factoring in Earth's curvature. Radio waves propagate in straight lines until they reach the horizon where they continue into space. The communication range for a two-way radio reaches its technical limit at the point where the horizon is visible. Antenna height remains an essential factor for this calculation.
To calculate the line-of-sight distance to the horizon for a given antenna height, we use the formula: The distance to the horizon in kilometers equals 3.569 multiplied by the square root of the antenna height measured in meters. Figure 1 illustrates this concept.
When a radio antenna stands 6 feet tall (1.8288 meters), the horizon reaches 4.83 kilometers (2.99 miles) from Point B which is depicted in the illustration. The calculation operates under the assumption that the receiving antenna stands at ground level while increasing its height would result in a longer line of sight.
The illustration marks Point C as the location of a second radio which uses a 6-foot antenna. This situation allows for theoretical communication across nearly 6 miles. Handheld two-way radios used by two people on flat ground without obstructions will allow communication up to 4 to 6 miles.
The advertised range of some radios being 25 miles or more may leave you wondering about their validity. They could reach this level of performance when specific conditions are met. The tower positioned on the mountain summit is represented by Point D in Figure 1. The additional height of the antenna when positioned on the tower surpasses a large portion of the Earth's curvature which leads to extended communication distances.
The communication range of two-way radios is influenced by several elements including atmospheric conditions and the specific frequency alongside physical barriers. The power output of the radio contributes significantly to its performance.
Two-Way Radio Power
The power output of a two-way radio which is measured in watts represents a vital factor that determines its communication range. FM radio stations often advertise their power output as being 50,000 or 100,000 watts. A handheld business-type two-way radio broadcasts at a low power level of 1-5 watts while vehicle mobile radios operate within a broader range of 5 to 100 watts. Increased wattage results in a longer transmission distance for the radio.Why is this the case? The laws of physics dictate that radio waves experience signal loss during transmission just as water loses pressure through pipes and electricity loses power across wires. The transmission range becomes longer when the power output at the source becomes greater because it compensates for signal loss.
Battery-powered handheld radios do not benefit from increased wattage because higher power output leads to quicker battery depletion. Higher wattage results in faster battery drain. Optimal radio performance requires finding a balance between power output and battery life.
Radio Frequencies
The operating frequency along with the surrounding environment determines how far a two-way radio can communicate.
Two major frequency bands are commonly used in two-way radios: The primary frequency bands utilized in two-way radios include Ultra High Frequency (UHF) and Very High Frequency (VHF). One frequency band does not surpass the other in effectiveness since both come with their own set of strengths and weaknesses. Your specific application determines which radio you should choose.
Two-way radios use radio waves that operate in various frequencies to carry their communications. Selecting a specific frequency on a radio receiver lets you detect the desired signal. Radio waves operate over cycles and "Hz" denotes Hertz which describes frequency as cycles per second.
The measurement of radio frequencies uses kilohertz (kHz) which stands for 1,000 cycles per second and megahertz (MHz) which stands for 1,000,000 cycles per second. The relationship is as follows: 1,000,000 Hertz = 1,000 kilohertz = 1 megahertz.
The term "wavelength" appears in radio wave discussions. The term originates from the early practice of measuring radio wave frequencies based on the span between two successive wave peaks rather than counting cycles per second. Lower frequencies produce longer wavelengths.
Transmission range for two-way radios depends on wavelength under specific conditions. The ability of a radio signal to travel longer distances is generally enhanced by lower frequencies because they correspond to longer wavelengths.
Since longer wavelengths which represent lower frequencies can penetrate deeper they serve as the communication medium for submarines. VLF radio waves with a frequency of about 330 kHz can reach through seawater down to 20 meters below the surface enabling submarines at shallow depths to communicate using these frequencies.
This information suggests VHF could be the best selection for two-way radio communication because its lower frequency allows signals to travel longer distances. However, this isn't necessarily true. VHF signals travel greater distances and penetrate better through obstacles but do not perform optimally when used inside buildings.
Imagine trying to send a signal across a metal commercial building from one side to the other with a metal wall and a three-foot doorway between the two sides. Radio waves are generally unable to travel through metal because metal blocks them.
Radio waves at UHF frequencies have a wavelength of approximately 1.5 feet while VHF radio waves have a wavelength of about five feet. The UHF signal, which features a narrower wavelength, passes through the door while the VHF signal exceeds the doorway width and gets reflected.
Your microwave oven provides an illustrative example. The door made of glass features a metal mesh pattern with tiny openings. The metal mesh door of a microwave oven traps microwaves which measure several inches in wavelength yet allows visibility because light waves possess microscopic wavelengths.
Think about the difference between moving through the building using a five-foot-wide pole compared to a foot-and-a-half-wide pole. The narrower pole will encounter fewer obstacles.
Wireless signals manage to pass through materials such as drywall, masonry, human bodies, furniture, and wall paneling but their strength diminishes as they pass through these substances. Denser objects reduce the signal more. Though VHF waves penetrate obstacles better than UHF waves, this characteristic alone does not make VHF frequencies optimal for indoor environments.
A three-foot-wide metal object in front of the transmitting radio makes VHF transmission more effective in this scenario. The VHF signal has the ability to bend around obstacles while the UHF signal encounters a complete blockage. VHF signals at lower frequencies have the capability to travel around large obstacles and penetrate materials such as brick and stone.
The majority of communication applications perform better with lower radio frequencies when needing to achieve longer transmission distances. A standard VHF TV station uses 100,000 watts of power to maintain a signal range of approximately 60 miles. To match the range of a VHF station a UHF station must transmit with 3,000,000 watts of power.
No definitive answer exists about which radio band performs better between VHF and UHF. Choosing radio technology requires understanding multiple variables since your particular requirements determine the best option. Below you will find comprehensive details about each technology to assist you in your decision-making process.
Populated areas benefit from UHF's increased channel availability which reduces interference from other systems. The reduced range of UHF when compared to VHF under most conditions proves advantageous as it helps to lessen interference from far-off radio sources.
The ability of VHF to penetrate physical barriers such as walls exceeds that of UHF but does not result in improved building coverage. UHF's shorter wavelength enables it to travel through building spaces more effectively. A UHF signal successfully moves through a small doorway whereas a VHF signal may encounter obstruction.
Comparison of Indoor Range
This excerpt from a leading two-way radio manufacturer's brochure shows the expected range performance of their handheld VHF and UHF radios."Coverage estimates: Signal coverage reaches about 4+ miles when operating at full power with a clear line-of-sight and no obstructions. The VHF indoor coverage extends to roughly 270,000 square feet while UHF reaches about 300,000 square feet. VHF signals will penetrate about 20 floors vertically while UHF signals can reach up to 30 floors. Note: The stated range and coverage values in this product are approximate and not assured.
The extended wavelength inherent to VHF signals inhibits their ability to penetrate through walls, buildings or uneven terrains which lowers their effective range in these scenarios. The problem becomes irrelevant when the necessary distance is only a few hundred feet. The range limitations of VHF signals can be partially resolved by installing an external antenna at the VHF base station.
Operating on UHF frequencies necessitates obtaining an FCC license yet many business band VHF frequencies also need licensing. VHF MURS radios operate without needing a license.
UHF's shorter wavelength enables the use of smaller antennas compared to identical VHF radios which enhances portability. VHF portable radio manufacturers frequently develop shorter antennas for their products.
VHF Radio
VHF technology serves FM radio and television broadcasts and two-way radios while the commercial radio band operates between 130 and 174 MHz.VHF radios face line-of-sight limitations similar to UHF radios yet experience slightly more restrictions. UHF frequencies deliver superior performance when compared to VHF waves in areas with dense trees and rocky terrain. In open terrain without physical obstructions VHF waves achieve nearly double the range of UHF waves which makes VHF the preferred choice for extended outdoor communication.
VHF radios usually work best for outdoor operations when combined with an indoor base station that uses an external antenna. Antennas positioned at greater heights increase the transmission range of signals. UHF radios could outperform VHF radios in transmitting signals through dense tree coverage.
MURS Radio
Two-way radio use now operates in the VHF 150 MHz band through FCC's authorization of the MURS spectrum. Users can select conversations from radios that match their privacy code through the use of 190 unique privacy codes available across 5 channels on the MURS system. No license is required for MURS products.MURS systems allow for extended range capability through the addition of external antennas which can be positioned on structures or towers. Manufacturers assert that using an external antenna increases the effective radiated power fourfold which allows MURS intercoms to send signals over several miles based on terrain conditions and antenna elevation reaching up to 60 feet above ground.
Summary
Short-range communication systems benefit from using both UHF and VHF technologies. Relaying UHF signals through repeaters extends coverage but requires a complex setup. Cities throughout various regions provide repeater services that require monthly payments. Most applications require only VHF or UHF radios without additional equipment.
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