Monday, February 1, 2010

UHF Versus VHF

There are two major formats for two-way radios. They are Ultra High Frequency (UHF) radio and Very High Frequency (VHF) radio. Neither frequency band is inherently better than the other. They each have their pluses and minuses. Both formats are effective ways to communicate with another person. But how do you decide which one will fit your needs? Let's go over the key components of both frequencies to help you decide.

UHF Radio

UHF equipment operates between the frequencies of 300 MHz and 3000 MHz. Until recently, it wasn't widely used. Now, the UHF radio frequency is used for GPS, Bluetooth, cordless phones, and WiFi.

There are more available channels with UHF so in more populated areas UHF may be less likely to have interference from other systems. If you are in an area where population is thin, VHF should work fine for you. If you are in an area where interference from other radios may be an issue, UHF transmitters and receivers could be your best choice. UHF is better at penetrating physical barriers like walls, buildings, and rugged landscape. Anything that obstructs a radio wave, will weaken a radio signal. UHF lessens that effect. Though it may not travel as far, UHF radio waves will penetrate obstacles better than VHF.

To highlight the differences in indoor range, below is an excerpt from a brochure of a leading two-way radio maker on the predicted range of one of their lines of handheld VHF and UHF two-way radios:

"Coverage estimates: At full power, line-of-sight, no obstructions the range is approximately 4+ miles. Indoor coverage at VHF is approximately 270,000 sq ft and 300,000 sq ft at UHF. Expect about 20 floors vertical coverage at VHF and up to 30 floors at UHF. Note: Range and coverage are estimates and are not guaranteed."

VHF waves are not very good at penetrating walls, buildings and rugged landscape. Therefore range will be significantly reduced for VHF radios in these environments. That may not necessarily be a problem if the range needed is only a few hundred feet. You can also add an external antenna to an indoor VHF base station that will reduce or eliminate this problem.

UHF equipment is usually more expensive. The components need to be finely tuned and are more expensive to construct. This does not mean it's necessarily better, just more expensive.

One advantage of UHF transmission is the physically short wave that is produced by the high frequency. That means the antenna on the radio can be shorter than an equivalent VHF radio.

VHF Radio

VHF equipment operates between the frequencies of 30 MHz and 300 MHz. FM radio, two-way radios, and television broadcasts operate in this range.

Both UHF and VHF radios are prone to line of sight factors, but VHF a little more so. The waves make it through trees and rugged landscapes, but not as well as UHF frequencies do. However, if a VHF wave and a UHF wave were transmitted over an area without barriers, the VHF wave would travel almost twice as far. This makes VHF easier to broadcast over a long range. If you are working mostly outdoors, a VHF radio is probably the best choice, especially if you are using a base station radio indoors and you add the external antenna.

Since VHF has been around longer and isn't as complicated to make, equipment is usually cheaper when compared to similar UHF equipment. One disadvantage to this equipment can be its size. Since the frequency waves are bigger, an antenna must be bigger.

VHF radios also have a smaller number of available frequencies. Interference with other radios could be more likely to be a problem. However, the FCC recently made this less of a problem when they opened up the MURS frequencies. The 150 MHz frequency is a Citizens Band radio spectrum that is called the MURS service. MURS stands for Multi-Use Radio Service. This service is for use in the United States and Canada. It is a low power, short range service in the VHF 150 MHz Citizens Band radio spectrum. There are 5 channels in the MURS frequencies with 38 privacy codes under each one that enable you to only pick up conversations on your code. The FCC does not require users of products for MURS to be licensed.

With MURS you can add a larger or external antenna to improve range. If you want to put an antenna on top of your house or business, you can do it with MURS. Some antenna manufacturers claim an external antenna can increase the effective radiated power of a transmitter by a factor of 4. These MURS intercoms can transmit up to four miles, and perhaps more with an external antenna depending on the terrain.

One benefit of VHF wireless radios is that battery life is almost always better than for similar UHF units. For handheld radios this is a plus.

VHF equipment is usually lower cost for those on a budget. Equipment can be more economical than similar UHF products.

In summary, if you are planning on using your two-way radios mainly inside buildings, then UHF is likely the best solution for you. If you are mainly using your two-way radios for communication outside, then VHF would be a good choice. Either radio technology can work for you if you don't really have a long range to cover. In that case you may want to choose VHF for it's lower cost.

Behaviour of radio waves

There are a few simple rules of thumb that can prove extremely useful when making first plans for a wireless network:

The longer the wavelength, the further it goes
The longer the wavelength, the better it travels through and around things
The shorter the wavelength, the more data it can transport
All of these rules, simplified as they may be, are rather easy to understand by example.

Longer waves travel further
Assuming equal power levels, waves with longer wavelengths tend to travel further than waves with shorter wavelengths. This effect is often seen in FM radio, when comparing the range of an FM transmitter at 88MHz to the range at 108MHz. Lower frequency transmitters tend to reach much greater distances than high frequency transmitters at the same power.

Longer waves pass around obstacles
A wave on water which is 5 meters long will not be stopped by a 5 mm piece of wood sticking out of the water. If instead the piece of wood were 50 meters big (e.g. a ship), it would be well in the way of the wave. The distance a wave can travel depends on the relationship between the wavelength of the wave and the size of obstacles in its path of propagation.

It is harder to visualize waves moving “through” solid objects, but this is the case with electromagnetic waves. Longer wavelength (and therefore lower frequency) waves tend to penetrate objects better than shorter wavelength (and therefore higher frequency) waves. For example, FM radio (88 to 108MHz) can travel through buildings and other obstacles easily, while shorter waves (such as GSM phones operating at 900MHz or 1800MHz) have a harder time penetrating buildings. This effect is partly due to the difference in power levels used for FM radio and GSM, but is also partly due to the shorter wavelength of GSM signals.

Shorter waves can carry more data
The faster the wave swings or beats, the more information it can carry every beat or cycle could for example be used to transport a digital bit, a '0' or a '1', a 'yes' or a 'no'.

There is another principle that can be applied to all kinds of waves, and which is extremely useful for understanding radio wave propagation. This principle is known as the Huygens Principle, named after Christiaan Huy-gens, Dutch mathematician, physicist and astronomer 1629 - 1695.

Imagine you are taking a little stick and dipping it vertically into a still lake's surface, causing the water to swing and dance. Waves will leave the center of the stick -the place where you dip in -in circles. Now, wherever water particles are swinging and dancing, they will cause their neighbour particles to do the same: from every point of disturbance, a new circular wave will start. This is, in simple form, the Huygens principle. In the words of

“The Huygens' principle is a method of analysis applied to problems of wave propagation in the far field limit. It recognizes that each point of an advancing wave front is in fact the center of a fresh disturbance and the source of a new train of waves; and that the advancing wave as a whole may be regarded as the sum of all the secondary waves arising from points in the medium already traversed. This view of wave propagation helps better understand a variety of wave phenomena, such as diffraction.”

This principle holds true for radio waves as well as waves on water, for sound as well as light -only for light the wavelength is far too short for human beings to actually see the effects directly.

This principle will help us to understand diffraction as well as Fresnel zones, the need for line of sight as well as the fact that sometimes we seem to be able to go around corners, with no line of sight.

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