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.
Monday, February 1, 2010
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 wikipedia.org:
“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.
Wednesday, January 13, 2010
Standing Wave Ratio (SWR)
A standing wave ratio bridge is used to measure the standing wave ratio, or SWR. SWR is an indication of how well the radiating part of an antenna is matched to its feed-line or how well the tuner is matching the antenna system. Most amateurs pay far too much attention to SWR. An SWR reading below 2:1 is acceptable, because the mismatch is so small that the feed-line loss can be ignored. If you are using a modern transceiver, its power may fold back to a lower power output above this SWR level.
When you have mismatch between the feed-line and the antenna, part of the power feeding the antenna system reflects back toward the tuner and the transmitter. The part of the power going toward the radiating part of the antenna system is called forward power. The part reflected back down the feed-line is called reflected power. The larger the mismatch the larger the reflected power will be.
If the feed-line and antenna are not matched, waves traveling toward the radiating part of the antenna system meet the waves being reflected back down the feed-line. The waves interfere with each other, and at certain points along the feed-line, the amplitudes of both waves combine. This will result in a current maximum to be found at that point; and at that point, the current will appear to be standing still. The length of feed-line and the frequency will determine where this point occurs. At another point, the forward and reflected waves interfere, and they subtract from each other. At that point, there will be a current minimum. If you could visualize this phenomenon, you would see a series of current maximums and minimums standing still along the feed-line. This is why we refer to them as standing waves. At different points along the feed-line, where you have high current, you will have low voltage, and where you have low current, you will have high voltage. At any point along the feed-line, multiplying the voltage times the current will equal the power in Watts. When the feed-line is matched to the antenna, current and voltage remain the same all along the feed-line because there is no reflected current to interfere with the forward current.
As happens with the current, the voltage will also appear to be standing still. The voltage maximums and voltage minimums will not be at the same locations as the current maximums and minimums. SWR is the ratio of the maximum voltage to the minimum voltage on the line. It is called "Voltage Standing Wave Ratio" or VSWR, but we shorten it to just SWR. There is also a current SWR or ISWR, and it is the same value as the VSWR. For example, if the standing wave voltage maximum is 200 volts and the minimum voltage is 100 volts, the VSWR will be 2:1. If the voltage maximum and voltage minimum are equal, the SWR will be 1:1. If the voltage minimum is zero, the SWR is infinite.
In measuring SWR at the transmitter, you need to realize that feed-line losses affect the SWR readings. If the feed-line losses are high, much of the power reflecting back from the antenna will be lost, and the SWR reading on the meter will indicate it is lower than it actually is. If a feed-line is so lossy that it consumes all forward and reflected power, it will measure an SWR of 1:1.
When measuring SWR on an antenna having a small amount of reflected power, the length of the feed-line between the bridge and the antenna may affect your SWR reading. An example of this is a 70-ohm antenna being fed with 50-ohm coax.
Different lengths of feed-line will give you small differences in SWR readings because at certain lengths, the mismatched feed-line starts to act like a series matching section. In the case of a 70-ohm antenna fed with 50-ohm coax, if the feed-line is a half wave long, the SWR will measure 1.4:1. At some particular length of feed-line and on one frequency, the SWR will measure 1:1 because that length of that feed-line transforms the impedance to make a match. Some hams have adjusted their feed line length to get a perfect match. This is called "tuning your antenna by tuning your feed-line." With other feed-line lengths, you will measure something different. Suppose the impedance of the feed-line and the antenna are perfectly matched. Then there is no reflected power. You will get a 1:1 reading on the SWR-bridge with any length of feed-line.
There is a myth that reflected power is burned up as heat in the transmitter. The reflected power coming back down the feed-line sees an impedance mismatch at the transmitter or tuner and it reflects back up again. The reflected power does not get back into the transmitter. Because the reflected power reflects back and forth, the radiating part of the antenna system absorbs most of the power being reflected back up each time. All of it eventually is radiated except for the power lost in the feed-line. The losses in a real feed-line will burn up some of the power on each pass. This is why the feed-line loss increases with SWR.
Built-in tuners are found in most modern transceivers. If yours doesn’t have one, then you can use an outboard tuner to give the transceiver a proper load. The place you want a 1:1 SWR is between the output of a transceiver and antenna or between the transceiver and the input of a tuner in order for the transmitter to deliver its maximum power. Because built-in tuners are in most modern transceivers, many hams use them to match antenna systems having high loss.
When you have mismatch between the feed-line and the antenna, part of the power feeding the antenna system reflects back toward the tuner and the transmitter. The part of the power going toward the radiating part of the antenna system is called forward power. The part reflected back down the feed-line is called reflected power. The larger the mismatch the larger the reflected power will be.
If the feed-line and antenna are not matched, waves traveling toward the radiating part of the antenna system meet the waves being reflected back down the feed-line. The waves interfere with each other, and at certain points along the feed-line, the amplitudes of both waves combine. This will result in a current maximum to be found at that point; and at that point, the current will appear to be standing still. The length of feed-line and the frequency will determine where this point occurs. At another point, the forward and reflected waves interfere, and they subtract from each other. At that point, there will be a current minimum. If you could visualize this phenomenon, you would see a series of current maximums and minimums standing still along the feed-line. This is why we refer to them as standing waves. At different points along the feed-line, where you have high current, you will have low voltage, and where you have low current, you will have high voltage. At any point along the feed-line, multiplying the voltage times the current will equal the power in Watts. When the feed-line is matched to the antenna, current and voltage remain the same all along the feed-line because there is no reflected current to interfere with the forward current.
As happens with the current, the voltage will also appear to be standing still. The voltage maximums and voltage minimums will not be at the same locations as the current maximums and minimums. SWR is the ratio of the maximum voltage to the minimum voltage on the line. It is called "Voltage Standing Wave Ratio" or VSWR, but we shorten it to just SWR. There is also a current SWR or ISWR, and it is the same value as the VSWR. For example, if the standing wave voltage maximum is 200 volts and the minimum voltage is 100 volts, the VSWR will be 2:1. If the voltage maximum and voltage minimum are equal, the SWR will be 1:1. If the voltage minimum is zero, the SWR is infinite.
In measuring SWR at the transmitter, you need to realize that feed-line losses affect the SWR readings. If the feed-line losses are high, much of the power reflecting back from the antenna will be lost, and the SWR reading on the meter will indicate it is lower than it actually is. If a feed-line is so lossy that it consumes all forward and reflected power, it will measure an SWR of 1:1.
When measuring SWR on an antenna having a small amount of reflected power, the length of the feed-line between the bridge and the antenna may affect your SWR reading. An example of this is a 70-ohm antenna being fed with 50-ohm coax.
Different lengths of feed-line will give you small differences in SWR readings because at certain lengths, the mismatched feed-line starts to act like a series matching section. In the case of a 70-ohm antenna fed with 50-ohm coax, if the feed-line is a half wave long, the SWR will measure 1.4:1. At some particular length of feed-line and on one frequency, the SWR will measure 1:1 because that length of that feed-line transforms the impedance to make a match. Some hams have adjusted their feed line length to get a perfect match. This is called "tuning your antenna by tuning your feed-line." With other feed-line lengths, you will measure something different. Suppose the impedance of the feed-line and the antenna are perfectly matched. Then there is no reflected power. You will get a 1:1 reading on the SWR-bridge with any length of feed-line.
There is a myth that reflected power is burned up as heat in the transmitter. The reflected power coming back down the feed-line sees an impedance mismatch at the transmitter or tuner and it reflects back up again. The reflected power does not get back into the transmitter. Because the reflected power reflects back and forth, the radiating part of the antenna system absorbs most of the power being reflected back up each time. All of it eventually is radiated except for the power lost in the feed-line. The losses in a real feed-line will burn up some of the power on each pass. This is why the feed-line loss increases with SWR.
Built-in tuners are found in most modern transceivers. If yours doesn’t have one, then you can use an outboard tuner to give the transceiver a proper load. The place you want a 1:1 SWR is between the output of a transceiver and antenna or between the transceiver and the input of a tuner in order for the transmitter to deliver its maximum power. Because built-in tuners are in most modern transceivers, many hams use them to match antenna systems having high loss.
Subscribe to:
Posts (Atom)
Apparatus Assignment
