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 SWR - Standing Wave Ratio The most important accessory that you will own is a SWR Meter. If you haven't got one, get one now!  A typical CB SWR Meter with
the necessary PL259 to PL259 Patch Lead
as supplied by Truck King Any aerial should be matched to the wavelength being used for it to be at its most efficient. This is particularly true when the antenna is being used to transmit. For an antenna to be at its most effective and not have the potential of causing damage to the transmitter (the CB radio) it must be matched to the transmitter. This is done by checking the VSWR (Voltage Standing Wave Ratio), usually simply referred to as SWR. The "standing wave ratio" in the feeder (the coaxial cable) is measured with an SWR meter and is an indication of how well the antenna is tuned to the particular frequency in use and matched to the feeder. CB antennas are designed to appear as an impedance of 50 Ohms when the antenna is resonant - i.e. tuned to the transmission frequency. The resonance of an antenna depends on its physical length - a relationship to the actual wavelength of the frequency being transmitted - and having the correct loading (any coils that may be included in the design of the antenna). If an antenna is not tuned to be resonant at the wavelengths being used then it will not operate at its optimum efficiency because not all of the radio frequency energy being produced by the transmitter (the CB radio) will be radiated by the antenna, so the range of transmission will be reduced. Perhaps more importantly than this, however, is that some of that wasted energy will be reflected from the antenna back towards to transmitter, measured by the resulting standing waves, and this can cause damage to the radio. Usually the SWR is adjusted by changing the length of the antenna, this may involve a sliding section that is held in place by a grub screw or clamp, or by physically reducing the length of the antenna by cutting it ( just a few millimetres at a time!), or by an electrical adjustment on the aerial such as moveable rings. How To Perform SWR Measurements and Antenna Adjustments Before you start your measurements and adjustments check a few things: 1/ Antenna: Make sure that you buy the best quality antenna that you can, the general rule is the bigger the antenna the better your signal will be transmitted. You will get much better results by spending an extra £20 or £30 on a higher quality antenna than spending £20 or £30 on a more expensive radio - which will not benefit your range at all! 2/ Installation: Install the antenna carefully and according to the manufactures instructions. If possible ensure that the antenna is adjusted to the correct length so that it will be as near to resonance (tune) as possible. Ensure that the antenna is not near any other objects or obstructions as they will have an effect on the SWR reading. This is especially true of antennas mounted on vehicles, so ensure that the vehicle is parked out in the open and not near other vehicles buildings or under a car port, for example. Also make sure that the doors, bonnet and boot lid are closed. It is much better to mount a mobile antenna on the roof where it will be well clear of obstructions - mounting it lower down on a wing or bumper will probably cause a high SWR reading. 3/ Cable: Use the highest quality coaxial cable possible. Ensure that it is of 50 ohm impedance, of course. RG58C/U is the standard quality cable used (6mm diameter requiring PL259 plugs with 6mm cable entry) although some prefer to use the thicker and higher quality RG8 Mini, this has a thicker braid screening which minimises losses and could help reduce any interference to other equipment (requires PL259 plugs with 7mm cable entry). 4/ Connections: Bad connections can be the source of many frustrating and confusing antenna problems, it is extremely important to make sure that all plugs and joints are clean and fault-less and that solder joints are well made and not 'dry'.  Different SWR
meters may
vary in appearance but the controls and settings are essentially the
same. The example SWR meter shown above can measure SWR and power,
though some SWR meters do not have the power measurement setting.
An SWR meter will wave two SO239 sockets on the back or on the side, these are the same as the socket on the rear of a CB radio. The SO 239 sockets allow the meter to be connected between the radio and the antenna. Using a short PL259 to PL259 patch lead, connect the CB radio to the input of the SWR meter, usually marked RTX (on the left hand side in the above example). Connect the antenna to the terminal on the SWR meter marked ANT (right hand side on the above example). For SWR meters that do not have a power scale there will be a switch that can be moved between "Set" and SWR. For SWR meters that include the facility for measuring power output there will be two switches; one for changing between power measurement "PWR" and SWR measurement "SWR" and another switch that can be moved between FWD (forward) and REF (reflected).
Ideally the SWR readings should be below 2.0 across the band, but this depends upon the 'bandwidth' of the antenna being used. The 80 EU and UK channels cover a band width of just over 1 MHz (1000 kHz) from EU channel 1 on 26.965 MHz to UK channel 40 on 27.991 MHz. Most antennas will have a sufficient bandwidth to cover all of these 80 channels with an SWR of below 2.0, but there are some antennas that have less bandwidth, e.g. 700 kHz and therefore might not cover all 80 channels with a SWR of less than 2.0.  Losses Due To VSWR Mismatch There are many misconceptions and myths surrounding SWR. There are some that will tell you that the VSWR must be less than 1.5 across all the channels for the antenna to be effective In fact losses are less than some might expect and experts agree that as long as the antenna presents a VSWR of less than 2.0 across the channels the antenna will be fine. At an SWR of 1.0:1 ( which is practically impossible to achieve) the loss will be 0dB. At an SWR of 1.5:1 the loss will be 0.25 dB At an SWR of 1.75:1 the loss is still only about 0.4 dB At an SWR of 2.0:1 the loss will only be 0.6dB At an SWR of 3.0:1 the loss will be 1.25 dB. A transmitter should not be operated into an antenna system that presents an SWR of 3.0:1 however. I would add that if using high power, it would be wise to attempt to attain an SWR as near to 1.5:1 as possible - or at least only use those channels that have an SWR of about 1.5:1 or less. The following table was compiled by the Firestick Antenna Company and shows the effect of SWR for a transmitter with 4 watts of transmitted power.
Having difficulties matching your antenna SWR? Any well designed, good quality and properly installed antenna should be able to be matched across all 80 CB channels with an SWR of less than 2:1. If the SWR is unacceptably high the obvious first step is to check that the antenna is properly adjusted and of the correct length. Make sure that the antenna is mounted well clear (at least 1 meter, or more) of any other metallic objects such as other aerials or metal roofs. If it's a mobile antenna mount it on the roof or gutter, rather than on the wing or bumper. For a home base antenna, install it clear of other objects and preferably above the height of the roof. If these adjustments do not produce a good SWR, then check the coaxial antenna cable for any sharp kinks, snags and other damage. Coax cable should have smooth bends not sharp kinks. It can be easy to damage a cable when pulling it through holes, for example, and a damaged cable will make for a poor antenna system. Always use the the very highest quality 50 ohm coaxial cable. e.g. Mil spec' RG58 C/U is the usual choice (6mm diameter requiring PL259 plugs with 6mm cable entry) , though you could also opt for the lower loss, higher quality Mini RG8 (7mm diameter requiring PL259 plugs with 7mm cable entry). Check that the PL259 plugs at each end of the cable are properly and securely fitted. A common problem is that the centre conductor is badly soldered to the centre pin of the plug. Ensure that the plugs are securely located in the rig and antenna sockets. These steps should help find the problem. However if you are trying to match over a greater range of frequencies than the normal 80 CB channels, for example if 'freebanding', some antennas may not have sufficient bandwidth to accommodate this. In such cases an Antenna Tuning Unit (ATU) could help match the antenna to the rig. Read more about antenna tuning units here.  MFJ 945E
Antenna Tuning Unit
A Note About Antenna Gain You will often encounter antenna gain expressed as dBi rather than dBd. Referencing any given antenna to dBi will always give a value of 2.15dB higher than referencing it to dBd thereby making the antenna seem better than it really is! This should be regarded as a marketing exercise by the antenna manufacturer since the "i" refers to an isotropic radiator which is a 'point source antenna' that is infinitely small. The isotropic antenna is purely theoretical and cannot physically exist in reality. The best measurement by which to judge any given antenna is to compare it against to a dipole. This will be expressed as a dBd figure. Additionally mistrust any information that just gives a gain figure as **dB. This is meaningless as the dB figure must be referenced to a known antenna - e.g to a dipole, i.e. dBd. Even having said all this, there are some unscrupulous antenna manufacturers and sellers that give entirely unrealistic and electrically impossible gain figures, whether dBd or dBi. You will, for example, often see the Solarcon A99 (a.k.a. the Antron 99) advertised with a gain of 9.9 dBi. This is simply ludicrous, electrically impossible and is a lie. The A99 antenna is a good for what it is - an easy to assemble and install device that should withstand bad weather and strong winds very well - however the A99 will have little if any gain over a dipole or a 1/2 wave Silver Rod. I know, I own one, I like it. The Solarcon A99 is a perfectly fine antenna if you disregard the hype. I does not have anywhere near 9.9 dBi gain and therefore cannot work miracles! You'd have to have an enormous multi-element directional yagi antenna to achieve that sort of gain, so the claims made for the A99 are utterly preposterous! See more about the A99 at this website: http://www.n1wpn.net/antron_99_exposed.htm More About Wavelengths Radio transmitters, such as CB radios, produce radio waves. Radio waves a part of the 'electromagnetic spectrum' (The electromagnetic spectrum also includes microwaves, infra red (heat), visible light, ultra violet and x-rays). All waves in the electromagnetic spectrum, including radio waves, travel at a fixed speed of 300,000,000 meters per second (three hundred million meters per second) commonly referred to as "the speed of light". The rate at which a radio wave oscillates up and down is termed as its frequency and can be expressed as the number of peaks that pass by a specific point every second (e.g. point X in the diagram below) and is expressed in Hertz (Hz) after Heinrich Hertz. The distance between each successive peak of a radio wave is termed as its wavelength. The wavelength and the frequency are inter-dependent - change one and the other must change accordingly. This is because of the three parts of the equation; wavelength, frequency and speed, the speed that radio waves travel that is fixed, therefore the two remaining parts, wavelength and frequency must be the variables. i.e. a radio wave with a longer wavelength will have a lower frequency and a radio wave with a shorter wavelength will have a higher frequency. If the wavelength is increased the number of peaks that can pass point X must be lower so the frequency of the radio wave will be lower. Conversely if the wavelength is reduced the number of waves that can pass the single point must increase causing a higher frequency. The calculation to convert a frequency to a wavelength is: 300,000,000 / frequency (Hertz) = wavelength (metres) example: 300,000,00 / 27,600,000 Hertz = 10.86 metres (or 300 / frequency (megahertz) = wavelength (metres) e.g. 300 / 27.6 MHz = 10.86 metres) [ N.B. 27 MHz = 27,000,000 Hertz = 27,000 KHz ] The calculation to convert a wavelength to a frequency is: 300,000,000 / wavelength (metres) = frequency (Hertz) example: 300,000,00 / 11.1 metres = 27,027,027 Hertz (or 300 / wavelength (metres) = frequency (megahertz) = e.g. 300 / 10.75 metres = 27.9 Mhz) The other consideration of a radio wave is the amount of energy that it is carrying i.e how strong, this is the Amplitude, and is the height of the wave, as seen in the diagram below:  Diagram
showing a radio wave
Radio waves are just a small part of "The Electromagnetic Spectrum". The electromagnetic spectrum includes: Radio Waves; Infra Red Waves (heat); Light (visible spectrum), Ultra Violet, X-Rays and Gamma Rays) is divided up here is a simple summary: VLF - Very Low Frequencies - (very long Waves) 3 to 30 kilohertz LF - Low Frequencies - 30 to 300 Khz (Includes the 'LONG WAVE' Broadcast band that has such stations as BBC Radio Four on 198 kilohertz and RTE on 252 kilohertz) MF - Medium Frequencies - 300 to 3000 Khz (Includes the 'MEDIUM WAVE' broadcast band from 520 to 1602 kilohertz, that has such stations as BBC 5 Live on 693 kilohertz, Talk Sport on 1053 kilohertz, Virgin Radio on 1215 kilohertz) HF - High Frequencies - ('Short Waves') 3 to 30 megahertz - Used for some utilities, aircraft, includes some broadcast bands that carry international broadcast radio, e.g. The BBC World Service, Voice of America and Voice Of Russia, etc., also contains a number of Amateur Radio band assignments and, of course, the CITIZENS BAND at 11 meters, that's around 27 megahertz (27,000 kilohertz). VHF - Very High Frequencies - 30 to 300 megahertz. Includes VHF/FM broadcast band from 88 to 108 megahertz; Military uses around 30 to 88 MHz, Cordless telephones, Aircraft Band between 118 and 132 MHz; An amateur radio assignment at 144 megahertz; DAB Digital Radio broadcasts at around 217 to 230 megahertz, among lots of other uses including broadcast television in some countries. UHF - Ultra High Frequency band 300 to 3000 megahertz: Includes broadcast television - analogue PAL and Digital Terrestrial Television (Freeview) at 400 to 854 megahertz; an Amateur Radio assignment at 430 megahertz; an allocation for PMR446 licence free walkie talkies at 446 megahertz(0.5 watts), GSM mobile phones and wi-fi. SHF - Super High Frequencies - 3000 - 30000 megahertz (3 to 30 gigahertz). (Incidentally Sound waves are NOT part of the electromagnetic spectrum.)  More CB Radio information from Right Channel Radios USA Top of page ^
 Remember in a
time of national emergency the telephone networks are
likely to fail, CB radio could be your only point of contact with the
outside world.
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