CIS Radio Intermediate Technical. CIS Radio Intermediate Technical – Subjects covered:- Technical Basics
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Units & Symbols – Electrical Circuits – Conductors & Insulators – Power & Resistance – Ohms Law – Uses for resistors – Alternating Currents & Voltages – Different frequencies – Radio Frequencies – Wavelengths – Other radio users
A simple transmitter – Modulation
A simple receiver – Detector
Feeders & Antennas
Feeders – Antennas (Dipole, ½ wave, 5/8th wave. Yagi and End Fed) – Polarisation – Matching Antennas – Antenna Tuning Unit – Standing Waves & SWR meters – Baluns – Dummy Loads
Spreading out – Buildings – Range – The ionosphere – Frequency & time of day
How is interference caused – What can be done to minimise interference – Earthing – Choice of antenna – Power and modes of transmission – Immunity – The neighbours – Other hazards
High Voltage – High current – Mains & earths – Protective Multiple Earthing – Accidents & emergencies – Antennas and feeders – Car batteries – Headphones – Other hazards
e.g. starter motor for car is about 100 Amps
e.g. electric kettle is about 10 Amps
Current is the quantity of electrons moving through a wire every second - think of the volume of water going through a pipe.
Symbol is the letter IAmps
The current is a measure of how much electricity is flowing. The unit of measure is the amp
The current is made up of millions of small particles called electrons which are electrically charged and move around the circuit carrying the electricity.
These electrons need enough room to move and if a larger current is expected to flow a larger diameter wire is needed.
The electrons get their energy from the supply or battery
The electrons have more energy when they enter a device than when they leave it
The potential difference across a device shows this difference in energy and is the energy being transferred to it often simply called the ‘voltage’
Potential Difference is measured in a unit called the Volt. If more volts are measured across a device it means the flow of electrons (current) is transferring more energy to that device.
A 4 cell battery has a potential difference of 6V. It will give the electrons four times as much energy as a single cell (1.5V) and will cause the electrons to flow Quicker.
Note: If we use mains supply the potential difference is 230V. Enough to cause serious injury or even kill you.
You should be familiar with the following symbols -
Resistors are coded using colours -
When you load a battery into an electronic device, you\'re not simply unleashing the electricity and sending it to do a task. Negatively charged electrons wish to travel to the positive portion of the battery and if they have to rev up your personal electric shaver along the way to get there, they\'ll do it. On a very simple level, it\'s much like water flowing down a stream and being forced to turn a water wheel to get from point A to point B.
Whether you are using a electrical mains or a battery to produce electricity, three things are always the same:
1. The source of electricity must have two terminals: a positive terminal and a negative terminal.
2. The source of electricity (whether it is a generator, battery or something else) will want to push electrons out of its negative terminal at a certain voltage. For example, one AA battery typically wants to push electrons out at 1.5 volts.
3. The electrons will need to flow from the negative terminal to the positive terminal through a copper wire or some other conductor. When there is a path that goes from the negative to the positive terminal, you have a circuit, and electrons can flow through the wire.
A simple circuit with a variable
resistor to increase or decrease
the bulbs brightness
You can attach any type of load, such as a light bulb or motor, in the middle of the circuit. The source of electricity will power the load, and the load will perform whatever task it\'s designed to carry out, from spinning a shaft to generating light.
Electrical circuits can get quite complex, but basically you always have the source of electricity (such as a battery), a load and two wires to carry electricity between the two. Electrons move from the source, through the load and back to the source.
Moving electrons have energy. As the electrons move from one point to another, they can do work. In an incandescent light bulb, for example, the energy of the electrons is used to create heat, and the heat in turn creates light. In an electric motor, the energy in the electrons creates a magnetic field, and this field can interact with other magnets (through magnetic attraction and repulsion) to create motion
This circuit shows a switch,
motor and battery
An electric circuit is the name given to the way electrical devices are connected. By connecting a battery to a light bulb we make a circuit
Consisting of the battery, connecting wires and the bulb
The battery provides the electrical energy. The electricity flows out of the positive side (+) along the connecting wire and into the bulb and back along a wire to the negative side (-). Electricity needs a complete circuit or path to flow round. Some electronic devices are very sensitive to which way around the battery is connected. The light bulb is a thin filament of wire in a glass bulb where all the air has been sucked out (a vacuum). If electricity is passed through the bulb the filament glows white hot and gives off light and heat. It does not matter which way the electricity flows through the bulb for it to work. It has no polarity.
Care must be taken that the battery is of the correct
voltage for the bulb or it will be either to dim or to
bright and possibly blow.
Is called a conductor and conducts electricity because the electrons can move freely. Metals are conductors.
The wire should have a plastic sheath or covering. This is called an insulator and does not conduct electricity. Electrons cannot move in an insulator. Wood, rubber, glass and ceramics are also insulators but NOT if they get wet.
Electricity may still be able to flow through any surface water and can still give you an electric shock. Be very careful in wet conditions
In a conductor, electric current can flow freely, in an insulator it cannot. Metals such as copper typify conductors, while most non-metallic solids are said to be good insulators, having extremely high resistance to the flow of charge through them. "Conductor" implies that the outer electrons of the atoms are loosely bound and free to move through the material. Most atoms hold on to their electrons tightly and are insulators. In copper, the valence electrons are essentially free and strongly repel each other. Any external influence which moves one of them will cause a repulsion of other electrons which propagates, "domino fashion" through the conductor.
Simply stated, most metals are good electrical conductors, most non-metals are not. Metals are also generally good heat conductors while non-metals are not.
Most solid materials are classified as insulators because they offer very large resistance to the flow of electric current. Metals are classified as conductors because their outer electrons are not tightly bound, but in most materials even the outermost electrons are so tightly bound that there is essentially zero electron flow through them with ordinary voltages. Some materials are particularly good insulators and can be characterized by their high resistance, plastic, wood, glass etc.
e.g. starter motor for car is about 100 Amps
e.g. electric kettle is about 10 Amps
Current is the quantity of electrons moving through a wire every second - think of the volume of water going through a pipe.
Symbol is the letter IAmps
Resistance is the measure of how difficult it is for electricity to flow.
The symbol for resistance is Rand is measured in ohms
In electrical terms a device that restricts the flow of current is called a resistor. How much difficulty it presents to the flow of the current is called its resistance. The higher the value the more resistance. If a voltage remained the same then higher resistance will mean a lower current flowing. Increasing the voltage would therefore increase the Current.
This relationship is called V=IxR or Ohms Law
(so the voltage across a resistance is the product of the resistance and the current through it)
Divide both sides of the top equation by I
V / I = R
(so the resistance of a circuit is given by the voltage across the circuit divided by the current through it)
Or divide both sides of the top equation by R
V / R = I
(so the current through a circuit is given by the voltage across the circuit divided by the resistance)Ohms Law
In electrical circuits it is often necessary to deliberately limit the flow of current. A resistor is used to insert some resistance into a circuit. A low value resistor will have little effect on the flow of current and the bulb will glow quite brightly. A higher value resistor will have a greater limiting effect on the current and the bulb will be much less bright.
If the resistance is too high then the bulb may not glow at all.
a device transfers the energy we deliver to it.
Power is simply the voltage times the current.
Power is measured in a unit called the watt
The symbol is P
So P = V x I (watts = volts x amps)
A 1 watt light bulb transfers 1 unit of energy every second to heat and lightPOWER
Alternating current, AC or a.c., keeps changing direction, first one way and then the other. AC is easier to generate and easier to change from one voltage to another. Your mains electricity is AC. The generator at the power station is a large coil of wire rotating around a powerful magnetic field. If we look at just one wire in the rotating coil, it is first going up through the field, then, half a turn later, coming down. Whilst the wire is going up the voltage and current generated has one polarity and moments later when its coming down the voltage and current are of opposite polarity.
The electricity produced by a bicycle dynamo is AC for the same reason. It is easy to produce and perfectly good for powering the light bulbs. The only problem is when the rider stops the lights go out unless there is a battery powered backup.
The alternator in a car also produces AC but that is no use to charge the battery or run the many electronics like the radio. Special electronics are required to convert the AC into DC. That recharges the battery and powers all the electrical items including lights (which could use AC or DC)
Alternating currents and voltages do not suddenly switch from one polarity to the other. They build up to a peak in one direction, then reduce back to zero before building up in the opposite direction. This smooth waveform is called a sine wave.
easier to generate (e.g. dynamo on bike)
easier to step up and down to different voltages
1 cycle (one direction then the other direction) per second is called 1 Hz
Mains supply is 50HzAlternating Current AC
Direct current or DC is the type of electricity that is produced by batteries, static, and lightning. A voltage is created, and possibly stored, until a circuit is completed. When it is, the current flows directly, in one direction. In the circuit, the current flows at a specific, constant voltage (this is oversimplified somewhat but good enough for our needs.) When you use a flashlight, pocket radio, portable CD player or virtually any other type of portable or battery-powered device, you are using direct current. Most DC circuits are relatively low in voltage. e.g. your car\'s battery is approximately 12 V. That\'s about as high a DC voltage as most people will ever use.
Sound is also an alternating signal carried by the movement of air. A sound of 50Hz is a very low note, as much felt as it is heard. Human hearing ranges from about 100Hz (a low note) to 15kHz (a very high note). For high quality music a full range of frequencies is desirable but for speech a much narrower range is sufficient. Usually 300Hz to 3kHz. Typical of a good telephone line. Electrical signals in the leads to a loudspeaker are alternating currents and voltages and many frequencies may be present at the same time, depending on the sounds. A clean whistle will be a single note and a single electrical frequency whilst music is likely to contain many notes and hence many frequencies.
Audio communication (electrical signals in wires etc) 300Hz - 3kHz
HF, VHF, UHF radio signals are up to 1000MHz (Radio Frequency - RF)Some Typical Frequencies
Sine waves are produced by oscillators - think of a swinging pendulum.
Electromagnetic waves are waves consisting of vibrating electric and magnetic fields. Various frequencies of theses waves are known as the Electromagnetic Spectrum
Radio frequencies or RF are generated by feeding alternating electrical signals to an antenna. These frequencies are much higher than we can hear. While the whole of the electromagnetic wave spectrum covers a huge range of frequencies, radio waves themselves extend over a very large range as well. Again it is useful to be able to easily refer to different sections of the spectrum. To achieve this different designations are given to different areas. The frequencies that are covered are split into sections that vary by a factor of ten, e.g. from 3 MHz to 30 MHz. Each section is allocated a name such as high frequency and these areas are abbreviated to give terms like HF, VHF and so forth that are often used. Often talk is heard of VHF FM, or UHF
The VHF and UHF refer to the areas of the radio spectrum where these transmissions take place.
For ease of reference these frequencies are divided up into bands.
Waves with low frequencies or long wavelengths are known as radio waves and are produced by causing electrons to vibrate in an antenna.
Molecular excitation produces microwaveand infrared waves which have little higher frequency than radio waves.
A higher frequency of molecules make up the visibleand ultraviolet regions of the spectrum. A very small portion of the electromagnetic spectrum is visible to the human eye. It is the ultraviolet radiation in sunlight that tans or burns are skin.
The next step of higher frequency waves are called X-rays.
Lastly gamma rays are the highest frequency in the spectrum.
All electromagnetic waves travel at the same speed in a vacuum. This speed is called the speed of light and is designated by the letter C and = 299 792 458 m/s or 186000 miles a second
A wavelength is the distance in between the repeating units of a wave, as measured from one point on a wave to the corresponding point in the next unit. For example, the distance from the top -- called the crest -- of one wave unit to the crest of the next is one wavelength. Wavelength is often designated by the Greek letter lambda λ. Wavelength is inversely proportional to the frequency of a wave. In other words, the shorter the wavelength is, the more wave units will pass in a given amount of time.
So, the WAVELENGTH of a wave is the distance between the same point on two consecutive cycles
In wireless systems, this length is usually specified in meters, centimetres or millimetres. Wavelength is inversely related to frequency. The higher the frequency of the signal, the shorter the wavelength. If f is the frequency of the signal as measured in megahertz, and w isthe wavelength as measured in meters, then w = 300/f and conversely f = 300/w
There is a fixed relationship between frequency and wavelength so that for radio waves, the frequency times the wavelength is equal to the speed of light. 186000 miles a second or 700 million miles an hour
To convert frequency to Wavelength, select a point on the X or horizontal axis of the graph, say 100MHz and look vertically upwards to the diagonal line. Now look to the Y or vertical axis and read off the wavelength. In this case 3m
We share the use of the radio spectrum with many other radio user\'s. Sometimes these other users are on adjacent frequencies and where it is possible to do so, we share these frequencies with other users.
For instance, the SCC shares frequencies with the MOD. SCL14 for example on 6992.5 kHz which is just below the bottom end of the amateur 40m or 7Mhz band which covers 7000 – 7200 kHz.
in the frequency spectrum
the 14Mhz or 20m band
The transmitter generates the radio waves with which we communicate. It has the potential to cause problems and it is important you understand a bit about how it works. A simple transmitter consists only of a method of generating the correct radio frequency plus an antenna. To send a message you switch it either ‘on’ or ‘off’ in an arranged way. Morse code is the most well known code used to send letters or the alphabet, numbers and punctuation marks simply by switching on and off. The need to send voice or pictures meant that a method of superimposing the information on the radio signal had to be devised. This device is called the modulator
Modulation is the process of getting the radio signal to carry an audio signal. The signal before modulation is usually called the ‘carrier’.
There are two ways of modulating the carrier. The first is to vary the amplitude of the carrier in time with the audio signal called Amplitude Modulation or AM. The second is to vary the frequency of the carrier in time with the audio signal called Frequency Modulation of FM.
1) Audio Stage 2) Modulator
3) Frequency Generator (or Oscillator)
4) RF Power Amplifier
+ Microphone at left and antenna at right
The signal from the microphone is quite weak and needs to be amplified in the audio stage box 1. Box 3 is the frequency generator. This produces the frequency which the transmitter will use to transmit the signal. This generator must be carefully designed and made to ensure it works on the correct frequency. If not on the right frequency the person you are talking to will not hear you. Importantly, you may be transmitting on someone else\'s frequency, even outside the band in use! This could cause problems if it was a frequency used for safety etc. Box 2 is the modulator which takes the radio signal from Box 3 and mixes it with the audio signal from Box 1 to produce a modulated radio signal. This signal is not usually powerful enough to transmit so we use a RF Power Amplifier to amplify the signal. Box 4 This signal is then fed to your antenna.
In AM the top waveform is the audio signal from the
The carrier is the HF (closer spaced) wave of constant
amplitude in the middle.
The process of AM produces a wave of the same
frequency as the carrier, but its amplitude varies in
Time with the audio signal seen at the bottom.
The top wave form is the signal from the microphone.
The steady, HF wave is the carrier.
FM produces a modulated wave of constant amplitude, but with the frequency varying in time with the signal from the microphone. The frequency does not vary much and the radio is tuned to the centre of the frequency variations of the signal
Care must be taken with either type of modulation not to ‘over modulate’, that is, not to have too strong a signal from the microphone or audio stage feeding the modulator. This is usually caused by turning the ‘microphone gain’ up too far. The effect is similar to turning up the volume on a receiver except the effects are heard at the distant end of your radio contact. Shouting into the microphone can also cause the same problem.
Excessive AM will make the peaks of the modulated carrier too large and reduce the troughs to zero. This distorts the audio which will sound rough at the receiver. It will also cause interference to radio receivers tuned to adjacent channels. This is important as it is a condition of your licence that you do not cause interference to other radio users.
Excessive FM is also to be avoided and may cause interference to neighbouring Users as well as risking poor quality audio signals to your intended recipient.
RF Power stage (Box 4) amplifies the radio signal. The power output must be connected to a correctly matched antenna. Use of the wrong antenna can damage the transmitter.
Excessive amplitude modulation causes distorted output and interference to adjacent channels. Excessive frequency modulation will cause interference to adjacent channels. So you need to set microphone gain carefully.
The radio signal is picked up by the antenna, which converts the radio signal into electrical signals on the feeder and fed along the feeder to the input of the receiver Box 1 which contains the tuning which selects the
wanted signal from all the hundreds of signals picked up by the antenna
And RF amplification which amplifies the wanted signal to bring it up to a
Suitable level for Box 2 which contains the detector. This recovers the original modulating signal. It extracts the original audio signal from the modulated signal as the carrier is no longer required. Detection is also called de-modulation. The audio amplifier in Box 3 ensures the audio signal is powerful enough to drive the loudspeaker or headphones Box 4.
The wanted radio signal is then selected by tuning the receiver to the correct frequency.
Antenna at left - Feeder from antenna to receiver
1) Tuning and RF amplifier 2) Detection / Demodulation
3) Audio Amplifier 4) Loudspeaker/Headphones
The type of detector used must be suitable for the method of modulation being used by the transmitter.
This can be demonstrated using a radio receiver by setting the mode switch to the wrong type of modulation. The correct mode like CW, SSB, FM etc needs to be chosen to correctly recover the original audio signal.
If is was a data signal sent at the transmitting end then a suitable detector is needed and the selection of the correct mode (e.g. upper sideband USB or lower sideband LSB or FM) at the receiving end is just as important but may be harder to determine by ear.
The wire connecting a transmitter to an antenna is called the feeder. This carries the powerful radio frequency signals which will radiate from any wire. To prevent radiation from the feeder it is usually made as coaxial cable. This has a centre conductor which carries the signal and an outer screen which confines the signal within the cable. This outer screen is usually braided to provide a good continuous covering. The inner conductor may be a single wire or several strands twisted together. Two ‘feeders’ are shown here – RG58 (50 Ohm) and twin feeder (450 Ohm)
The correct type of plug must be used
Coax braid is continuous through plug’s outer metallic case. BNC typically used at VHF and low power. PL259 at HF and higher power. The screen of the coaxial cable must be properly connected to the body of the plug to ensure the screen and plug form a continuous shield for the inner conductor which should be soldered to the centre pin of the plug.
It needs to be designed for the frequency or wavelength in use and there are five that we need to consider, the dipole, 1/4 wave ground plane, the Yagi, the 5/8th wave and the end-fedAntennas
¼ Ground Plane
A dipole antenna is a radio antenna that can be made of a simple wire with a center-fed driven element. It consists of two metal conductors of rod or wire, oriented parallel and collinear with each other (in line with each other), with a small space between them. The radio frequency voltage is applied to the antenna at the centre, between the two conductors. These antennas are the simplest practical antennas from a theoretical point of view. They are used alone as antennas, notably in traditional "rabbit ears" television antennas and as the driven element in many other types of antennas, such as the Yagi. Dipole antennas were invented by German physicist Heinrich Hertz around 1886 in his pioneering experiments with radio waves.
The dipole is a basic antenna and is half a wavelength long. This means the size
of the dipole and all other antennas must be suitable for the frequency it is
intended to use it for.
If it is mounted vertically it radiates equally in all directions. If mounted horizontally,
More common at HF it radiates well from the sides but not off the ends. Given the
Choice it should be side on to the desired direction of maximum signal but this
may not always be possible
This antenna gets its name from the fact that the radiating elements are ¼ wavelength long or λ/4. The radiating or active element is always vertical. Four horizontal wires called radials form the groundplane – an earthed surface which acts like a mirror to radio waves.
The transmitted signal is ‘omni-directional’ radiating equally in all directions. It does not radiate vertically.
Highly directional antennas such as the Yagi are commonly referred to as "beam antennas" due to their high gain. However the Yagi design only achieves this high gain over a rather narrow bandwidth, making it more useful for various communications bands (including amateur radio) but less suitable for traditional radio and television broadcast bands. Amateur radio operators ("hams") frequently employ these for communication on HF, VHF and UHF bands.
The Yagi or Yagi-Uda RF antenna or aerial is one of the most successful RF antenna designs for directive applications. It is used in a wide variety of applications where an RF antenna design with gain and directivity is required. It has become particularly popular for television reception, but it is used in very many other applications where an RF antenna design is needed that has gain.
The full name for the antenna is the Yagi-Uda antenna. It was derives it name from its two Japanese inventors Yagi and his student Uda. The RF antenna design concept was first outlined in a paper that Yagi himself presented in 1928. Since then its use has grown rapidly to the stage where today a television antenna is synonymous with an RF antenna having a central boom with lots of elements attached.
This is a development of the ¼-wave groundplane. It is
better at directing signals towards the horizon, rather
than up in the air. It is always mounted with the active
element vertical and is omni-directional. The vertical
element is 5/8 of a wavelength long and due to its size
it is more often used on VHF and UHF frequencies
where the wavelengths are shorter.
The coil at the base is part of the matching of the antenna
to the coaxial cable.
The 5/8 wave has a slight gain over the 1/4 wave antenna . Like the 1/4 wave, the 5/8 wave is also used vertically and gives an omni-directional radiation pattern, but the thing to note is that the 1/4 wave has no coil, whereas the 5/8 wave requires the coil at the base of the antenna for impedance matching.
Just like the λ/4 wave the 5/8λ antenna is a favourite choice for mobile working, especially on 2 metres and above, due to enhanced performance over the λ/4 wave but still with a relatively small size. As with the λ/4 the ground plane radials are replaced by the vehicles\' bodywork
The end fed wire is simply a random length of wire attached to the centre of a coax feeder or, more usually, linked directly onto the rear of a suitable ATU that can take single wire. This is a poor antenna as it is not tuned to any particular frequency and thus generally performs badly relative to a dipole. It is unlikely to be a ¼ or ½ wavelength long and matching it will be a problem. A device called an ‘antenna tuning unit’ will match the wire and enable the antenna to accept power from the transmitter. A likely minimum length of the wire for this antenna would be around 80 feet but is often much longer.
The antenna is often set up with the far end fixed to a pole or tree and the end closest to the transmitter secured by a length of rope to a wall or chimney of a house with the end dropping down to the transmitter. This results in high voltages or currents close to the house and the strong radiation can upset TV’s and other electronic equipment. If this antenna is your only choice it is better to feed it at the far end and bury the feeder.
A Yagi is directional
A long wire can have high voltages or currents close to the feed point
A ground plane is omni directional
5/8th wave is better at directing signals to the horizon
Remember:- The antenna system must be suitable for the frequency of the transmitted signal. If it isn’t it will not match the transmitter and will not work effectivelyAntennas (again)
Depending upon how the antenna is orientated physically determines it\'s polarisation. An antenna erected vertically is said to be "vertically polarised" while an antenna erected horizontally is said (not so surprising) to be "horizontally polarised". Other specialised antennas exist with "cross polarisation", having both vertical and horizontal components and we can have "circular polarisation".
Note: When a signal is transmitted at one polarisation but received at a different polarisation there exists a great many decibels of loss. This is quite significant and is often taken advantage of when TV channels and other services are allocated. If there is a chance of co-channel interference then you can try a different polarisation. Have you ever noticed vertical and horizontal TV antennas in some areas. Now you know why.
A vertical dipole or Yagi will radiate a vertical radio wave and it will also best receive vertical waves.
It does not matter which polarisation you choose as long as both the transmit and receive antennas are the same. However, groundplane and 5/8 antennas are always vertical. The
mobile antennas mounted to vehicles are vertical for
There is an amateur convention at VHF and UHF that
when using FM the antenna is vertical as most mobile
operation uses FM and SSB operation uses Horizontal polarisation.
Effective radiated power or ERP is the product of power and antenna gain.
For example, 50 Watts to a Yagi with gain of 4 is 200 Watts ERP in the direction the Yagi is pointing. This is equivalent to 200 Watts into an antenna with no gain (which radiates equally in all directions)Effective Radiated Power (ERP)
It can allow a single antenna to operate on several bandsAntennas & ATU’s
If the size of the antenna is correct, ½ wavelength long at the wanted
frequency if it is a dipole for example, then the antenna will match the
transmitter and feeder. This means that the power from the transmitter
will be properly radiated by the antenna.
If the antenna is used on the wrong frequency (not the correct size)
some of the power will reflected back down the feeder instead of being
radiated by the antenna.
An antenna tuner, transmatch or antenna tuning unit (ATU) is a device connected between a radio transmitter or receiver and its antenna to improve the efficiency of the power transfer between them by matching the impedance of the equipment to the antenna. An antenna tuner matches a transceiver with a fixed impedance (typically 50 ohms for modern transceivers) to a load (feed line and antenna) impedance which is unknown, complex or otherwise does not match. An ATU allows the use of one antenna for a broad range of frequencies. An antenna plus matcher is never as efficient as a naturally resonant antenna due to additional induced losses on the feed line due to the SWR (multiple reflections), and losses in the ATU itself, although issues of pattern and capture area may outweigh this in practice. An ATU is actually an antenna matching unit, as it is unable to change the resonant frequency of the aerial. Note that similar matching networks are used in other types of
equipment, such as linear amplifiers to
SWR is used as an efficiency measure for transmission lines, electrical cables that conduct radio frequency signals, used for purposes such as connecting radio transmitters and receivers with their antennas. A problem with transmission lines is that impedance mismatches in the cable tend to reflect the radio waves back toward the source end of the cable, preventing all the power from reaching the destination end. SWR measures the relative size of these reflections. An ideal transmission line would have an SWR of 1:1, with all the power reaching the destination and no reflected power. An infinite SWR represents complete reflection, with all the power reflected back down the cable. The SWR of a transmission line can be measured with an instrument called an SWR meter and checking the SWR is a standard part of installing and maintaining transmission lines.
The SWR is usually defined as a voltage ratio called the voltage standing wave ratio or VSWR.
The SWR meter or VSWR meter measures the standing wave ratio in a transmission line. The meter can be used to indicate the degree of mismatch between a transmission line and its load (usually a radio antenna), or evaluate the effectiveness of impedance matching efforts.
Note: An SWR meter does not measure the actual impedance of a load (i.e., the resistance and reactance), but only the mismatch ratio. To measure the actual impedance, an antenna analyzer or other similar RF measuring device is required.
Allowing the operator to adjust the ATU until the antenna system is
Matched and the reflected power to the transmitter is minimised.
If the reading on the meter has unexpectedly increased it means
That more signal is being reflected back from the antenna. The most
likely cause is damage to the antenna or moisture in the connectors.
A dipole is a good match only when cut to the correct length. At other
Lengths the match will be poorer depending on how far from the
Ideal length it is.
If some power is reflected back there will be a “standing wave” on the feeder.
Tolerable levels of SWR are up to about 2:1
High SWR’s measured at the transmitter end mean a fault in the antenna or feeder not the transmitter.So we now know that -
A balun is a device that joins a balanced line (one that has two conductors, with equal currents in opposite directions, such as a twisted pair cable) to an unbalanced line (one that has just one conductor and a ground, such as a coaxial cable).
A balun is a type of transformer: it\'s used to convert an unbalanced signal to a balanced one or vice versa.
Baluns isolate a transmission line and provide a balanced output. A typical use for a balun is in a television antenna. The term is derived by combining balanced and unbalanced.
In a balun, one pair of terminals is balanced, that is, the currents are equal in magnitude and opposite in phase. The other pair of terminals is unbalanced; one side is connected to electrical ground and the other carries the signal.
‘balanced’ antenna and requires two signals, one for each half of
the dipole. The signals are balanced because when the wave on
one side is going up its going down on the other.
Coaxial cable is not electrically symmetrical and is called unbalanced.
It has one centre live conductor and an earthed screen and is not
suitable for connecting directly to a dipole. A balun takes the signal
from the coaxial cable and converts it to two signals suitable for
feeding to the dipole.
If the coaxial cable is connected directly to the dipole, RF current will
flow back down the screened of the cable. This current will radiate and
the screening properties of cable will be upset. Since this cable runs
back into your operating position radiation will take place inside the
room and may cause interference to electrical equipment
or even your neighbours.
Dummy load is not actually an antenna, it dissipates all transmitted power in a form of heat. So what\'s the use of it? Well, it is presents an ideal match for an output of your transmitter (usually 50ohms). Since all power (virtually 100%) is transverted into heat there won\'t be any interference to your neighbours while you do tuning and testing. This is what dummy load is usually used for; testing and tuning transmitters. If you don’t have dummy load, you can build one easily from a BNC or other RF connector and the proper wattage/value of CARBON resistor(s). DO NOT USE WIREWOUND OR METAL FILM RESISTORS! A useful one can be constructed with 4 -220 Ohm 1/4 watt resistors in parallel (220/4 = 55 Ohms) with center conductor to outershell (ground) of an RF connector. That is pretty close to 50 Ohms and if you use 1/4 watt resistors you get a nifty 2 Watt Dummy Load for testing your equipment without an antenna. Commercial Dummy loads are available.
A dummy load is a screened resistor, capable of absorbing all the power from the transmitter and presenting a good match i.e. no power reflected. It must be well screened to minimise unwanted radiation and is is connected instead of an antenna to allow the transmitter to be set up and tested without radiating a signal.
Schematic diagram showing the propagation of high-frequency (shortwave) radio waves by reflection off the ionosphereSpecific ionization conditions vary greatly between day (left) and night (right), causing radio waves to reflect off different layers of the ionosphere or transmit through them, depending upon their frequency and their angle of transmission. Under certain conditions of location, ionization, frequency, and angle, multiple “skips,” or reflections between ionosphere and Earth, are possible. At night, with no intervening layers of the ionosphere present, reflection off the F layer can yield extremely long transmission ranges.
Propagation is the technical term for how radio waves behave once they have left the antenna. Radio waves travel in a straight line unless they are reflected off a suitable surface or are refracted rather light going through a prism.
Radio waves also spread out from an antenna. Close to the antenna they are concentrated so a receiving antenna will pick up a strong signal. Further away the signal will be weaker. Too far away and the signal will be too weak to receive.
Radio waves, spreading from the centre of a dipole
Radio waves can penetrate buildings like an xray can penetrate skin but bones will leave a shadow. Some of the energy is lost in penetrating the building. In a basement or middle of a large building where there are several walls to pass though the signal may be too weak to be of use.
The penetrating ability of radio waves depends on their frequency. For lower frequencies in the MW and HF bands the wavelengths are large and the buildings ‘appear’ fairly small in comparison. Such waves penetrate the building quite easily but have difficulty with obstacles like mountains. At higher frequencies like VHF/UHF the wavelengths are much shorter and the buildings comparatively much bigger causing more of a problem to the VHF and UHF waves.
There is a small advantage to higher frequencies. If the wavelength is smaller than a window, the window appears as a big hole for it to get through, then there will be a reasonable signal with a window on the side facing the transmitter.
The best range with VHF and UHF radio services is achieved when the transmit antenna is mounted high up and clear of obstructions like trees and buildings. It also helps if the receiving antenna is sited in a similar way.
Broadcast transmitters have very tall masts supporting the antenna for that very reason. It also helps in getting the signal into dips behind hills and into valleys. Such places would otherwise be in shadow of a transmitting antenna and maybe not receive the signal.
The range you can achieve with a radio signal depends on a number of factors. A more powerful transmitter will have a greater range though this is not always as noticeable. Consider a torch beam. At double the distance the circle of light is twice as wide and twice as high. It has 4 times the area to cover. Each part of a wall its shining on only gets a quarter of the light. To get the same strength you started with you need four times the power!
In our terms it is much more effective to use a Yagi antenna to focus all our transmitted power in the right direction than getting a bigger transmitter. Also the Yagis ‘gain’ is effective on receive so picks up weaker signals better.
Frequency can effect the range too. The higher the frequency the bigger trees and buildings appear to the wave and more wave is lost penetrating them. Hills cause shadows and the curvature of the earth also has an effect making the hills in the middle of the path appear taller. At VHF and even more so at UHF the range is not normally further than ‘line of sight’ and depending on the terrain that may be from 10 or 20 km up to 60 or 80 km in open country from a hill top. Handheld radios down amongst buildings even 1 km may be difficult.
Radio waves get weaker as they spread out. (Same amount of power has to cover a greater area)
At VHF and UHF hills cause shadows.
Radio waves get weaker penetrating buildings - but windows are more transparent to radio waves.
The range of a signal at VHF depends on the antenna height, a clear path and the transmitter power.
Higher antennas are better than higher power - they improve reception as well.
Outdoor antennas will perform better than indoor antennas.
At VHF/UHF, range goes down as frequency goes up.
Line of sight at VHF/UHF is a little further than the horizon because of very slight bending in the atmosphere (like a mirage). Hills and buildings cause path loss.
VHF/UHF range is generally little
more than line of sight.
The ionosphere is a part of the upper who\'s layers of partially conductive gas occur from about 70 km to 400 km in altitude. These layers are formed by the action of ultra violet light from the sun interacting with air molecules in the upper atmosphere. At this time you only need know that the strength and level of the ionisation varies with the time of day – The amount of sunlight. It also varies with the seasons from summer to winter.
The ionosphere can refract or bend radio waves in the same way as a lens bends light some thousands of miles away from the transmitter. The key point is that the signal can be heard far over the horizon and well beyond the range of what would be achieved by a wave travelling direct to the receiver.
The ionosphere is important for radio wave (AM only) propagation....ionosphere is composed of the D, E, and F layers the D layer is good at absorbing AM radio waves D layer dissapears at night.... the E and F layers bounce the waves back to the earth
this explains why radio stations adjust their power output at sunset and sunrise
When the ionosphere is strong or highly ionised it can bend radio frequencies back to earth than when it is weak. During the day frequencies as high as 30MHz or more may be returned whereas during the night this may be as low as 3MHz. In the summer the highs and lows are more modest and often the highs are higher and the lows lower.
The highest frequency that will return to earth is the Maximum Usable Frequency or MUF and this depends on the time of day and the season.
Any one band may only offer communications for a few hours each day. When this happens the band is said to be ‘open’. As the morning progresses it may be necessary to move up one or two bands until early afternoon when you may have to drop down again to maintain communications. Further moves down will be needed as the evening and night progresses.
World-wide propagation is possible by ionospheric or ‘sky wave’ paths and a single ‘hop’ can be up to 4000km. The radio wave can bounce off the earths surface allowing multiple hops and world-wide coverage.
These layers reflect radio waves (below about 30MHz back to the ground). Higher frequencies normally pass through (so the Klingons can watch our UHF TV).
Effective communication relies on these reflected waves.
The earth can then reflect the wave back up for another go.
So - HF radio waves can bounce around the world.
But it depends on the time of day, the frequency and the time of the year.
Each HF band will only support propagation to a particular place at certain times when it is said to be “open”.
Transmitters can cause interference to nearby electronic and radio equipment.
Radio receivers can also suffer from interference.What is Electro Magnetic Compatibility or EMC
There are so many different scenarios that it is impossible to say in advance just what the effect may be
But problems can be minimised by putting antennas as far away as possible (including high up) and by using balanced antennas at HF.
Even though your TV is not affected others further away could still be affected.
At HF, horizontal dipoles are less likely to cause problems than other types.
The more power a station runs, the more likely it is to cause interference.How is interference caused?
Direct pick up - Field strength, the strength of the radio wave, may be too high for the effected equipment. More of a problem on VHF/UHF. Reduce this by moving the transmitting antenna further away
RF conducted in mains cables- RF signals may leak out of the transmitter along its power supply leads. Fit filters on the power leads to the transmitter
RF from the Antennapicked up and conducted into different devices - Move antenna away from the house and fit filters to the effected device.
RF fed back into mains earth wires – Sort out earthing arrangements in the shackWhat can be done to minimise interference?
AKG RF filter BB1switch this into your TV\'s antenna lead if you suffer from RF breakthrough. keep your neighbours happy
An ‘inline’ mains filter
A ‘clip-on’ Ferrite RFI / EMI / TVI Filter used for Noise Suppression of electrical interference. Can be clipped on to power or coaxial cables and is available in various sizes.
Many mains filters employ Ferrite materials. These have some useful properties and can be employed in various ways. It consists of a block of ferrite material that can be clamped around a mains cable and surround a section of the cable. This means it is exposed to the electromagnetic fields around the cable.
Clip on’ ferrite blocks are easy to buy and use. However they tend to work best at high frequencies unless you have a large chunk of ferrite extending along the cable. There are however some alternative arrangements which can give high Common Modeseries impedance (the interference voltages on the two wires are identical, and they produce the same amount and direction of current on each wire). These are based on using rings or loops of ferrite materials. In
fact these filters are basically similar to the clip on block.
In the radio shack there are two separate reasons for earthing equipment, especially transmitters.
At HF an effective option to reduce interference is to use a balanced antenna such as the dipole. A balun will be needed if coaxial feeder is used and the RF on the earthed outer of the feeder is minimised if the feeder drops symmetrically away from the dipole. That is, it drops at right angles and not alongside one of the dipole halves.
End fed HF antennas always need an effective earth path. Even so, the fed-end has a high current or voltage and is prone to EMC problems such as direct pickup and pickup by mains and telephone wiring. If an end-fed must be used it is best to run the feeder down the garden and fee the antenna from the far end away from the house. A good earth on the feeder braid at this point is important.
Modes of transmission
power levels used by the amateur foundation licence (10w) the likelihood
of problems is not too high.
The different modes AM, FM, SSB and data tend to cause different
levels of interference. The modes which have a constant output are less
likely to be a problem.
FM This is the most benign since there are no changes in level at all
SSB This is the worst. The level varies continuously in time with the
transmitted voice and the interference caused sounds like a
distorted voice which is subsequently annoying.
Data Many of the data modes have a fairly constant power level and
often cause problems
Morse CW if well keyed with smooth changes from ‘on’ to ‘off’ can be
(CW) can be reasonable. Much depends on the quality of designs
of the transmitter and the keying circuits.
Immunity of most types of equipment can be improved by fitting external chokes or filters in mains leads, loudspeaker cables, and TV antenna leads etc. These must go as close to the affected device as possible.
Anything fitted to the mains wiring must be properly made for the purpose (but ferrite rings OK). Don’t mess with mains wiring and DO NOT fit home made items to the mains
Information about getting and fitting chokes and filters is readily available from the RSGB.
RF earth connection is to provide a path to ground for radio signals and prevent them entering mains wiring.
If neighbours have a problem with your transmissions you have got to deal with it and a cooperative approach is best. As to see what the problem is and carry out tests to see how the problem can be resolved. Maintaining good will is essential if a satisfactory solution is to be found. It is likely you will need to stop transmitting until you can fine the cause of the interference. Often though it is usually inadequate immunity of the effected device that is the problem. If that’s the case it may be better to get independent assistance.
Remember, if you do have an interference problem outside your own home -
Your local office of the RA may call if neighbours
complain and your station can be inspected and
are willing to risk a £50 charge.
High Current supplies also carry a risk, even if only low voltages are
A short circuit can result in currents high enough to overheat wires
and start a fire or cause a burn. Many batteries especially rechargeable
batteries can give surprisingly high currents which will cause wires to
become red hot.
Rings and metallic watches should be removed when dealing with sources
Of high current.
Brown is live
Blue is neutral
Yellow/green is earth
Make sure cable clamp in plug is clamping the outer insulation and not the individual wires.
Have no “whiskers” protruding inside plug.
Only work inside mains equipment if it is disconnected from the mains (don’t rely on switches)UK Mains plug
The mains earth is a safety earth designed to protect you if faults develop which would otherwise result in exposed metalwork becoming live. This protection also relies on the correct fuse being fitted so that the fuse will ‘blow’ before anything else becomes too hot. It is not acceptable to just fit a 13A fuse in every plug. A thin mains flex will overheat at well below 13A. Fit the correct fuse and do not use higher values even temporarily.
At this CIS level assume that it is inappropriate to wire mains plugs and work inside mains powered items. You should still be able to recognise if a plug is safe.
The reasons are beyond this CIS level but a leaflet is available on the CIS website and from the RSGB EMC committee www.rsgb.org/emc
Note: If your house has “Protective Multiple Earthing” then you need to get a leaflet from your electricity company and may need to take special extra measures e.g. bond your RF earth to the house earth.PME
In the unlikely event of an accident in your radio room, there is always
the possibility that the person has suffered an electric shock and may
still be in contact with live mains.
If you then touch the casualty then you could also suffer a shock and
become a casualty yourself.
Your first action should be to isolate the mains supply. This is best
achieved if all your sockets in the room can be isolated by one
single off switch that is clearly marked and everyone knows.
DO NOT TOUCH THE CASUALTY UNTIL YOU KNOW THAT THEPOWER IS OFF
Help should be summoned. Anyone who has suffered
an electric shock should receive medical attention.
Antennas should always be mounted so they cannot be walked into
and out of reach of being touched
RF burns are electric shocks from feeders and antenna elements carrying RF power. The burn may not be particularly painful but can be quite deep. The full extent of the injury may not be known for sometime. Antennas must not be touched when you are transmitting and insulated but unscreened antennas and wires can cause almost as much damage as touching bare wires. Antennas and feeders must be securely mounted and well away from overhead power lines. The wind loading on an antenna can be quite high during a gale which may bring it down causing immediate injury or causing a trap to be walked into later. It may blow or fall against overhead power lines resulting in all the exposed metalwork in the shack becoming live. You may not notice this risk from inside the shack. High antennas may have a ‘higher risk’ of lightening strikes. Planning permission may be needed before a large mast is installed. Erecting the antenna can also be hazardous. Working at any height carries with it the risk of falling so always have a second person around to summon help if required. Not all risky activities are obvious so an adult must be present when anyone is up a ladder. Those on the ground should have head protection in case tools are dropped.
Think about what might happen if they sag.
Make sure they are strongly mounted to withstand birds, winds, etc.
Never put anything near overhead power cables.
Antenna erection is hazardous. Things can break and fall down and always have one adult present and always have a second person to summon help.
Antennas should not normally be touched when transmitting (except low powered hand-held equipment).
High power can cause arcing and burns - also don’t leave them touching trees!Remember:-
The main risk from car batteries is the high current available. However
charging them causes the battery to give off hydrogen which can be
explosive in confined spaces so they should not be charged indoors.
Safer, sealed batteries should be seriously considered. If the battery
is tipped over they may leak fluid which is a highly corrosive acid and
any spills will need to be mopped up immediately.
Splashes on the skin require immersion in running water for several
minutes (15 minutes is recommended) and splashes to the eye need
immediate and constant irrigation, then medical attention
Firstly, it is very easy to cause hearing damage. Particularly laud noises
can cause damage and pain quickly. Changing channels after listening
to a very quiet station may cause momentary pain or discomfort which,
if repeated often, will lead to damage or hearing loss in the future.
Secondly, but of more concern is listening too loud. It may take a year
or more for this to add up to a loss of hearing but by then it will be too
late.Try turning the volume down until it is too quiet and then turn it up a
little. Some try the opposite way and turn the volume up until it is too
loud. This should be avoided but may take a deliberate effort on your
part to achieve.
Another risk, when servicing equipment, is that the headphones may
complete a the electrical circuit and enhance the effects of an electric
DO NOT wear headphones unless you are seated
and operating your equipment normally.
Wearing headphones carries two risks
1. It is easy to cause hearing damage e.g. loud noises when tuning or high volume for prolonged periods
2. When servicing equipment the headphones may completetheelectrical circuit
The shack may contain a number of potential
hazards apart from the risk of electric shock.
Tools have sharp edges causing cuts.
Soldering irons get hot and can cause serious burns.
Wires trailing across floors can trip people or drag equipment on the floor or expose live parts.
Overhanging wires are liable to snag people and other items.
Wires, especially mains wires running under carpets will become frayed over time and the damage may not be noticed until it is too late and a fire or electric shock is occurs.