Electromagnetic Waves

An electromagnetic wave has three components.

1. Magnetic field.

2. Electric field

3. A direction of travel.

The magnetic and electric components are at 90 degrees to each other and both are at 90 degrees to direction of travel.

Radiating a Signal

In order to create an electromagnetic wave in the atmosphere capable of travelling a fair distance it is necessary to feed the signal from the transmitter to an aerial or antenna.
The basic antenna is usually taken to be a piece of wire or rod half a wavelength long electrically.
 

		=  150           metres
		f(MHz)

In reality the actual length is less than this due to.

a).  The velocity of propagation in the wire is less than in free space  - velocity factor.

b).  Presence of objects near to the wire.

c).  The diameter of the wire.

Normally 5% is allowed for these factors.

The antenna has to be connected to the transmitter via some sort of cable or feeder.  The point of connection is known as the feedpoint which is usually the centre in a conventional dipole.
The connection point is important as the impedance (voltage to current ratio) changes with the feedpoint as shown below.  The diagram shows the relationship accross a full wavelength of wire.  A dipole antenna is a half wavelength.

At certain points in the antenna large currents flow whereas at others there is a build up of voltage with little current flow.  We say that the impedance of the antenna varies along its length being high at some points where high voltages exist and low where high currents flow.

Transmission Lines ~ Feeders

These are used to feed the signal from the transmitter to the antenna.  and vice versa when receiving.

Two main types are used
balanced - usually two wires separated by insulaters.
unbalanced -  such as coaxial cables.

In a balanced system neither side is earthed - in an unbalanced system one side is in effect earthed.
Feeders of either type are said to have a characteristic impedance.
This is the impedance which exists at any point along the cable if it is terminated with the same impedance.  Calculation of the impedance of a feeder is not required for the RAE and will not be dealt with here.

However if a line is terminated with an impedance (Z) other than its characteristic impedance standing waves due to resonance may occur in the feeder similar to those in an aerial wire.  The impedance then varies along the feeder.
The impedance of open and closed length as shown below should be learnt for the RAE.
 
 

It can be seen from the above that a piece of coax feeder which is a 1/4 wavelength long at the operating frequency presents a short circuit at the other end at the operating frequency only.

If a transmitter had a spurious emission at low level and was radiating interference at 98 MHz a stub a 1/4 wave long can be connected to the transmit feeder and will attenuate this frequency whilst having little effect on the main signal at say 145MHz.

It can be seen from the foregoing that power is only carried along a transmission line if it is matched correctly i.e.

The characteristic impedance of the line is similar to that of the transmitter and the aerial.

A transmitter can be designed to work into a 75 ohm impedance aerial

We have already said that the impedance of an aerial varies along its length, this fact is used in aerial matching, the best feedpoint being chosen to give the correct impedance.

Standing Wave Ratio

If power from the transmitter is fed along a transmission line to the antenna and this is not matched to the transmission line and transmitter the power is not transferred efficiently to the aerial.

        S.W.R.  =  VMAX   =  IMAX      =    ZO  or  R
                          VMIN        IMIN           R        ZO

The theoretical S.W.R. in the fig above would be  1.5:1  Note that V.S.W.R. is always expressed as a ratio and there are no units.

The S.W.R. can be measured by the use of a meter.  This measures the RF current flowing along the line in both directions and makes a comparison between the two.  The circuit is as shown below.

I

In order to maximise this property aerials are made in a form where resonance occurs ~ this means that higher currents will flow and give increased radiation.
The aerial will radiate less signal at unwanted frequencies.
Resonance occurs when the aerial is a multiple of 1/4 wavelength in length.

eg 1/4 wave    1/2 wave.

One of the reasons why most of the amateur bands are arranged in multiples  of each other is illustrated by the fact that a dipole cut for 7 Mhz (40 metres) will radiate at the third harmonic on 21 Mhz  (15 metres) when it is 1 and a half wavelengths long.

Trap Dipole

If it is intended to use a dipole on several bands traps may be incorporated

Here the LC tuned circuit of the traps shortens the aerial when working at the resonant frequency of the trap.
At a lower frequency the trap does not shorten it but brings in the outer sections and increases the length.
These  sections shown as A and B in the diagram are shorter than one might expect because the traps increase the electronic length of the aerial.

Another example where a the length is electronically increased is the 5/8 ths wave which is often used on two metres.  Here the base loading coil adds an 1/8 l to the aerial which effectively makes it resonant as 3/4 l.

1.  Vertical antennas.
All the vertical antennas are omnidirectional but the angle of radiation varies.
Usually the lower this is the better in amateur radio.
see below different directional properties.
Horizontal Antennas are usually preferred for DX working and by use of rapid switching between the two in phase circular polarization cn also be acheived.

    Above dotted lines show the principal radiation direction from

a) 1/2 wave dipole
b) 3/2 waves
c) large multiple of half waves.
d) 1/2 wave with reflector
e) 1/2 wave with reflector and directors  (Yagi or beam).

The effect of adding directors is to create a narrower angle of radiation with power concentrated into a more narrow beam.  The power  at the center of the beam is found to have increased so we say that the antenna has a gain or increased E.R.P.  (effective radiated power).

The effective radiated power is measured in watts.

The gain is measured in decibels.
Decibels are a logarithmic scale and are always measured relative to some starting point or unit.

A small i Dbi compares the increase to an isoptropic source.

A small w as in Dbw compares power to one watt.
thus 26dbw = 400 watts

Power increases by 10 times for each 10 decibels of gain.
                                  2 times for each   3 decibels.
 
 

		+1Db	1.3 times power
		+3Db	2 times power
		+6Db	4X
		+10Db	10 X
		+13Db	20 X
		+20Db	100 X
		+26Db	400 X

The decibel notation is used because it compares the power with an original level.
Consider

"Power has increased by 1 watt."

This could mean an increase from 100 to 101 watts or 0.25 to 1.25 watts

using the decibel notation :-
 

		Number of decibels =	10 log10 W2
			W1

Example:
What would the decibel increase be if the power is increased from 0.25 watts to 1.25 watts
 

		Decibel increase =	10 log10 W2	=	10  log10   5    = 7dB.
			W1

Next consider the increase from 100 watts to 101 watts
 

		Decibel increase =	10 log10 W2	=	10  log10 101     =  0.04 dB
			W1		100

The power of a radio transmitter is usually expressed in dbw  The small w means that the power is expressed as an increase in power compared with 1 watt.

ie

400 watts  =     10 log10 400    =  26dBw
                                         1

Why not check this yourself with the windows calculator?  Choose view | scientific to give access to logs.

Shunts and Multipliers

The sensitivity of a meter movement is usually quoted as n milliamps or microamps.
Where n is very small the meter movement is very sensitive and will draw a smaller current from a circuit under test than a less sensitive meter which requires a large current to move the needle

Using a meter to measure voltage.

Suppose a meter draws 1 milliamp for full scale deflection (f.s.d.) and it is desired to make the meter give f.s.d. for 100 volts.
 
 

		I =	V		.^.   .001=100
			R		R
					.^.  R    = 100	=	100,000	ie 100k ohm
					.001		1

For accurate work or where the resistance of the meter movement is itself very high this must also be taken into consideration.
 

		Rs   =	Rm   x	V     -	Rm
				Vm
						where	Rs =	resistance of multiplier
		or	Rs   =	Rm	(V - 1)		Rm =	resistance of meter movement
					Vm		V =	required voltage for f.s.d.
							Vm =	voltage accross meter
								movement for f.s.d.

Current Measurements

In order to increase the range of current to be measured by a meter a shunt resistor is wired accross the meter movement.

The resistance of the shunt which is made from thick wire or a plate of a suitable alloy to give a stable low resistance is given by :-
 
 

		Rs   =	Rm
			n  -  1
				where	Rs =	resistance of shunt
					Rm =	resistance of meter
					n =	scale multiplying factor

Sometimes the resistance of a meter will not be known,  it can be found experimentally by passing a constant current through it to give a reading on the meter.  A resistor is then connected accross the meter and its value is varied till the meter reading is halved.  This resistor is then of equal value to the internal resistance of the meter.

Meter Protection

~  Meters and in fact many other delicate circuits often use two diodes accross there input terminals which take any excess current and helps protect the device.
It has already been noted that a silicon diode has a very high resistance until about 0.5 volt is applied to it voltages in excess of this are shunted through the diodes thus avoiding damage to te meter.

Transistor Voltmeter

Consider making measurements on the following circuit which is quite common as a microphone amplifier.

    A multimeter has a resistance of 20,000 ohms per volt  ~  the 5 volt range is suitable for making the test for 1 to 6 volts on the transistor base so the the resistance is 100,00 ohms  (5 X 20,000).
in this range.  As it happens this is also similar to the base emitter junction of the transistor.  As a result the reading on the meter is about half that we expect and we might wrongly conclude that a fault was present.

    To overcome this problem we need a meter with a very high input resistance approaching infinity.
This results in very little current being drawn from the circuit so that its operation is not affected.

This is accomplished by using an amplifier circuit in front of the meter to boost its sensitivity.

Valves,  biplar transistors and latterly F.E.T's have been utilised for this purpose.  Modern I.G.F.E.T.  (insulated gate Field effect transistors)  have an input resistance of many tens of megohms and are often used in electronic voltmeters.

A simplified arrangement is shown here.

N.B. At zero volts 0.5 amp flows through 4k7 which gives a voltage at point B of 2.35 volts.
The voltage at the gate of the F.E.T. will be about the same and no current will flow.

Power Supplies

In this section we look at how to obtain DC voltages from AC mains.

Amateur radio equipment draws its power from a variety of supply voltages which tend to be governed by the types of devices used.

For instance a 2 metre PA running in Class C may use a transistor which gives 25 watts out for 2 watts in at 24 volts. Reducing the voltage reduces the power gain drastically so that at 12 volts the same device may only give 10 watts out. Increasing the supply voltage will increase the gain but the device will break down if too much voltage is applied.

In the power supply the voltage is determined largely by the mains transformer. The secondary voltage V_S is given by:-
 
 

V_S = No. of turns on secondary X V_P
            No. of turns on primary

V_P = primary voltage

Rectifying circuits

1) Half Wave - produces 50 Hz ripple

C₁ is known as the reservoir capacitor. It's purpose is to store energy during the positive half cycle and supply the load during the negative half cycle.

C₁ is usually very large typically 10,000 mF per amp drawn from the supply.

The diode only conducts for that part of the half cycle when the positive voltage is above the of the charge in the capacitor. A high current of short duration flows through the diode. - this is often limited by a series resistor to prevent an excessive surge at switch on when the capacitor is fully discharged.

Capacitors for this application must not only be of the correct working voltage and capacitance but also be able to withstand a high ripple current without overheating.

Peak inverse voltage across rectifier.

This is usually the pk-pk voltage of the AC supply.

Bridge Rectification

Here an arrangement of four diodes gives full wave rectification. This means that both negative and positive half cycles are conducted through the diodes and charge the reservoir capacitor twice as often.

- the frequency of the ripple is therefore twice that of half wave rectification i.e. 100 Hz.

Used for high voltages at low currents often to drive CRT's in oscilloscopes.

Advantage is that the transformer can be made smaller because less turns are required on the secondary and less insulation. High voltages can cause breakdown in transformer windings.

Sometimes an earthed metal screen is placed between the windings of a transformer. Its function is to remove interference from the mains and in the case of a transmitter it helps prevent signal being radiated by the mains. The screen is made of thin copper and does not form a continuous layer right round the transformer as this would act as a shorted turn and cause the transformer to overheat.

Regulated power supplies

In the circuit below a transformer reduces the AC mains voltage to about 12 volts AC. This is converted to DC by a rectifier bridge . A single half wave rectifier may be used but a bridge is more efficient. The resultant voltage is about 18 DC. This is reduced and stabilised by the regulator circuit to give a stable 12-14 volt DC at the output. Often an IC with as few as three connections forms the regulator circuit.

Notes on Safety

Low voltage power supplies are often capable of delivering high currents.

Although DC voltages less than 50 volts are unlikely to cause serious electric shock the high currents involved can cause metallic objects to get very hot if a short circuit occurs. Always switch off the power before using metal tools in equipment and take care not to wear rings when working on equipment where there is a risk from exposed terminals.

All equipment in the radio shack should be connected to a common switch. Other members of the household should be aware of its position so that it can be turned off quickly if a problem occurs.

The earthing of the mains outlets should be checked to ensure that it conforms to IEE regulations.

All wiring should be properly insulated and high voltage connections must not be exposed.

Capacitors in power packs should have a suitable bleeder resistor across there terminals so that they do not become a shock hazard when the equipment is serviced. The RSGB recommends that this applies to high voltage capacitors over 0.01mF. The size of the bleeder resistor should be 1/C megohms.

Indicator lamps showing when mains is on should be installed and maintained on equipment.

Double pole mains switches should always be used with the correct type of fuses. Switches should be off when fuses are changed.

If metal cased equipment has inspection covers which can be easily opened the use of micro switches which turn off the power when opening is recommended.

Test prods and lamps should be insulated.

Attention to floor coverings is important. Rubber or suitable insulating materials help prevent serious shocks. Damp increases the likelihood of electric shock.

It is always best to switch off before making adjusments. If live adjustments to equipment cannot be avoided always use one hand and keep the other in your pocket. Always use tools with insulated handles.

Do not wear headphones when making internal adjustments to live equipment.

Ensure metal cases of microphones; Morse keys; are properly connected to the chassis of equipment.

Do not use meters with metal adjusting screws or control knobs with metal grub screws and shafts on high voltage equipment.

Mains and other high voltages should be avoided on antennas. An Rf choke will provide a suitable DC path to earth.

RF voltages can be a hazard if a person comes in contact with a high voltages node on an antenna. Transmitting antennas should be suitable sited and all cables insulated.
 

Measurement of RF power ~ Dummy Loads and Modulation Monitors.

In order to extract power from a transmitting device it is necessary to provide a good impedance match to the device.

Failure to do so causes excessive power to be dissapated in the same way as a battery which is overloaded.

Also because of the nature of RF standing waves can be produced in the transmission lines at points of mismatch and energy is reflected back to the transmitter increasing heat dissipation still further.

Aerials and feeders have to be carefully designed to match to the transmitter.

If tests are to be carried out on a transmitter it is necessary to provide a dummy load of the correct impedance otherwise maximum output power will not be obtained and the equipment could suffer damage.

Dummy loads are made from non inductive resistors which must be capable of dissipating the required power. At H.F. they are often enclosed in an oil filled container. The oil helps dissipate the heat. Another requirement is that the load should be screened as unwanted radiation (transmission) may occur during tests. Non inductive loads are used to avoid resonances which would occur with stray capacitances and produce impedances which varied with frequency. V.H.F. loads are carefully designed to avoid this.

Power Measurement

A thermocouple meter (RF ammeter) is the best way of measuring R.F. power and is often used in conjunction with a dummy load. The thermocouple device consists of a glass envelope containing a filament or heater which is close to a thermocouple. This develops an E.M.F proportional to the heat generated which is read on the meter.

The thermocouple unit may be enclosed in the meter or may be part of a combined dummy load, this latter arrangement is helpful at V.H.F. where leads to a meter could cause problems because of there inductance and ability to radiate.

Causes of interference to other users either amateur or other services due to incorrectly aligned or set up transmitters are as follows.

1.  Key clicks on C.W.   -  keying the wrong stage
2.  Overdriving of the P.A. or other stages.
3.  Incorrect bias level applied to linea stages e.g. amplifiers which should be linea running in Class C.
4.  Spurious emissions due to poor choice of mixing frequencies.
5.  or oscillation of P.A. or other stages due to instability.

1. Key Clicks on C.W.
 
 

The coil capacitor and resistor above are arranged to slow the rise and fall times when the key is operated thus acting as a key click filter.
Also it is good practice to key the lowest power stage possible but not the oscillator which should run continuously at a stable frequency.
Keying the oscillator produces an effect known as "chirp" which is caused by the oscillator taking time to stabilise each time the key is pressed.

Overdriving of the modulating or P.A. (power amplifier) stage in telephony produces.

a)   Flat topping (squaring)
of envelope on a.m. or s.s.b.
This is rich in harmonics and produces side bands outside wanted bandwidth -- a wide signal.
An oscilloscope or modulation monitor which is a specialised form of the former reveals.

b) Over modulation of F.M. or P.M.(phase modulation)   -  excessive deviation produces unwanted extra sidebands.
-- a wide signal.

c)  Incorrectly set up P.A. Stages producing non linearity. ie due to faulty or incorrect setting up a linea runs in Class C.

Unwanted harmonics are produced
-- wide signal with distortion

d)  Spurious emissions due to insufficient rejection of frequencies involved in mixing.

Self oscillation of P.A. stages -- feedback due to poor design inadequate decoupling etc.

Production of harmonics often by overdriving the P.A.
This can be detected and then avoided by the use of an absorbtion wavemeter to check for harmonics and other spurii.

Holders of an amateur  license should undertake such checks from time to time.

Filtering of Harmonics and Spurii

Harmonics of the wanted ouput frequency may fall on or near T.V. and F.M. radio channels leading to complaints grom neighbours For example 5 x 144 MHz = 720 MHz  which is channel 2 U.H.F. television.

On h.f. harmonics often fall inside another amateur band i.e. 3.53MHz x 2 = 7.06 MHz

Harmonics are generated in high power P.A. stages especially if running in Class C although linea Class A AB and B amplifiers will also produce harmonics especially if driven to there limits.

Spurious Oscillations
 

Sometimes due to poor design or choice of components an amplifier stage in a transmitter will burst into oscillation or a product of mixing may produce an undesirable frequency.
These oscillations are known as spurii and may occur at any frequency unlike harmonics which are related to the carrier frequency.

Filters may be used to remove both harmonics and spurii.
R.F. Filters consist of a passive network of capacitors and inductors which exhibit certain characteristics as the input frequency is varied.

The filters of most interest in amateur radio are
(a) Low pass filters which pass all frequencies below that specified.
~ attenuate frequencies above

(b) High Pass filters which pass all frequencies above that specified.
~ attenuate frequencies below.

(c) Band Pass Filter
~ Pass those frequencies which lie between a lower and upper limit and attenuate the rest.
These are made using low and high pass and passing the signal through both.

Generally the most common reason why interference occurs to TV sets with modern rigs is not so much that the tranmitter is radiating out of band but that the signal is extremely strong in the vicinity of the QTH.  I have found that if interference occurs it can usually be largely prevented by inserting a belling lee style adapter high pass filter in the path of the aerial feeder where it enters the TV or video recorder.  Also aerial boosters often used to improve Channel 5 reception are often the cause of the problem.  If such a device is fitted fit a high pass filter here too.  They only cost a couple of quid.

Sometimes a low pass filter may need to be fitted between the transmitter and antenna.  This will remove harmonics and spurii generated in the transmitter.  This device has to handle the full output power from the rig and could cost a couple of hundred pounds especially if you get a modified licence which permits higher power.

Absorption Wavemeter
 

The circuit below constitutes an absorption wavemeter and can be used for detecting harmonics and spurii .
  ~   it is a condition of the amateur license that one is available for checking for unwanted emissions from time to time.  Details of such tests shall be recorded in the station log.

Heterodyne Wavemeter

Consists of calibrated variable frequency oscillator with slow motion drive.  To check the frequency accuracy the signal from the oscillator is picked up on a receiver (which need not be calibrated accurately) and a crystal oscillator is switched on.  Zero beat with the crystal oscillator should be heard at multiples of the crystal fequency.  Usually every 100kHz.

Mains born Interference

This is not uncommon and in attached properties can be a problem with neighbours.
Regard should be made to the following.

a)  Transmitters should not use the mains earthing system.

b)  Filters may be used to minimise interference.

c)  Aerials should not be close to mains wiring.

Using Decibels

Decibels are used in amateur radio and electronics for relative measurements. The scale is logarithmic and is uses base 10 logarithm.

Although we can show the relationship as

power decibels = 10 * log₁₀ power in watts.

The value given should be shown as Db_w which means power is relative to one watt.

There is an easy approach to decibel problems which depends on remembering a few simple relationships.

For every 3dB the power doubles.

For every 10 dB the power increases 10 times
 
 

?     ~      What power in watts is 23 dbw 20/10 = 2 add 2 zeros

=100 watts

for 3 db the power doubles so 200 watts

If calculation is about voltage gain for instance in a receiver then this is given by

voltage gain decibels = 20 * log₁₀

The same rules apply then double it.

To get more practice using decibels and familiarise yourself with the relationship between dBw and watts load down the RAE calculater.  Practice with figures from the BR68 booklet converting powers allowed on different bands.  Note the restrictions in power for parts of top band and 6 metres.

Rae Calculator

You may need the vbrun200.dll for this also.  The program requires no installation simply copy to a folder along with the dll if required.
 
 

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